Free Theoretical and Percentage Yield MCQs with Answers

1. The theoretical yield of a reaction is

• A. the maximum mass of product calculated from the balanced equation and the limiting reactant
• B. the mass of product actually obtained in the laboratory
• C. always equal to the mass of the limiting reactant
• D. the mass of the excess reactant left over

Explanation: Theoretical yield is a calculation, assuming the reaction goes to completion with no losses and no side reactions, and it is the standard against which real performance is judged. What is actually collected is the actual yield and is nearly always smaller. Comparing the two as a percentage tells a chemist how efficient the process was.

2. A reaction has a theoretical yield of 25 g but only 20 g of product is obtained. The percentage yield is

• A. 125 per cent
• B. 80 per cent
• C. 5 per cent
• D. 20 per cent

Explanation: Percentage yield is actual divided by theoretical multiplied by 100, so 20 divided by 25 multiplied by 100 gives 80 per cent. Dividing the wrong way round gives 125 per cent, which should be recognised as impossible because a reaction cannot produce more than the equation allows. A figure above 100 per cent in real work means the product is still wet or impure.

3. The actual yield of a reaction is usually less than the theoretical yield for all of the following reasons EXCEPT

• A. some product is lost during filtration and transfer
• B. side reactions produce unwanted products
• C. the balanced equation was written incorrectly
• D. the reaction is reversible and does not go to completion

Explanation: Physical losses, competing side reactions and reversibility are all genuine reasons why a real yield falls short. A wrongly balanced equation is not a reason for a low yield at all; it simply makes the calculated theoretical yield wrong, so the comparison itself becomes meaningless. Recognising which factors are chemical and which are arithmetic is the point of the question.

4. How many grams of calcium oxide are produced when 1 mole of calcium carbonate decomposes completely, given CaCO3 gives CaO + CO2?

• A. 100 g
• B. 44 g
• C. 40 g
• D. 56 g

Explanation: The equation shows a one to one ratio, so 1 mole of CaCO3 gives 1 mole of CaO, whose molar mass is 40 + 16, that is 56 g. The value 100 g is the molar mass of the calcium carbonate that decomposed and 44 g is the mass of carbon dioxide released, and note that 56 plus 44 equals 100, which confirms mass is conserved.

5. Consider the given balanced chemical equation: 2H2 + O2 gives 2H2O. If 4 g of H2 reacts with 32 g of O2 to produce 28 g of H2O, what is the percentage yield of the reaction? (Molar mass of H2 = 2 g/mol, O2 = 32 g/mol and H2O = 18 g/mol)

• A. 63.6%
• B. 77.8%
• C. 87.5%
• D. 92.5%

Explanation: Four grams of hydrogen is 2 moles and 32 g of oxygen is 1 mole, which is exactly the 2 to 1 ratio the equation requires, so neither is in excess and 2 moles of water should form, that is 36 g. The percentage yield is therefore 28 divided by 36 multiplied by 100, which is 77.8 per cent. Checking for a limiting reactant before computing the theoretical yield is the step most often skipped.

6. A chemical reaction has a theoretical yield of 25 g, but only 20 g of product was obtained. What is the percentage yield of the reaction?

• A. 20%
• B. 25%
• C. 45%
• D. 80%

Explanation: Percentage yield is actual divided by theoretical multiplied by 100, so 20 over 25 multiplied by 100 gives 80 per cent. Dividing the other way round would give an impossible 125 per cent, which is the check worth making on every such answer. The missing 5 g is lost to transfer, side reactions or an incomplete reaction.

7. If % yield and actual yield is 80 and 20 g respectively, what will be theoretical yield?

• A. 20 g
• B. 25 g
• C. 30 g
• D. 40 g

Explanation: Rearranging the percentage yield formula, the theoretical yield is the actual yield multiplied by 100 and divided by the percentage, so 20 times 100 over 80 gives 25 g. The theoretical yield must always exceed the actual yield, so any answer of 20 g or less can be rejected immediately. This is the reverse of the earlier question in the same paper.