moles are easy you probably know that if there are numbers in front of something in a chemical equation you need that many atoms or molecules to react with the other reactant if there's no number there's an invisible one there so this equation tells us that for every particle of magnesium hydroxide not really a molecule as it's ionic but that's not important we need two particles of hydrogen chloride HCL or well hydrochloric acid when it's in solution we can also see that this would make one particle of magnesium chloride and one molecule of water the thing is we can't deal with individual atoms or molecules they're too small and there are too darn many of them to count all we can do is measure the mass or volume of reactants and products so how can we tell how many particles we have from the mass or volume enter moles the middleman between number of particles and mass or volume a mole is just a certain number of particles that we group these into much like a dozen is 12 it's just a way bigger number this number is actually 6.02 * 10 3 also known as avagard constant but we don't need to know that for now so what we said above is not only true for individual particles but also moles we need two moles of hydrochloric acid for every one mole of magnesium hydroxide if we wanted a complete reaction it's just ratios in chemistry we call this stochiometry the stochiometry of magnesium hydroxide to hydrochloric acid is 1:2 but we don't really care about the number of particles as we have an equation that lets us calculate how many moles of a substance we have have moles equals g over Rams G being the mass of a reactant or product and Rams being short for relative atomic mass but it can also be relative formula mass for compounds it's just a nice way of remembering it remember Ram is just the mass number on an element symbol on the periodic table so if I have 12 G of pure carbon how many moles of carbon do I have well moles equals g over Ram so 12 G divided by carbon's Ram of 12 gives us 1 mole one mole of carbon has a mass of 12 G remember that's just a huge group of individual particles what about if we had 24 G instead 24 G divided carbon's Ram of 12 equals 2 there are 2 moles of carbon in 24 G here's one for you I have 54 G of pure water how many moles is that don't forget to add up the Rams for the atoms in H2O pause the video and have a go the Ram or to be more precise the relative formula mass for water is 18 two lots of one for the hydrogen's plus Oxygen 16 moles equals g over RS so 54 ID 18 we have 3 moles moles allow us to predict how much of a reactant we need or how much of a product is made in a reaction by comparing with something else in the reaction so let's look at a question using the equation what mass of hydrogen chloride would be needed to fully react with 2. 9 G of magnesium hydroxide first let's get the Ram yes technically rfm of magnesium hydroxide but I'm just going to say Ram from now on 24 for magnesium plus 2 lots of 17 for the O so that's 58 Al together moles equals g over Ram so 2.9 ided 58 we have 0.05 moles Now using the stochiometry we can figure out how many moles of hydrogen chloride HCL we need what's the stochiometry it's 2: one in hcl's favor so that means we need double the moles of HCL so 0.05 * 2 = 0.1 moles we need 0.1 moles of HCL now what do we do with that we can turn that back into mass by rearranging our equation moving Rams to The Other Side by doing the opposite with it so multiplying g equals moles time Rams so 0.1 moles times hcl's Ram of 36.5 and that equals 3.65 g we'd need 3.65 G of HCL to react with 2.9 G of magnesium hydroxide can you see we went from Mass to moles to moles to mass okay time to take off the training wheels here's a question just for you what mass of aluminium chloride can be made from 124.80 okay let's see how you got on first let's get the relative formula masses the Rams for both substances were concerned with berium chloride is 208 aluminium chloride is 133.5 next we do our whole Mass moles moles Mass the number of moles of barium chloride moles equals g over Rams so 124.80 that gives us 0.6 moles then we look at the stochiometry to find out how many moles of the second thing we need it's 3 to2 we can see that's fewer moles of aluminium chloride so the ratio is 2/3 so 2/3 of 0.6 moles that's 0.4 mol of aluminium chloride finally finding Mass using our rearranged equation we get 53.4 G well done if you got that right now the thing is in order to technically be hydrochloric acid HCL needs to be dissolved in water to make a solution this is typically how neutralization reactions between an acid and an alkaline also called a bass go but not always but let's say that we actually used a volume of 200 cm cubed which is the same thing as 200 mlit what is the concentration of the acid how concentrated is the actual compound in the solution this is usually measured in gam per decim cubed or moles per decim cubed I always say units are your friends and it's the case here that slash means divide so even if you can't remember the equation the unit tells you what to do so the equation is concentration equals mass in grams or moles divided by volume in decimet cubed first things first we need to convert that volume from cm cubed into decim cubed there are 1,000 cm cubed in 1 decim cubed a decimet cubed is bigger so that means there will always be fewer decimet cubed compared to cenm cubed so what is 200 cm cubed in decimet cubed we want a smaller number so we divide by our conversion factor of a thand so 2 100 cm Cub / 1,000 equal 0.2 DM cubed next to use the concentration equation let's use the mass first 3.65 G / 0.2 DM cubed that gives us 18.25 G per decim cubed if we're using moles instead that's 0.1 moles divided by 0.2 decim cubed so that's 0.5 moles per decimet cubed this is more often than not how concentration is given that's why you could see this deviated to just 0.5 capital M we might say 0.5 Moler that's just short for moles per decim cubed titration relies heavily on moles for the whole method check out my practical video but all it is in essence is carrying out a neutralization reaction to calculate an unknown concentration of acid or Alkali using exactly the same principles we've used already so let's see if you can use what we've learned in a titration context here we go 50 mlit or 50 cm cubed of a 0.2 moles per decim cubed solution of magnesium hydroxide neutralizes 25 cm cubed of hydrochloric acid what is the concentration of the acid if you want to go yourself first pause the video in the next few seconds so once again we start by calculating moles for what we have the magnesium hydroxide we know concentration is moles divided by volume and you can see that from the unit can't you moles ided by decim cubed we must have the volumes in the same units though so let's convert the 50 cm cubed to decim cubed by dividing by a th we want a smaller number that's 0.05 decm cubed so rearranging we get moles equals concentration times volume and that gives us 0.01 moles next let's calculate how many moles of acid we need to neutralize you can write out the whole equation and balance but it's not necessary we know that to neutralize we must have the same number of H+ and oh minus ions there's two oh minus ions in the magnesium hydroxide but just one H+ ion in the HCL so we must have double the moles of acid the stochiometry is therefore 1 to two so doubling 0.01 moles of magnesium hydroxide we know we've had to use 0.02 moles of hydrochloric acid to neutralize finally we just need to use the concentration equation again to calculate what the concentration of the acid is concentration equals moles ided by volume volume so 0.02 mes ided by 0.025 DM cubed did you remember to convert the centimet Cub to decimet cubed that means the concentration of the acid must be 0.8 moles per decim cubed so I hope you found this helpful please leave a like and a comment if you did check out the 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