Stoichiometry CFA Review
Learning Goal: Identify and use mole ratios. *** Have to have a balanced equation to determine the mole ratio. The ratios
come from the compounds’ coefficients.
N2 + 3H2  2NH3
The ratio between nitrogen and hydrogen is 1 N2 to 3 H2
The ratio between hydrogen and nitrogen is 3 H2 to 1 N2
*** Make sure you put them in the order the way the question is worded
Practice Problems:
1. H2SO4  H2O + SO3
a. Ratio between water and sulfite?
b.
2.
3Mg + N2  Mg3N2
a. Ratio between nitrogen and magnesium?
b.
3.
Ratio between sulfuric acid (H2SO4) and sulfite?
Ratio between magnesium and nitrogen?
As2O3 + 3H2O  2H3AsO3
a. Ratio between arsenic trioxide and water?
b.
Ratio between water and arsenous acid (H3AsO3)?
Learning Goal: Able to use units to convert between moles and mass.
N2 + 3H2  2NH3
To convert between moles of two different compounds you must use a mole ratio.
If you have 5 moles of N2, how moles of H2 would you have?
5 moles N2
3 mole H2
= 1.67 mole H2
1 mole N2
How many grams are in 5 moles of N2? If you need to convert moles to grams, follow your units. *** Notice
you cancel out the units you do not need and remain with the units you do need to answer the problem.
5 moles N2
28.0 g N2
= 140 g N2
1 mole N2
If you have 5 grams of N2, how many moles would you have? (This problem is set up opposite to the one
above)
5 grams N2
1 mole N2
= 0.18 mol N2
28.0 g N2
Practice Problems:
3Mg + N2  Mg3N2
Molar Mass: Mg = _________________ N2 = _______________ Mg3N2 =___________________
4.
If you have 10 moles of magnesium, how many moles of nitrogen would you have?
5.
If you have 15 moles of nitrogen, how many moles of magnesium would you have?
6.
If you have 15 moles of nitrogen, how many grams of magnesium would you have?
7.
If you have 15 grams of nitrogen, how many grams of magnesium would you have?
As2O3 + 3H2O  2H3AsO3
Molar Mass: As2O3 =____________________ H2O= _________________ H3AsO3= ___________________
8.
If you have 32 moles of arsenic trioxide, how many moles of water would you have?
9.
If you have 47 moles of water, how many grams of arsenous acid (H3AsO3) would you have?
10. If you have 32 grams of arsenic trioxide, how many grams of water would you have?
Learning Goal: Use stoichiometry to determine Limiting Reactants, Amount of Excess Reactants, % Yield.
Sample Problem: There are 6.0 g of Mg and 10.0 g of N2. What is the limiting reactant and how much excess is there?
3Mg + N2  Mg3N2
Molar Mass: Mg = 24.3 g/mol
6.0 g Mg
1 mol Mg
24.3 g Mg
1 mol Mg3N2
3 mol Mg
101 g Mg3N2
1 mol Mg3N2
24.9 g Mg3N2
10.0 g N2
1 mol N2
28.0 g N2
1 mol Mg3N2
1 mol N2
101 g Mg3N2
1 mole Mg3N2
= 36.1 g Mg3N2
6.0 g Mg
1 mol Mg
24.3 g Mg
1 mol N2
3 mol Mg
28.0 g N2
1 mole N2
= 2.3 g N2
N2 = 28.0g/mol
The top equation
produced LESS
Mg3N2, therefore,
Mg is the limiting
reactant!
1.
Complete two stoichiometry problems (convert both reactants to a SINGLE product); the REACTANT with the least
amount of product is the Limiting Reactant
a. Once you know the limiting reactant, NEVER, and I mean NEVER, look at those 2 stoichiometry problems
again!!!!!!!!!
2. To continue on to find the Amount of Excess Reactant, do a stoichiometry problem starting with the limiting
reactant (Mg), and ending with the OTHER reactant (N2).
a. This OTHER reactant is called the EXCESS reactant (N2).
3. In this example, that means if we use 6.0 g of Mg, we will also use 2.3 g of N 2. In the original problem you were given
10.0 g of N2, but only used 2.4 g of N2. How much N2 is left in Excess?
10 g N2 – 2.3 g N2 = 7.7 g N2 in Excess
4.
Lastly, theoretical yield! If we would theoretically produce 24.9 g Mg3N2, but when you actually did the experiment
you got 21.8 g Mg3N2, what is your theoretical yield?
Theoretical yield =
21.8 𝑔 Mg3N2
24.9 𝑔 Mg3N2
Theoretical yield = 87.55 %
x 100
Practice Problems:
3Mg + N2  Mg3N2
11. Given 23 g of Magnesium and 15g of Nitrogen. What is the limiting reactant and how much excess is there?
As2O3 + 3H2O  2H3AsO3
12. Given 23 grams of As2O3 and 20 grams of H2O. What is the limiting reactant and how much excess reactant is there?
13. Based on your limiting reactant, how many grams of H3AsO3 would you have?
14. If you calculated that after this reaction that you should have 43.8 grams of H 3AsO3, but when you massed your
product you had 21.8 grams of H3AsO3. What was your % yield?
Practice Problems:
3Mg + N2  Mg3N2
15. Given 23 g of Magnesium and 15g of Nitrogen. What is the limiting reactant and how much excess is there?
As2O3 + 3H2O  2H3AsO3
16. Given 23 grams of As2O3 and 20 grams of H2O. What is the limiting reactant and how much excess reactant is there?
17. Based on your limiting reactant, how many grams of H3AsO3 would you have?
18. If you calculated that after this reaction that you should have 43.8 grams of H 3AsO3, but when you massed your
product you had 21.8 grams of H3AsO3. What was your % yield?