Grade 11 Chemistry Percentage Yield Problems/Solutions

Percentage Yields Problems

1. Powdered zinc metal reacts violently with sulphur (S₈) when heated to produce
zinc sulphide.


a. What mass of product can be produced with 25 g of zinc?


Unknown: mass of zinc sulphide
Given: mass of zinc metal = 25 g


Molar mass of zinc = 65.38 g/mol


Grams to moles:
25 g ZnS / 65.38 g/mol = 0.382 mol


0.382 mol Zn ÷ 1 mol Zn x 1 mol ZnS = 0.382 mol ZnS


Moles to grams:
0.382 mol ZnS x 97.446 g/mol = 37.22 g


∴ 37.22 g ZnS can be produced by 25 g of Zn.


b. What is the actual yield if the percentage yield was 80%?


Actual yield = (Percentage yield x Theoretical yield) / 100
= ( 80 x 37.22 ) / 100

, = 29.78 g


∴ 29.78 g ZnS was obtained (actual yield).


2. When 9 g of hydrogen reacts with oxygen, 73 g of water is produced.


a. What is the theoretical yield of water?


2 H₂ + O₂ → 2 H₂O


Theoretical yield = 9 g H₂ / MM[H₂]
= 9 g / 2.016
= 4.46 mol H₂
4.46 mol H₂ x 1 mol H₂ / 1 mol H₂O
= 4.46 mol H₂O
4.46 mol H₂O x 18.015 g/mol H₂O
= 80.35 g H₂O


∴ 80.35 g H₂O is the theoretical yield.


b. What is the actual yield?


Actual yield = 73 g


∴ 73 g H₂O is the actual yield.