in this video we will learn how to do stoichiometric calculations for chemical reactions balanced chemical equations provide us information on the ratio of molecules and the ratio of moles of reactants and products in a chemical reaction the given chemical equation 2 h2 plus o2 forms 2h2o can be read as two molecules of hydrogen and a molecule of oxygen react to produce two molecules of water it can also be read as two moles hydrogen plus one mole oxygen react to produce two moles of water the coefficients in a balanced chemical equation allow us to predict the amounts of reactants and products in a reaction for example we can predict how many molecules of the reactants hydrogen and oxygen are needed to produce 2.4 times 10 to the third molecules of water so we start our calculation with the given amount of water which is 2.4 times 10 to the third molecules for determining the number of molecules of h2 we can use the ratio two molecules h2o to two molecules of h2 so molecules h2o goes at the bottom to allow cancellation of the unit molecules h2o and then at the top we will put multi two molecules of h2 so upon calculation this gives us 2.4 times 10 to the third molecules of h2 which is consistent with the one-to-one ratio of molecules of hydrogen and water now let's calculate the number of molecules of oxygen so we again start with 2.4 times 10 to the third molecules of h2o again the molecules h2o will go at the bottom and in the balanced chemical equation for oxygen the coefficient is one so we will put one molecule of o2 at the top molecules h2o cancel and upon calculation this gives us 1.2 times 10 to the third molecules of o2 we can do a similar calculation but this time using the coefficients as ratio of moles how many moles of h2o is produced from 0.89 moles of o2 so we start with the known amount of the reactant which is 0.89 moles of o2 next we will look at the ratio of moles so from the coefficients that would be one mole o2 to two moles h2o so one mole o2 goes at the bottom two moles h2o at the top we get to cancel mole o2 the answer here is 1.8 moles water we have learned that the coefficients in the balanced chemical equation a ratio of moles or ratio of molecules or atoms of reactants and products in a chemical reaction previously we have learned how to do mole conversions for the next few exercises we will combine mole conversion and using the coefficients to solve some stoichiometry problems here is an example problem how many grams of hydrogen is needed to react with 1.57 grams of oxygen for the reaction that we used in previous examples this time we will solve the mass of one substance given the mass of another substance for this particular example we are given the mass of the reactant oxygen and we need to solve the mass of hydrogen that will react with it how are we going about that calculation let's take the reactant with the given mass substance a and the reactant for which we are calculating the mass of a substance b so we're going from mass of substance a to mass of substance b using the mass and molar mass of substance a we can calculate its moles using the coefficients in the balanced chemical equation we will go from moles of substance a to moles of substance b and from that calculated number of moles and molar mass of substance b we can determine the mass of substance b that reacts with a given mass of substance a let's go back to the problem we are given the mass of the reactant oxygen and we are to solve for the mass of the other reactant hydrogen we will start with what we know which is the 1.57 grams of o2 grams o2 can be converted to moles o2 by using the molar mass of o2 which is 32 grams per mole so the 32.0 grams goes at the bottom to cancel out the grams o2 and then mol o2 at the top from mole o2 we can proceed to mole of h2 by considering that one mole of o2 react with two moles of h2 according to the balanced chemical equation so the one mole o2 goes at the bottom cancel and the two moles h2 goes at the top and for moles h2 we can convert it to grams h2 by using the molar mass of h2 which is two grams per mole the moles go at the bottom and the two grams at the top so we have moles h to cancel our leftover units grams h2 and calculating this we have 0.196 grams of hd here is the flow chart for solving stoichiometric calculations involving chemical reactions the most common variation would be to solve from mass of one substance to the mass of another substance because this is what's applicable in a lab scenario there may be problems where you start or stop at any point in this flow chart the next example that we will do starts from moles of one substance asking for the number of molecules of another substance for the given reaction if 1.8 moles of oxygen reacts that's what we know how many molecules of h2o are produced we are solving for molecules of h2o to set up our equation we start with the 1.8 moles of o2 when you have moles of one substance it means that you can proceed to the other substance by using their mole ratio from the balanced chemical equation here we know that one mole o2 reacting there will be two moles of h2o produced so the number of moles of o2 would go at the bottom so one mole o2 and the number of moles of h2o goes at the top that's two moles h2o now we are at the substance being asked for which is h2o but we need the molecules and at this point we have the moles to convert from most molecules we use avogadro's number so the mole will go at the bottom and the molecules with avogadro's number would go at the top 6.02 times 10 to the 23rd molecules calculating this gives 2.2 times 10 to the 24th molecules in the next example we are given 1.25 times 10 to the 24th water molecules and we need to calculate the mass of h2 so we start with the 1.25 times 10 to the 24th h2o molecules we need to convert this to moles and for that we will use avogadro's number 6.02 times 10 to the 23rd molecules at the bottom and one mole h2o at the top next we will convert from moles h2o to moles h2 by using their mole relationship in the equation which is two moles h2o and two moles h2 next from moles of h2 we now have h2 as our substance all we need to do is to convert it to grams using the molar mass of h2 which is 2 grams per mole so that would be the mole at the bottom and 2 grams at the top this gives us 4.15 grams of each