hello this is Dr Henna aseel and this is about calculations involving mold so the first question is what is a mole you should realize that mole is the relative atomic mass in grams or the molecular mass in grams we know that the molecular mass is the total of the masses from the periodic table so if we have sodium sodium has an atomic mass or mass number of 23 so that means that if I have 23 grams of sodium that is regarded as one mole so it's a certain amount if I have sodium chloride you should realize that the molecular mass of sodium chloride or the mass of one mole of sodium chloride is the total of the mass numbers of sodium and chlorine so that comes out to 58.5 and that means that if I have 58.5 grams of sodium chloride that is regarded as one mole if I have twice of that then that is two moles and so on so the mole is the molecular mass in grams now we should also remember that one mole of any gas and this is we're talking about gases occupies a volume of 24 decimeter cubed at room temperature and pressure so if I have one mole of any gas its volume is 24 decimeter cubed at room temperature and pressure now Avogadro's Law says that equal volumes of gases contain the same number of particles so we have something called the Avogadro constant Avogadro's constant is 6.02 times 10 to the 23 and this says that one mole of a substance has this number of particles or this number of molecules one mole of the substance has 6.02 times 10 to the 23 molecules and that means if we want to get the number of moles one mole has 6.02 times 10 to 23 the number of moles would be a number of molecules over 6.02 times 10 to the 23 so this is from Avogadro's Law the point of this questions would be to calculate number of moles so we need to know how do we calculate number of moles well that depends on what we have so for example if he gives me a mass in grams he says I have 5 grams 10 grams 25 grams whatever and he's asking what is the number of moons then I use this equation number of moles is mass over molecular mass number of moles is Mass which has to be in grams over molecular mass and that means from this equation can I get Mass if I need to get it Mass would be number of moles times Mr or molecular mass but what if we don't have a mass in grams what if he's talking about gases and he gives me a volume and that has to be in decimeter cubed then I can get number of moles is volume over 24. remember that the volume in any of these equations has to be in decimeter cubed so if he gives it to me in centimeter cubed I have to divide by a thousand okay what if we're not talking about gases what if we're talking about sodium hydroxide solution hydrochloric acid solution then the number of moles in that case would be concentration times volume and again the volume has to be in decimeter cubed and that means if I'm asked to get concentration then the concentration of a solution is number of moles over volume so please keep these equations in mind we need to know these three equations number of moles is mass over molecular mass number of moles is volume over 24 only for gases number of moles is concentration times volume only for Solutions so he's going to give me a certain question and let's apply all of this so for example a simple question would be calculate the number of moles of sulfuric acid in 20.0 centimeter cubed of a one mole per decimeter Cube solution so the first question you're going to ask yourself is what does he give me he gives me 20 centimeter cubed remember that anything in centimeter cubed is the volume one mole per decimeter cubed anything in mole per decimeter Cube that is concentration so he's giving me volume and concentration and he's asking me to get number of moles so which of these equations do I use you remember we said if we have a solution we have concentration and volume then the number of moles is concentration times volume concentration is one mole per decimeter Cube now we said the volume has to be in what the volume has to be in decimeter cubed so if he gives it to me in centimeter cubed I have to divide by a thousand so this number of moles is 0.02 more another question says what is the concentration in mole per decimeter Cube so he's asking me for concentration of a solution containing 7.84 grams of phosphoric acid which is h3po4 in 400 centimeter cubed of solution so he's asking for concentration but he's giving me what he's giving me Mass 7.84 grams and volume 400 centimeter Cube so how am I going to get concentration in order to get concentration I need that last equation which says concentration is number of moles over volume but that means I need to get number of moles how do I get number of moles you should realize that he gave me mass so I'm going to use that very first equation number of moles is mass over molecular mass so the mass is 7.84 the molecular mass is the addition of all the masses so I have three hydrogens each has a mass of one plus I have one phosphorus with a mass of 31 all of this is from the periodic table plus oxygen which is 16 I have four of them so that comes out to be a certain number and that gives me 0.08 moles so that is the number of moles I can then use it to get concentration we said concentration is number of moles over volume again please remember the volume you cannot put it as it is it's in centimeter cubed so before you put it into the equation you have to divide by a thousand and that gives you the concentration okay usually he will give you an equation and he will ask something about the equation so for example here we have the equation of the reaction of sodium carbonate with hydrochloric acid to give sodium chloride plus water plus carbon dioxide gas now what is the volume of HCL needed to react with 5.3 grams of sodium carbonate if the concentration of HCL is this now you have to remember that a balanced equation gives you the ratio between the number of moles so that equation on top tells me there is no number in front of sodium carbonate so that is one so if I have one mole of sodium carbonate it reacts with two moles of HCL to give two moles of sodium chloride one mole of water one mole of carbon dioxide now what is he asking about he's asking about the volume of HCL but what does he give me so you look at what he gives me he gives me 5.3 grams of sodium carbonate so in all of these equations he will give me a certain information so now he's giving me the mass of sodium carbonate and then he's asking me about HCL but from the mass of the sodium carbonate I need to get number of moles so which of these equations am I going to use to get number of moles of what he is telling me so he's telling me 5.3 grams that is mass so I need to get number of moles of sodium carbonate using that very first equation Mass over molecular mass 5.3 over the molecular mass of sodium carbonate again I'll remind you that molecular mass sodium from the periodic table has a mass of 23 I have two of them so 23 times 2 plus 1 carbon with a mass of 12 plus uh three oxygens with a mass of 16 so this should give me the molecular mass of sodium carbonate I put 5.3 over that that gives me 0.05 mole of sodium carbate so he gives me information about something so here he gives me information about sodium carbonate I get from that information the number of moles then what is he asking me about he's asking me about hydrochloric acid so I look at the equation so the second step is I look at the equation the equation says that one mole of sodium carbonate needs two moles of HCL so if this is the number of moles of sodium carbonate 0.05 what would be the number of moles of HCL when the equation tells me that it should be twice of the sodium carbonate so it is two times the 0.05 this gives me the number of moles of HCL then what is he asking about he's asking about volume so how do I get volume and remember this is volume of a solution so the volume would be number of moles over concentration I got the number of moles 0.1 and he gives me the question that the concentration is 2 so that gives me a volume in decimeter Cube now if he wants it in centimeter you to change from decimeter Cube to centimeter Cube you multiply by a thousand so that means that the volume of HCL needed is 50 centimeter cubed are we following let's try another question here he says I have 9.8 grams of potassium chlorate and he already gives me the formula of potassium chloride because we did not talk about chlorate in this syllabus was heated to give potassium chloride and oxygen and first he wants me to Write the balanced chemical equation okay let's start potassium chlorate he already gives me the formula now how do I write the formula of potassium chloride potassium chloride potassium valency one chlorine valency one so this is potassium chloride plus oxygen I'm reminding you again that oxygen is diatomic that means I write it O2 then you have to balance how do you balance you increase the one that is less so if we have one potassium before the arrow one potassium after the arrow one chlorine before the arrow one chlorine after the arrow but the oxygen is less after the arrow so I need to increase it if I put 3 here and 2 there then there will be balanced but that means I have ruined the potassium chloride so that is the balanced equation then what is the question determine the mass of potassium chloride formed okay how am I going to get that I look at the information he gives me he gives me 9.8 grams of potassium chloride and we said from the information he gives me I need to get number of moles again which equation should I use the one that has mass so number of moles is mass over molecular mass that comes out to be 0.08 mole so I got the number of moles of what he gave me information about now what is he asking about he's asking about potassium chloride I look at the balanced equation the balanced equation tells me two of potassium chloride gives two of KCl that means the number of moles of KCl is the same as the number of moles of potassium chlorate so number of moles of KCl is the same as what I got then what is he what does he want he wants the mass how do I get Mass from the equation mass is number of moles times m r so I put the number of moles times the Mr of what I'm talking about KCl this gives me a certain mass of potassium chloride the other part of the question says determine the volume of oxygen produced so now we're looking at oxygen we already talked about kclo3 so kclo3 I have the number of moles of the KCl O3 now I look at the equation and I find that 2 of kclo3 would give three of oxygen so the point O eight will give how much that means that the number of oxygen is 0.08 times 3 over 2. this gives the number of moles of oxygen what is he asking about volume how do I get volume remember we're talking about volume of a gas so I'm going to use volume is equal to number of moles times 24 and that gives me a volume in decimeter okay another type of questions he gives me a reaction and he says 25 centimeter cubed sample of 0.2 mole per decimeter Cube potassium hydroxide was neutralized by 28.40 centimeter cubed of sulfuric acid so it's giving me information of potassium hydroxide it has 25 centimeter cubed which is volume 0.2 mole per decimeter Cube which is concentration so I have concentration and I have volume and this is a solution so which equation should I use number of moles is concentration times volume remember the volume has to be divided by a thousand so this gives me number of moles of Koh good then he's asking about what number of moles of sulfuric acid so I go back to the equation what does the equation tell me it says two moles of Koh react with one mole of sulfuric acid that means the number of moles of sulfuric acid is actually half of what we got for potassium hydroxide so that gives me number of moles of sulfuric acid and he is asking about what concentration of sulfuric acid so how do we get concentration of a solution that's the last equation there concentration is number of moles over volume we got the number of moles and he gave me the volume up there 28.4 centimeter cubed so I divided it by a thousand before I put it into the equation getting better let's try another one this is a reaction between ammonia and nitric acid to form ammonium nitrate and he's saying 25 centimeter cubed of ammonia of concentration 0.3 mole per decimeter Cube so it's giving me information about ammonia once reacted with nitric acid and 15 centimeter cubed of nitric acid was required calculate the concentration of nitrates again he gives me information about what about ammonia he gives me the volume 25 centimeter cubed of ammonia and concentration so which equation should I use these are solutions number of moles is concentration times volume so the 0.3 times the volume divided by a thousand that gives me number of moles of ammonia then he's asking about what nitric acid I look at the equation can you see the equation one mole of ammonia reacts with one mole of nitric acid and that means number of moles of nitric acid is the same as ammo then he's asking about calculate what calculate concentration concentration is number of moles over volume again the volume he said 15 centimeter cubed of nitrix so I divided by a thousand before I put it in and I get the concentration of okay a similar question here says he finds that 5 centimeter cube of 0.90 mole per decimeter cubed of potassium iodide solution reacts with eight centimeter cube of lead nitrate calculate the concentration of lead nitrate again let's go through this very quickly the first thing I need to do is get the number of moles of what he tells me about so he's telling me about potassium iodide number of moles of potassium iodide is solution concentration times wall so the concentration times the volume divided by a thousand gives me number of moles of potassium iodide then I look at the equation that's step two look at the equation what does the equation tell you that the number of moles of lead nitrate is half of the potassium iodide that gives me the number of moles now he wants concentration concentration is number of moles over volume are we okay with all of this and this gives me a concentration remember the units for concentration is mole per decimated are we okay let's try one more a chemist neutralizes 25 centimeter cubed of 0.5 mole per decimeter cubed HCL calculate the amount in moles of HCL so what was the number of moles of HCL number of moles what does he give me he gives me volume and concentration so number of moles is concentration times volume and then he's saying calculate the mass of calcium hydroxide so we got number of moles of HCL I look at the equation what does it tell me that the number of moles of calcium hydroxide is actually half of the HCL so I get number of moles of calcium hydroxide and then I can get the mass mass is number of moles times m is that okay another one he says calculate the volume in decimeter cubed of carbon dioxide formed when 0.03 mole of anhydrous magnesium carbonate is completely decomposed again he gives me here the number of moles of magnesium carbonate so I really don't need to sit down and calculate number of moles so I can look directly at the equation and you can see that the number of moles of magnesium carbonate is the same as the number of moles of CO2 so the number of moles of CO2 is 0.03 now what is he asking about volume of carbon dioxide okay how do we get volume of a gas volume is number of moles times 24 this gives me the volume in decimal okay this is another kind of question so please pay attention here he's saying 20 centimeter cubed of a hydrocarbon was burnt in 175 centimeter cubed of oxygen now first of all you remember what we mean by hydrocarbon hydrocarbon is something that has carbon and hydrogen so here we have carbon and hydrogen but the problem is we don't know C what H what so we're trying to get what is X and what is one and he's saying I started with 20 centimeter cubed of the hydrocarbon so that is 20 centimeter here uh when we do organic you will remember that burning of any hydrocarbon in oxygen will give carbon dioxide plus what so he has excess oxygen so it gives carbon dioxide and water and we're trying to get the balanced equation so that I can get X and Y okay so he says 20 centimeter cubed of a hydrocarbon was burnt in 175 centimeter cubed of oxygen this is excess this is not the actual volume that react now after cooling the volume of the remaining gases was 125 remember that the 125 is the total of what of the carbon dioxide that was formed plus anything remaining from the oxygen okay he added aqueous sodium hydroxide to remove the carbon dioxide you know that carbon dioxide is acidic so it reacts with the base and he found that after he removed the carbon dioxide he was left with 25 centimeter cubed of unreacted oxygen remember we had 125 he removed the CO2 and we're left with 25 that means the CO2 was how much a hundred and then 25 of the 175 remained that means how much oxygen reacted one-fifth so the volume of oxygen used was 150 the volume of carbon dioxide formed was 100 now he's saying deduce the formula remember that we said Avogadro's Law says one mole of any gas occupies the same volume so one mole of we can relate directly in the equation using number of moles or using volumes of gases remember we can relate directly in the equation number of moles or volumes of gaskets in all the previous questions we were relating number of modes here we have all gases so we're going to relate the volumes I don't need to calculate number of moles so if 20 centimeter cubed of the hydrocarbon reacts with 150 of the oxygen to give 100 of the CO2 I can simplify these ratios dividing by 20 I will get 1 of c x h y plus 7.5 of the O2 to give 5 of CO2 remember the 2150 100 I divided all through by 20 to simplify the ratios so this gives me the ratio of the number of moles so this gives 1 to 7.5 to 5. now looking at that that means that X is 5. and then if I balance all of this I will need to put 5 in front of the water and that means my H is 10. so this is the balanced equation and that means my hydrocarbon is c5h10 please pay attention to this okay this kind of question comes a lot in several papers okay another type of question says basic lead to carbonate has a formula x p b c o three y pboh twice where X and Y are whole numbers and we're trying to determine what is X and what is one and he's telling me that the leg carbonate alone of course his his heating all of this the basic lead to carbonate which is X with Y but the let carbonate alone will give carbon dioxide gas the lead hydroxide alone gives the water now he's giving me information when heated the basic lead to carbonate gate 2.112 gram of carbon dioxide and 0.432 grams of water so first of all he wants you to get the number of moles of each of course number of moles is mass over molecular mass so for the CO2 is the mass of the CO2 over the mass of one mole remember that the word mass of one mole means molecular mass means instead of sitting down to calculate carbon is 12 plus oxygen is 16 times 2 he already calculated it for me so mass of one mole of something is the molecular mass so we get number of moles of CO2 also we have a certain mass of water so again we're going to use the same equation Mass over molecular mass so now I have values for a number of moles remember to get X and Y I need to simplify this so if I divide through by the smallest this gives me a ratio of 2 to 1 and that means that the formula up there is actually two of lead carbonate one which is not written of lead hydroxide are we following okay now another thing that you will be required to calculate is percent Purity and you have to keep in mind that the percent Purity is the mass of the pure substance over the mass of the impure times a hundred now what does this mean let's take a look at this question he's telling me an excess of hydrochloric acid was added to 1.23 grams of impure barium carbonate now if he's giving me mass of impure something that means you're not going to use that mass for anything you're not going to use that Mass to calculate number of mood because actually it is not the actual mass of barium carbonate it is the mass of barium carbonate plus impurities so don't use the 1.23 grams when he says it is impure something don't use it to get number we're going to use the other information that he gives me okay what is he talking about he says the volume of carbon dioxide collected at and this is room temperature and pressure RTP is room temperature and pressure was 0.12 decimeter Cube so he's giving me the volume of carbon dioxide and he's saying calculate the percent purity from the volume of the carbon dioxide I will get number of moles so if he gives me volume of a gas how do we get number of moles number of moles is volume over 24 and then I look at the equation and I find that the equation says one mole of CO2 is equal to one mole of barium carbonate the number of moles are the same so the number of moles of barium carbonate is the same as that of carbon dioxide so that comes out to be the same number then we use the number of moles of barium carbonate to get Mass how do I get mass mass is number of moles times m r so this 0.985 is the actual mass of barium carbonate that we have but if he wants the percent Purity we said that is the mass of the pure 0.985 over the mass of the impure 1.23 times 100 and that means that my barium carbonate sample was only 80.1 percent few to calculate percent yield you should know that percent yield is actual yield over theoretical yield times a hundred what does that mean let's try a question he's telling me 4.8 grams of hydrogen reacted with nitrogen to give 12.8 grams of ammonia calculate the percentile now percent Yin that means I'm not going to use the 12.8 now for anything he is telling me information about 4.8 grams of hydrogen I will use that to get number of moles of hydrogen so that is mass over molecular mass remember we're talking about hydrogen that is H2 I'm reminding you that to get the molecular mass of something you do not include the balancing so when we say Mr of hydrogen it's H2 H2 has an MR of two you don't use that three in the balancing for anything to get number of moles now so he's saying number of moles of hydrogen Mass over molecular mass 4.8 over the two that is 2.4 moles and then I look at the equation and decide what would be the number of moles of ammonia well the equation says three moles of hydrogen have two moles of ammonia so the number of moles of ammonia is the 2.4 times 2 over 3. that gives me number of moles of ammonia now I need to calculate what is the theoretical mass of ammonia so from your calculation you get that you're supposed to get 27.2 grams of ammonia but what did he actually get he got 12.8 and that means his percent yield is what he got 12.8 over what he should get he should get 27.2 times a hundred that gives you your percent yield so let's take a look at this question he's saying hydrobromic acid reacts with magnesium carbonate to form a solution containing magnesium bromide crystals of hydrated magnesium bromide can be obtained from the solution and the question is an excess of hydrobromic acid is reacted with 0.125 mole of magnesium carbonate so he already gives me the number of moles I don't need to sit down and calculate number of moles of magnesium carbonate now show by calculation that the maximum theoretical mass of hydrated can be that can be made is 36.5 Yani he's saying calculate the mass that you should get okay so he says I have 0.125 mole of magnesium carbonate so I look at that equation on top to get the number of moles of the hydrated magnesium bromide that would be the uh same as the number of moles of magnesium carbonate looking at the equation on top you can see that number of moles of magnesium carbonate is the same as magnesium bromide then he says the mass should be what the mass is the number of moles times Mr so the 0.125 times the Mr of the hydrated magnesium bromide which he gives me there so this is the mass then he says in an experiment using point one two five mole of magnesium carbonate so that is the same number of moles from that first question with an excess of hydrobromic acid the mass of hydrate magnesium bromide obtained is 26.4 we just calculated that he was supposed to get 36.5 grams but actually when we do any experiment in the lab what we get is usually less than the theoretical yield or what we should get so in this experiment we should get 36.5 grams but when he did it the mass obtained is 26.4 so just two reasons why the actual mass obtained is less than the theoretical Mass we said always when we do an experiment in the lab so if I'm trying to prepare these crystals in the lab I'm supposed to get 36.5 but I will always get less so here for example he got only 26 now why do you think we get less than what we should get probably what we start with is some of it did not react so some magnesium coordinates did not react or that the magnesium carbonate we started with was impure these are two of the reasons why what we actually get in the lab is usually less than what we should get okay this question says copper carbonate is broken down by heating to from copper oxide and carbon dioxide and he gives me the equation and he says 31 grams of copper carbonate are heated until all the contents of the test tube have turned from green to Black of course you should know that copper carbonate is green copper oxide is black and he says the yield of copper oxide is 17.5 that means that when he did the experiment in the lab what he got was 17.5 grams and he's asking me what is the percentage yield in order to get the percentage yield I have to calculate how much he should have got so what is the theoretical math that he should have got okay he gives me information about 31 grams of copper carbonate I get the number of moles of copper carbonate copper a number of moles is mass over molecular mass this is the molecular mass of copper carbonate so the number of moles comes out to be 0.25 more and then we want to know copper oxide we look at the equation and we find from the equation the number of moles of copper carbonate is the same as number of moles of copper oxide so the 0.25 mole is number of moles of copper oxide and from that I can get mass mass is number of moles times Mr and we calculate that we are supposed to get 20 grams now what did he actually get he got 17.5 remember the word yield means the mass that was obtained so that means we said how do we get percent yield it is what he actually got 17.5 over the theoretical that we calculated so over what he should get which is 20 times 100 that gives me a percent okay so from these choices my answer is d another kind of question he has equations of strontium chloride made from strontium carbonate and he says in the above experiment 50 centimeter cubed of hydrochloric acid of concentration two mole per decimeter cubed was used 6.4 grams of strontium chloride was made so this is what he actually got calculate the percentage yield again he started with 50 centimeter cube of HCL concentration two more per decimeter cubes so I can use that to get number of moles number of moles is concentration times volume again don't forget to divide the volume by a thousand so that gives me the number of moles of HCL is 0.1 mole then he is asking about strontium chloride I look at the equation on top what is the relationship between number of moles of HCL and number of moles of strontium chloride the equation says two of HCL gives one of strontium chloride and that means number of moles of strontium chloride is half of what we have for HCL now then he says mass of one mole again I'm going to remind you that mass of one mole is the same as molecular mass so he already calculated masses of strontium plus chlorine plus hydrogen and so on to get 267 grams so you don't need to sit down and calculate molecular mass and he is asking for theoretical yield again I'm going to remind you the word yield means Mass so he's asking for the math I already have number of moles so the mass is number of moles times the m r that he already gives me so that means I'm supposed to get 13.35 grams but what did he actually get if you look on top he got only 6.4 grams so his percent yield is the 6.4 over what we calculated which is the theoretical yield times a hundred and that gives me the percent yield okay limiting reactants we use limiting reactants when he gives me information about both react so if he's giving me information for example I have nitrogen plus hydrogen to give ammonia in all the previous questions we had information about only one of the reactants and we use that to get anything he's asking about but here if he gives me information about both reactants I have to decide which one is excess and which one is limiting so the limiting reactant is the one that is not excess so if he gives me information 50 grams of nitrogen and 40 grams of hydrogen I calculate number of moles of nitrogen Mass over molecular mass and number of moles of hydrogen Mass over molecular mass and I get the number of moles of each then I look at the equation and decide which one is excess and which one is limiting so looking at the equation we can see that one mole of nitrogen needs to react with three moles of height and that means that the number of moles of hydrogen should be three times that of nitrogen so how much nitrogen do we have we have 1.79 mole let's say around two moles what would be the number of moles of hydrogen it should be six moles or less than six moles how much did he actually put he actually put 20 moles that is way too much more than needed so we see that the hydrogen is excess it is more than needed we need only about six moles and we put 20 so the hydrogen is excess and that means the nitrogen is limited and when he asks about anything else we use the information about nitrogen not the information about time so I'm not going to use the number of moles of hydrogen for any further calculations I look at the number of moles of nitrogen so he wants ammonia ammonia is twice that of nitrogen can you see that from the equation one mole of nitrogen would give two moles of ammonia so the number of moles of ammonia is twice what we have for nitrogen and that means I can get the mass of ammonia number of moles signs Mr another example here he says 0.096 grams of magnesium was added to 25 centimeter cubed of 0.4 mole per decimeter cubed hydrochloric acid again here he's giving me information about both the ACT so I need first to decide which one is excess so number of moles of magnesium what do we have 0.096 so I'm going to say Mass over molecular mass that gives me a certain number of moles of magnesium now we want number of moles of HCL what do we have concentration times volume that gives me number of moles of HCL and then I look at the equation in order to determine which one is excess I look at the equation the equation says that the number of moles of hydrochloric acid should be twice magnesium so if I have 0.004 mole of magnesium how much of each cell do I need I need twice of that that's 0.008 that is less than what he put He put more than what we need so the HCL is excess are we following all of this okay again calculate the mass of carbon dioxide given off when 20 grams of calcium carbonate react with 40 centimeter Cubit of hydrochloric acid of concentration this so he's giving me information about the calcium carbonate and the information about the hydrochloric acid and he wants mass of carbon dioxide so I need first to decide which one is excess so number of moles of calcium carbonate is the mass over the molecular mass what about the HCL what does he tell me about the HCL number of moles is the concentration which is two mole per decimeter two times the volume divided by a thousand of course so I get number of moles of HCL then I look at the equation what does the equation tell me it says that if the number of moles of calcium carbonate is one the number of moles of HCL should be true that means number of moles of HCL should be twice calcium carbonate okay so number of moles of calcium carbonate is 0.2 twice of that is 0.4 but I have much less added of the HCL so my HCL is limiting all my calcium carbonate is the excess because if I have 0.08 of HCL I will need only half of that from calcium carbonate so I need 0.04 mole he put 0.2 that's way too much so my calcium carbonate is excess all my HCL is limiting now he's asking about what mass of carbon dioxide well I'm going to to relate it to the HCL so looking at the equation the number of moles of carbon dioxide should be half of the HCL so that is 0.04 and from that I can get the mass number of moles times Mr another type of question he says 20 centimeter cubed of sulfuric acid with this concentration was added to 40 centimeter period of sodium hydroxide with this concentration so how many moles of sulfuric acid were added again concentration times volume how many moles of sodium hydroxide were added concentration times volume and then I look at the equation well the sulfuric acid should be half of the sodium hydroxide is the sulfuric acid half of the sodium hydroxide the sodium hydroxide is 8 times 10 to the minus three so half of it should be 4 times 10 to the minus three so obviously the acid is excess I need only 0.004 mole Which is less than the 6 times 10 to the minus 3 that he added then he has a question so we've decided that it is the acid that is excess in the solution then he's saying is the pH of the final mixture less than seven equal to seven or more than seven how do we know well we just said that the acid is excess that means I have a lot of sulfuric acid in the solution what is the pH of acid less than seven and that's the end of this part uh I hope this has clarified a lot about this chapter please practice all these questions again and uh thank you for listening