Percentage yield (Triple Only)

• 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.

• 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.

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%

Practice Questions