Percentage yield (Triple Only)
Lajoy Tucker
Teacher
Contents
Introduction & Definitions
Percentage Yield measures how much product is obtained in a chemical reaction compared to the theoretical maximum.
Definition: Percentage Yield = (Actual Yield/Theoretical Yield)×100
Actual Yield: The amount of product actually obtained from the reaction.
Theoretical Yield: The maximum possible mass of product, based on stoichiometric calculations.
Basic Principles
A 100% yield means the reaction produced all the product it theoretically could.
Common reasons for a percentage yield below 100% include:
Incomplete reactions.
Reversible reactions (the products turn back to reactants).
Side reactions forming unwanted products.
Loss during purification (e.g. filtration, evaporation).
Product remains in the reaction vessel.
Percentage Yield Calculations
Example 1:
Titanium can be extracted from titanium chloride by the following react
45g is the maximum mass of Ti that was calculated to be produced from reacting a known mass of
and Mg. Only 20g of Ti is produced from titanium chloride and excess magnesium.
Calculate the percentage yield
Answer:
Percentage Yield = (Actual Yield/Theoretical Yield)×100
= (20/45) x 100
= 44.4%
In some questions, reacting masses calculations must be used to first calculate the theoretical (maximum) yield).
Example 2 (challenging):
Question:
In the reaction:
Mg + 2 HCl → MgCl₂ + H₂
A student reacts 2.40 g of Mg with excess HCl. The mass of hydrogen gas collected was 1.80 g. What is the percentage yield? (Mr H₂ = 2.0, Mg = 24.0)
Answer:
Step 1: Moles of Mg used; 2.40 g ÷ 24.0 = 0.10 mol
Step 2: Theoretical moles of H₂ (1:1 ratio with Mg); 0.10 mol
Step 3: Theoretical mass of H₂; 0.10 mol × 2.0 g/mol = 0.20 g
Step 4: Use percentage yield formula; (0.18 / 0.20) × 100 = 90%
Check for Understanding
Recall
a) What does percentage yield measure in a chemical reaction?
Answer:
a) How much product is obtained compared to the theoretical maximum amount.
b) Write the formula used to calculate percentage yield.
Answer:
b) Percentage Yield = (Actual Yield ÷ Theoretical Yield) × 100.
c) State two reasons why the percentage yield of a reaction may be less than 100%.
Answer:
c) Any two from:
Incomplete reaction
Reversible reactions
Side reactions forming unwanted products
Loss during purification
Product remaining in reaction vessel
Application
d) A reaction has a theoretical yield of 45 g but only 20 g of product is obtained.
Calculate the percentage yield.
Answer:
d)
Percentage Yield = (Actual ÷ Theoretical) × 100
= (20 ÷ 45) × 100
= 44.4%
e) A student collects 75 g of aluminium from a reaction that has a percentage yield of 80%.
Calculate the maximum mass of aluminium that could have been produced.
Answer:
e)
Use formula: Percentage Yield = (Actual ÷ Theoretical) × 100
80 = (75 ÷ T) × 100
0.80 = 75 ÷ T
T = 75 ÷ 0.80
T = 93.75 g
= 94 g (to 2 s.f.)
Challenge
f) In the reaction:
Mg + 2HCl → MgCl₂ + H₂
A student reacts 2.40 g of Mg with excess hydrochloric acid and collects 0.18 g of hydrogen gas.
Calculate the percentage yield. (Mr Mg = 24.0, H₂ = 2.0)
Answer:
f)
Moles of Mg = 2.40 ÷ 24.0 = 0.10 mol
Ratio Mg : H₂ = 1 : 1
Theoretical moles of H₂ = 0.10 mol
Theoretical mass of H₂ = 0.10 × 2.0 = 0.20 g
Percentage Yield = (0.18 ÷ 0.20) × 100
= 90%