Guide to Stoichiometry
By Kent Nguyen
What is Stoichiometry and why do we use it?
2 slices of bread + 1 slice of cheese + 1 slice of ham= 1 sandwich
Now, say you have 6 slices of bread, 3 slices of cheese, and 3 slices of ham; you are able to make 3 sandwiches. However, if you have 6 slices of bread, 3 slices of cheese, and only 2 slices of ham you only have enough ingredients to make 2 sandwiches because there is not enough ham.
Stoichiometry uses these ratios in order to figure out how much of each reactant you're going to need in order to make a chemical reaction, or vice versa, how much product you will end up making in the chemical reaction.
Starting a stoichiometric equation
Say we are given sodium sulfate + Barium hydroxide. This is double replacement because there are two compounds that react together.
Here is the balanced equation: Na2SO4 + Ba(OH)2 > 2NaOH + BaSO4.
So we won't have to stop and do this during the actual stoichiometry part, we need to find the molar masses of all the reactants and products.
Na2SO4 = 142.042 g/mole NaOH = 39.997 g/mole
Ba(OH)2 = 171.342 g/mole BaSO4 = 233.39 g/mole
Mole to Mole Conversions
9.24 g Na2SO4| 2 moles NaOH
| 1 mole Na2SO4
Then you multiply the top and divide by the bottom and you end up with 18.48 moles of NaOH.
Mass to Mass Conversions, Limiting/Excess Reactants, and Theoretical/Percent Yield
12.1 g Na2SO4 | 1 mole Na2SO4 | 1 mole BaSO4 | 233.39 g BaSO4
| 142.042 g Na2SO4 | 1 mole Na2SO4 | 1 mole BaSO4
You multiply by the top and divide by the bottom and you end up with 19.86 g of BaSO4.
You are given 12.3 grams of Ba(OH)2
12.3 g Ba(OH)2 | 1 mole Ba(OH)2 | 1 mole BaSO4 | 233.39 g BaSO4
| 171.342 g Ba(OH)2 | 1 mole Ba(OH)2 | 1 mole BaSO4
You multiply by the top and divide by the bottom and you end up with 16.75 g of BaSO4.
The limiting reactant is Ba(OH)2 because it yields less than Na2SO4. The excess reactant is Na2SO4 because it yields more than Ba(OH)2. The theoretical yield is 16.75 g BaSO4 because it is less than 19.86 g BaSO4.
To find the percent yield, all you have to do is divide the actual yield by the theoretical yield and multiply by 100.
Say the actual yield was 12.4 g BaSO4.
12.4/16.75 * 100= 74.03% The percent yield is 74.03%.