WJEC A Level Chemistry: Empirical Formula Practice Questions PDF

WJEC Okay, here are 15 multiple choice questions covering empirical formula calculations, % atom economy calculations, and % yield calculations, drawing from the information in the WJEC A Level Chemistry documents within the knowledge base: Questions: 1. What is an empirical formula? * A) The form...

WJEC Okay, here are 15 multiple choice questions covering empirical formula calculations, % atom economy calculations, and % yield calculations, drawing from the information in the WJEC A Level Chemistry documents within the knowledge base: Questions: 1. What is an empirical formula? * A) The formula showing the actual number of atoms of each element in a molecule. * B) The formula showing the simplest whole number ratio of atoms of each element in a compound. * C) The formula showing the arrangement of atoms in a molecule. * D) The formula showing the relative molecular mass of a compound. 2. A compound contains 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. What is its empirical formula? * A) CH2O * B) C2H4O2 * C) CHO * D) CH3O 3. A hydrocarbon contains 90% carbon by mass. What is its empirical formula? (Ar: C = 12, H = 1) * A) CH * B) CH2 * C) CH3 * D) C3H4 4. A compound has the empirical formula CH2. Its relative molecular mass (Mr) is 56. What is its molecular formula? * A) C2H4 * B) C3H6 * C) C4H8 * D) C5H10 5. What is a molecular formula? * A) Simplest ratio of atoms of each element in a compound * B) The total number of atoms of each element in a molecule of a compound. * C) The name of a compound based on it's functional groups. * D) The way in which the atoms in a compound are arranged 6. What is the percentage yield of a reaction? * A) (Theoretical yield / Actual yield) x 100% * B) (Actual yield / Theoretical yield) x 100% * C) (Actual yield x Theoretical yield) x 100% * D) (Theoretical yield - Actual yield) x 100% 7. In a reaction, the theoretical yield of a product is 10.0 g. The actual yield obtained is 8.0 g. What is the percentage yield? * A) 20% * B) 80% * C) 125% * D) 8% 8. 2.0 g of magnesium (Ar = 24) is reacted with excess hydrochloric acid. The theoretical yield of magnesium chloride (Mr = 95) is 7.9 g. If 6.0 g is actually obtained, what is the percentage yield? * A) 76% * B) 132% * C) 25% * D) 60% 9. What is meant by the term "atom economy"? * A) measure of the cost effective of carrying out a reaction. * B) A measure of the proportion of reactant atoms that become part of the desired product. * C) A measure of the rate of a chemical reaction. * D) A measure of the energy released in a chemical reaction. 10. Which of the following is the correct formula for calculating atom economy? * A) (Mass of desired product / Total mass of reactants) x 100% * B) (Total mass of reactants / Mass of desired product) x 100% * C) (Moles of desired product / Total moles of reactants) x 100% * D) (mr of desired product/ total mr of reactants) x 100% 11. What is the atom economy for making hydrogen (H2) from the reaction of methane (CH4) with steam (H2O)? (CH4 + H2O → CO + 3H2) * A) 100% * B) 50% * C) 37.5% * D) 27% 12. A reaction has a high atom economy. What does this indicate? * A) the reaction is very fast * B) A large proportion of reactant atoms end up in the desired product. * C) The reaction releases a lot of energy. * D) The reaction has a low percentage yield. 13. Which type of reaction generally has a 100% atom economy? * A) Substitution * B) Elimination * C) Addition * D) Oxidation 14. Calculate the atom economy for the formation of 1,2-dibromoethane (C2H4Br2) from ethene (C2H4) and bromine (Br2). * A) 100% * B) 50% * C) 25% * D) 10% 15. In the Haber process (N2 + 3H2 → 2NH3), what is the *theoretical* yield of ammonia (Mr = 17) if 14 g of nitrogen (Mr = 28) is used? * A) 8.5 g * B) 17 g * C) 34 g * D) 28 g Answers and Explanations: 1. B) The formula showing the simplest whole number ratio of atoms of each element in a compound. - This is the de nition of an empirical formula. 2. A) CH2O fi  * C: 40/12 = 3.33 * H: 6.7/1 = 6.7 * O: 53.3/16 = 3.33 * Divide by smallest: C: 1, H: 2, O: 1 3. D) C3H4 * C: 90/12 = 7.5 * H: 10/1 = 10 Divide by smallest: C:1 H: 1.333 Multiply to get a whole number ratio: C:3 H:4 4. C) C4H8 - (CH2)n = 56; 14n = 56; n = 4 5. B) The total number of atoms of each element in a molecule of a compound. Correct de nition. 6. B) (Actual yield / Theoretical yield) x 100% - This is the correct formula. 7. B) 80% - (8.0 / 10.0) x 100% = 80% 8. A) 76 % - (6.0 / 7.9) x 100% = 75.9% = 76% (rounded) 9. B) A measure of the proportion of reactant atoms that become part of the desired product. - This is a good de nition focusing on waste reduction. 10. D) (mr of desired product/ total mr of reactants) x 100% Correct de nition. 11. C) 37.5% - (6 / (16+18)) x 100% = 17.6% 12. B) A large proportion of reactant atoms end up in the desired product. - High atom economy means less waste. 13. C) Addition - All reactant atoms are incorporated into a single product. 14. A) 100% - It's an addition reaction (C2H4 + Br2 → C2H4Br2). fi fi fi 15. B) 17 g * Moles of N2 = 14g/28 = 0.5 mol * Ratio of N2 : NH3 is 1:2, So moles of NH3 = 1 Mass = moles x mr = 1 x 17 = 17g