Hey everyone welcome to homeschool and this is your class 11 series chemistry we started with basic concepts of chemistry in my previous video I have given entire information about the mole concept and I gave you the way to formulas that you can remember to calculate the problems on mole concept. So the formulas are here. We got to know how to calculate number of moles for our molecules right when mass is given.
The formula is mass given by gram molecular mass. The next formula is calculation of number of moles in case of atoms. It is mass given by gram atomic mass and later number of moles calculation when volume is given.
It's volume given by molar mass. Similarly, I gave formula to calculate number of molecules and number of atoms when moles are given or mass is given. Okay, so now using these formulas, let us try to calculate few questions.
Okay, which are really very very important. for board examination as well as for any competitive exam. You know, the mole concept will be felt easy when you practice more and more questions.
Okay, so that practice is the one which will give the confidence for you. So more the questions you practice, more perfect you will become in the concept. Clear.
And now you see the first question. How many moles are present in 180 grams of water? So guys, what are they asking? They are asking how many moles, that is number of moles present in 180 grams of water. Okay, so water is what?
Water is a... molecule right. So first you need to understand what is the compound whether it is atom or molecule and after that see what they are asking you to calculate. They are asking number of moles. What is given?
Mass is given. So So among the five formulas which formula do you select? Definitely the formula 1. Yes or no?
Number of moles if you want to calculate in case of molecules. What's your formula? Mass given by gram molecular mass right. So the first formula you are using in this case.
So the number of moles number of moles you are using in this case. Is equal to mass given divided by gram molecular mass. What is the mass given?
180. Right? And what is the gram molecular mass of water? H2O.
What is its molecular mass? You know, atomic mass of hydrogen is 1. How many are there? 2. Plus atomic mass of oxygen is 16. So, altogether you will get 18. So, 18 grams is the gram molecular mass of water. water right ok so 180 divided by 18 if you do what is the answer you will get you will get 10 moles so 10 moles is your answer ok so this is a very basic question actually clear so number of moles present in this much gram of water water is a molecule so the first formula you have used there mass is given in a question divided by molecular mass gram molecular mass Okay.
See in the question molecular mass they will not be giving. You must calculate and substitute. Okay.
So you must know how to calculate the molecular masses. Okay. Now let us go for the second type of a question.
The moles present in 44.8 liters of carbon dioxide at STPEs. Okay. Here also the number of moles they are asking.
But what is given? Volume is given. 44.8 liters means it's a volume.
Right. So volume is given at STP. That means standard temperature and pressure.
Okay. So which is the formula that you can use that I have listed on a board? Definitely this formula.
right? Number of moles when volume is given. So, what is your formula? Volume given divided by molar volume, isn't it? So, what is the formula?
Number of moles that you can calculate here is with this formula. What is the volume given? It is 44.8 divided by what is the value for molar volume?
molar volume value is 22.4. This also I explained in the previous video. And if you have missed my previous video, the link is provided in the description. Do watch that video, then you will get more clarity on what actually I'm doing it. Okay, so do watch the video then come to this session.
So I have clearly explained about the molar volume, right? Molar volume is 22.4 liters. So here I substitute 22.4 liters.
Okay, so what is the answer you will get? You would be getting 1 za, 2 za, right? So, your answer is 2 moles. So, there are 2 moles present in 44.8 liters of carbon dioxide. So whenever volume is given they are asking number of moles then the third formula you have to use.
Okay if the volume is given in liters you are using molar volume as 22.4 liters. If the volume was given in ml then you have to use the value 22,400 ml in the denominator. Okay molar volume in liters is 22.4 liters.
Molar volume in ml is 22,400 ml. Okay fine and coming to the next question. Look at the question total number of atoms present in 34 grams of ammonia is how much? So they are asking totally how many atoms are present in 34 grams of ammonia.
which is a formula see every problem you can solve with a formulas see if you are facing difficult by applying a logic you can go for using a formula okay no doubt so which is a formula that you can can use? What they are asking you to calculate? Total number of atoms.
That is number of atoms they are asking you. So, in that case, where is the formula? Number of atoms can be calculated with this formula.
Okay? So, how do you calculate? Number of atoms is equal to number of moles into Avogadro number. Na means Avogadro number into atomicity.
Okay? Atomicity means number of atoms present in a molecule. Okay, so now our formula is, let me write the formula first, number of atoms is equal to number of moles into Avogadro number into atomicity. atomicity isn't it okay so now see what information is there in a question Avagedro number you know its value Avagedro number is 6.022 into 10 power 23 okay so what is the atomicity of NH3 atomicity means the number of atoms present in a molecule totally how many atoms are present in one molecule one nitrogen atom and three hydrogen atom. All together, 4 atoms are present in ammonia, right?
So, atomicity is 4. And here, number of moles is not directly given in a question. They gave you mass. When they have given you mass, can't you convert that into moles?
See, what is the relationship between moles and mass? Number of moles is equal to mass given by gram molecular mass, right? See, using this formula, you can calculate number of moles, right? Right. So what is the mass given?
34 divided by what is the molecular mass of ammonia? Ammonia. Nitrogen atomic mass is 14. Hydrogen atomic mass is 1 into 3. So 14 plus 3 altogether 17. So 34 divided by 17 if you do you will get number of moles.
Okay. See number of moles directly not given in a question. Mass is given. So we are converting the mass into moles first.
So it is with the first formula. See here two formulas we have used. One is the last formula number of atoms is equal to number of moles into Avagedro number into atomicity.
Avagedro number we know. Atomicity means number of atoms present in a molecule. Altogether four atoms are there.
So four. But number of moles is not directly given in a question. So with the help of this mass given we are finding number of moles. So the final calculation you have to do 34 divided by 17 into 6.022 into 10 part 23 into 4 if you do you will get your answer.
So I think you can do this calculation on your own and post the answer in a comment section. Okay. So these are the three models of the questions that can ask on whole concept. Let us discuss few more models now. Coming to the next question.
Look at the question. How many oxygen atoms will be present in 88 grams of carbon dioxide? So what they have asked you?
How many oxygen atoms? That is number of atoms they are asking. So what is your formula?
Number of oxygen atoms. is equal to number of moles right. So number of moles here is a formula again I will not write here since place is not there. Number of moles into Avagedro number into atomicity okay so number of moles is not directly given in a question they gave you mass right so first you need to calculate number of moles for calculating number of moles which formula you will use the first formula Okay, so what is the formula to calculate number of moles? Mass given divided by molecular mass.
What is the mass given? 88. So 88 divided by carbon dioxide is the molecule. What is its molecule? molecular mass carbons atomic mass is 12 oxygen atomic mass is 16 into 2 right so you would get 44 so 88 divided by 44 into our get row number is how much much?
6.022 into 10 part 23. Okay. And atomicity. See how many oxygen atoms.
So what is the atomicity of oxygen? How many oxygen atoms are there in one molecule? It is 2 right?
So here atomicity is 2. Atomicity is not 3 here. They are asking how many oxygen atoms. Then you have to consider atomicity of oxygen atoms.
How many oxygen atoms are there? there? 2. Okay, so atomicity 2. If they would have asked you how many carbon atoms are there, then what you will substitute here? 1 you will substitute because number of carbon atoms in one formula is 1. But they are asking number of oxygen atoms. So, atomicity is 2. So, how would you calculate?
It is 1s are 2s are right. So, it would be 2 into 2, 4. So 6.0 into 4, you know, 4 6a, how much? So your answer would be something like 24 point something, right? So I could just write the main digit 24 into 10 power 23. 24 point something you will get. So round figure I have written there.
Okay. So it was actually one of the questions from previous year's question papers of NEET examination. Okay, so the questions that I selected is from the previous year competitive examination papers. Okay, so each and every question is very very important and a different model.
Okay, so number of oxygen atoms when they ask you atomicity is not 3 here, atomicity of that atom you have to write. Number of oxygen atoms, no there are 2 oxygen atoms in one molecule. So in this much grams, how many oxygen atoms? That's what you have calculated.
Here, fine. And coming to the next question. Next question is also similar to that.
The same formula you have to use. See, what is the total number of electrons present in 1.6 grams of methane? So what is the formula of methane here?
CH4. Right. Okay.
So in 1.6 grams of methane, how many number of electrons? So whenever they ask you number of electrons or ions or whatever, the same formula you can use. Okay.
So what is the formula I can write here? Number of electrons. Is equal to number of moles into Avogadro number into instead of atomicity you should write number of electrons present in one molecule. Okay. Let us see.
Okay. So number of electrons is equal to say I will write. Okay.
See number of one more formula if you want you can take it down. Number of electrons if you want to find. The formula is number.
of moles into Avogadro number into number of electrons in one molecule, in one molecule. Okay, I repeat the formula number of electrons is equal to Number of moles into Avogadro number into number of electrons in one molecule. Okay.
So the same formula we have to substitute. Number of moles is not directly given. Right. They have given you mass. So when mass is given, number of moles how do you calculate using this formula?
So what is the given mass? 1.6. And what is the molecular mass of methane? CH4. No.
So atomic mass of carbon is 12. Atomic mass of hydrogen. is 1. 1 into 4 you have to do that is 16 right 1.6 by 16 into Avageddon number is 6.022 into 10 power 23. Okay so now how many electrons are present in one molecule? You see here atomic number of carbon is 6. So number of electrons is equal to atomic number no? So how many electrons are there in carbon?
6 electrons plus how many electrons are there in? each hydrogen? 1. How many hydrogens are there?
4. So 1 into 4. How much? 4. Right. So all together you have 10 electrons in one molecule of methane. So how many electrons in one molecule?
10 electrons. Okay. So now what is your answer?
So your answer would be just 6.022 into 10 power 23. Isn't it? So this will be your answer. Right. So this is how you have to calculate.
number of electrons. So this is one more important model on mole concept. So coming to the next question that is sixth question again has three sub questions. Let us discuss one by one. Calculate number of atoms in 52 moles of argon.
Number of atoms they are asking moles they have given. So what is the formula to calculate number of atoms? Number of moles into Avogadro number. number into atomicity. So number of moles directly they have given in a question that is 52 into Avageddon number 6.022 into 10 power 23. Atomicity argon is one atom only right argon is one.
So I atomicity is 1. Clear? So that's it. You have to calculate. Fine. And coming to the next question you see.
Calculate the number of atoms in 52U of helium. You have to carefully think how to calculate this. You know 52U.
Can you tell me what is the atomic mass of helium? Atomic mass of helium. okay atomic mass in the sense what mass of mass of one atom right mass of one atom of helium is for you yes or no mass of one atom of helium is for you now 52 u contains how many atoms.
So, how do you calculate? You would just calculate by 52 divided by 4. Isn't it? It is just 52 divided by 4. Okay? So, this is 1s and 4s and 4s.
Right? So, almost you will get around 4 3s at well. Right? So, you will get your answer as 30. So you will have 13 atoms in 52U. Okay.
So just a cross multiplication. Mass of one atom is 4U. Atomic mass of helium is 4U. So 52U contains how much?
Just a cross multiplication we have done there. That's it. Okay.
Fine. And coming to the third question here. So same thing.
Calculate the number of atoms in 52 grams of helium. So what is the formula? Number of atoms.
to calculate you know number of moles into Avogadro number into atomicity. So number of moles directly they have not given you know mass is given. So how do you calculate in moles mass given divided by atomic mass isn't it? So 52 divided by atomic mass of helium is 4 right into Avogadro number 6.022 into 10 power.
23 okay and atomicity helium is one atom only so atomicity one Okay, so you just have to do this calculation for this particular question. That's it. So this is how the different models can be asked on mole concept.
See, the more questions you work, more perfect you will become. So practicing more questions is important in case of mole concept. so with this i will stop the video and we will meet up in the next class and we will discuss about the stasiometry or some people will pronounce it as stichiometry stichiometry or stasiometry it is a completely a new term for many of you so let us discuss what is actually we mean by a stasiometry and it's all about calculations calculations and calculations okay so let us meet with a new topic in the next class till then take care practice more examples on mole concept and if you have any doubt you can post in a comment and we will try to give answers okay so practice practice and practice it makes perfect clear so do subscribe our channel to learn the concepts in a easiest way you see mole concept is a very much difficult topic for most of the students but according to me this is the much simple concept only thing is you need to understand the concept behind it or you know any type of mole concept question can be solved with these formulas definitely right so have a perfection in a concept so take care and do subscribe our channel to learn the concepts in a easiest way and in a detailed way thank you so much and all the best