Consider the above reaction where 6.1 g of Benzoic acid is used to get 7.8 g of m-bromo benzoic acid. The percentage yield of the product is __________
(Round off to the Nearest Integer).
[Given : Atomic masses : C : 12.0 u, H : 1.0 u, O : 16.0 u, Br : 80.0 u]

Solution

Chemistry ma’am · 3 steps

Percentage yield is about moles first: find how much product 6.1 g of acid could give, then compare.

1. Molar masses. Benzoic acid, C₆H₅COOH (C₇H₆O₂), is 84 + 6 + 32 = 122 g mol⁻¹. m-Bromobenzoic acid, C₇H₅BrO₂, is 84 + 5 + 80 + 32 = 201 g mol⁻¹.
2. Theoretical yield. The reaction is 1 : 1, so 6.1/122 = 0.05 mol of benzoic acid can give at most 0.05 mol of product. That is 0.05 × 201 = 10.05 g.

   % yield = (actual mass)/(theoretical mass) × 100 = 7.8/10.05 × 100 = 77.6

3. So. The percentage yield is 77.6%, which rounds to 78.

The common slip: 128. Dividing the product mass by the benzoic acid mass directly, 7.8/6.1 × 100, gives 128, which is impossible. The product is heavier, so you must go through moles.

The idea to keep. For percentage yield, never compare grams of reactant with grams of product. Convert to moles, use the balanced equation, convert back to grams of product, then divide.

Answer from the NTA final answer key. Solution by PrepWiser.