ACSBR 7E Chemical Calculations (Extensions) - Assignment ANS

Anglo-Chinese School (Barker Road) A Methodist Institution Founded in 1886 CHEMISTRY DEPARTMENT OF SCIENCE Name: ANSWERS ( ) Class: SEC 3 CHEMICAL CALCULATIONS (EXTENSIONS) – ASSIGNMENT ALEX LEE & SAMSON WOON/2010 PAGE 1 OF 4 PURE/COMBINED Multiple-Choice Questions [20 Marks] TOTAL SCORE / 30 Write in your selected answer for the multiple-choice questions in the boxes provided. 1 2 3 4 5 6 7 8 9 10 A A B C C A D D B B 11 12 13 14 15 16 17 18 19 20 D C D C D B D A A C 1. The ratio of the number of atoms in 2 moles of oxygen molecules to the number of atoms in 4 moles of helium is A 1 : 1 B 1 : 2 C 1 : 4 D 2 : 1 2. Which of the following contains the same number of atoms as 25.5 grams of ammonia? A 2.0 mol of CO2 B 3.0 mol of He C 4.0 mol of CH4 D 6.0 mol of O2 3. A 0.25 mol sample of an unknown metal Q is burnt in oxygen. The oxide formed was found to have a mass of 18 grams. Given that the relative atomic mass of Q is 64, what is the oxide formula? A QO B Q2O C QO2 D Q2O2 4. 0.1 mol MSO4 combines with 5.4 g of water to form the hydrate MSO4.nH2O. What is n? A 1 B 2 C 3 D 4 5. Which of the following contains the greatest mass of nitrogen for every 1 gram of substance? A (NH4)3PO4 B Mg(NO3)2 C NH3 D NH4NO3 6. Which of the following contains the greatest mass of nitrogen for every 1 mole of substance? A (NH4)3PO4 B Mg(NO3)2 C NH3 D NH4NO3

ALEX LEE & SAMSON WOON/2010 PAGE 2 OF 4 7. The empirical formula of an organic molecule was found to be CH2O. What other information is required in order to find its molecular formula? A melting and boiling points C products of combustion B percentage composition D relative molecular mass 8. Which of the following statements about empirical formula is false? A Ionic compounds are always represented by an empirical, and not molecular, formula. B Percentage composition by mass of a compound can be found from its empirical formula. C The empirical formula of a compound can be found from its percentage composition by mass. D Whether a substance has a simple or giant structure can be seen from its empirical formula. 9. An unknown chloride of phosphorus was found to contain 5.6 g of phosphorus and 32.0 g of chlorine. What is its empirical formula? A PCl3 B PCl5 C PCl6 D P2Cl5 10. An oxide of sulfur contains 1.5 times the mass of oxygen than sulfur. What is the empirical formula for this oxide? A SO2 B SO3 C S2O3 D S3O2 11. In an experiment to find the empirical formula of a metallic oxide, a strip of the unknown metal X is first weighed. Next, the strip of X is allowed to react completely in a covered crucible, and the residue allowed to cool. After cooling, the residue is then extracted and weighed. The results for the experiment are as shown. mass of metal X 3.0 grams mass of residue 5.0 grams Assuming that the relative atomic mass of X is 32, find the empirical formula of the oxide of X. A XO B XO2 C X2O3 D X3O4 12. Pentane fuel is combusted in excess oxygen as shown: C5H12 (l) + 8 O2 (g) ⎯⎯→ 5 CO2 (g) + 6 H2O (l) What is the total volume of gas remaining if 0.24 grams of pentane is allowed to react with 800 cm3 of oxygen? Assume that all volumes are measured at room conditions. A 160 cm3 B 400 cm3 C 560 cm3 D 880 cm3 13. 2.00 grams of solid calcium carbonate is allowed to react with 50.0 cm3 of 0.200 mol dm-3 hydrochloric acid, as shown below. What mass of calcium carbonate remains after the reaction? CaCO3 (g) + 2 HCl (aq) ⎯⎯→ CaCl2 (g) + H2O (l) + CO2 (g) A 0.25 grams B 0.50 grams C 1.00 grams D 1.50 grams

ALEX LEE & SAMSON WOON/2010 PAGE 3 OF 4 14. Silver ions react with chloride ions. Ag+ (aq) + Cl- (aq) ⎯⎯→ AgCl (s) It is found that 5.0 cm3 of a 0.1 mol/dm3 solution of the chloride of metal X requires 10.0 cm3 of 0.1 mol/dm3 silver nitrate for complete reaction. What is the formula of the chloride? A XCl B X2Cl C XCl2 D XCl4 15. Sulfur trioxide, SO3, is prepared by reacting equal masses of sulfur and oxygen. What percentage of the excess reagent remains unreacted? A 25.5 % B 28.8 % C 31.1 % D 33.3 % 16. 48.0 g of impure carbon combusts in an excess of oxygen to form 21.8 dm3 of carbon dioxide, measured at room temperature and pressure. What is the percentage purity of the carbon? A 30.3 % B 22.7 % C 45.4 % D 53.8 % 17. A solution is made up by dissolving 1.25 g of impure sodium hydroxide in water and making it up to 250 cm3 of solution. 25.0 cm3 of this solution is neutralized by 30.0 cm3 of 0.100 mol/dm3 HCl. What is the percentage purity of the sodium hydroxide? A 9.60 % B 26.0 % C 48.0 % D 96.0 % 18. 0.2 mol of aqueous magnesium chloride was mixed with an excess of aqueous silver nitrate in a beaker. 36.6 grams of precipitate was formed. What is the percentage yield for this reaction? A 63.8 % B 78.4 % C 85.0 % D 89.7 % 19. The conversion of ethene to ethanol can be represented as follows: A ⎯⎯→ B ⎯⎯→ C ⎯⎯→ D ⎯⎯→ 2 C + 2 H2 C2H4 C2H5OH CH3COOH CH3COONa 38.4 g of C (excess H2) 36.0 dm3 (at r.t.p.) 64.4 g 2.0 dm3 of 0.65 M 1.2 mol Which reaction, A, B, C or D, has the greatest percentage yield? 20. The first step in the Ostwald Process for producing nitric acid is as follows: 4 NH3 (g) + 5 O2 (g) ⎯→ 4 NO (g) + 6 H2O (g) If 150 g of ammonia reacts with 150 g of oxygen gas to give 87 g of nitric oxide, what is the percentage yield for this reaction? A 33 % B 49 % C 77 % D 100 %

ALEX LEE & SAMSON WOON/2010 PAGE 4 OF 4 Structured Questions [10 Marks] 21. An 16.0 g sample of an unknown oxide of phosphorus contains 9.0 grams of oxygen by mass. (a) By showing appropriate working, find the empirical formula of this oxide. [2] mass ratio of P : O = 7 : 9 mole ratio of P : O = 7/31 : 9/16 = 0.226 : 0.563 = 1 : 2.5 = 2 : 5 Hence the empirical formula is P2O5. [1 mark presentation, 1 mark answer] (b) Given that relative formula mass of the oxide is 284, find its molecular formula. [2] Molecular formula is P2nO5n. Mr of P2nO5n = 2n (31) + 5n (16) = 62 n + 80 n = 142 n = 284 Hence, n = 2 Molecular formula is P4O10. [1 mark presentation, 1 mark answer] (c) Would you expect this phosphorus oxide to have a high or low boiling point? Explain how you arrived at your answer. [3] Low boiling point [1]. This phosphorus oxide has a simple covalent structure [1]. We can deduce this as its molecular formula is not the empirical formula, whereas marcomolecules are represented by their empirical formulae (or OWTTE). [1] 22. In the industrial extraction of iron, haematite, an iron ore mainly consisting of iron(III) oxide, is heated in the presence of carbon monoxide, as shown. Fe2O3 (s) + 3 CO (g) ⎯→ 2 Fe (l) + 3 CO2 (g) A 6.00 kg sample of haematite reacts with excess carbon monoxide produce 3.50 kg of iron. Calculate the percentage purity of the haematite. [3] mol of Fe = mass ÷ Ar = 3500 ÷ 56 = 62.5 mol [1] mol of Fe2O3 = ½ mol of Fe = ½ (62.5) = 31.25 mol mass of Fe2O3 = mol × Mr = 31.25 × 160 = 5000 g [1] % purity = (5000 ÷ 6000) × 100% = 83.3 % (3 s.f.) [1] END