Free Fundamental Concepts of Chemistry MCQs with Answers
24 Fundamental Concepts of Chemistry MCQs from Chemistry, each with the correct answer and a written explanation of why it is correct. Free and unlimited, with no account needed.
24 questions · page 3 of 3
21. 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.
Correct answer: 77.8%22. Consider the equation H2 + O2 gives H2O. What volume of hydrogen gas is required to produce 1 mol of water at standard temperature and pressure?
- A. 11.2 dm3
- B. 22.4 dm3
- C. 10 dm3
- D. 58 dm3
Explanation: Balanced properly the equation is 2H2 plus O2 giving 2H2O, so one mole of water needs one mole of hydrogen, and one mole of any gas occupies 22.4 dm3 at STP. The figure 11.2 dm3 is half a mole and is the trap for anyone who halves the ratio the wrong way. Molar volume is independent of the identity of the gas, which is Avogadro's law.
Correct answer: 22.4 dm323. 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.
Correct answer: 80%24. 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.
Correct answer: 25 g