CHEMISTRY QUESTIONSHEETS AS Level AS TOPIC 2 FORMULAE, EQUATIONS & MOLES Questionsheet 1 Questionsheet 2 Questionsheet 3 Questionsheet 4 Questionsheet 5 Questionsheet 6 Questionsheet 7 Questionsheet 8 Questionsheet 9 Questionsheet 10 Questionsheet 11 Questionsheet 12 Questionsheet 13 Questionsheet 14 FORMULAE AND MOLECULAR EQUATIONS IONIC EQUATIONS MOLES EMPIRICAL FORMULAE MOLECULAR FORMULAE USES OF FORMULAE YIELD CALCULATIONS CALCULATION OF REACTING MASSES CALCULATIONS INVOLVING GASES CALCULATIONS INVOLVING THE IDEAL GAS EQUATION EXPERIMENT TO FIND THE FORMULA OF A METAL OXIDE EXPERIMENT TO DETERMINE THE FORMULA OF AN HYDRATED SALT EXPERIMENT TO DETERMINE OF THE RELATIVE MOLECULAR MASS OF A GAS BY DIRECT WEIGHING USING THE IDEAL GAS EQUATION EXPERIMENT TO DETERMINE THE RELATIVE MOLECULAR MASS OF A GAS 17 marks 15 marks 14 marks 16 marks 17 marks 16 marks 16 marks 17 marks 21 marks 41 marks 12 marks 9 marks 12 marks 10 marks Authors Trevor Birt Donald E Caddy Andrew Jones Adrian Bond Editors John Brockington Stuart Barker John Brockington Kevin Frobisher Andy Shepherd Stuart Barker Curriculum P ress www.curriculum-press.co.uk Curriculum Press Licence Agreement: Paper copies of the A-Level Chemistry Questionsheets may be copied free of charge by teaching staff or students for use within their school, provided the Photocopy Masters have been purchased by their school. No part of these Questionsheets may be reproduced or transmitted, in any other form or by any other means, without the prior permission of the publisher. All rights are reserved. This license agreement is covered by the laws of England and Wales Curriculum Press March 2008. Curriculum Press Bank House 105 King Street Wellington Shropshire TF1 1NU
TOPIC 2 Questionsheet 1 FORMULAE AND MOLECULAR EQUATIONS a) Explain why the formula of aluminium sulfate is Al 2 (SO 4 ) 3 ; not AlSO 4. b) (i) Write down the formulae of the following compounds. Copper(II) sulfate... [1] Sodium hydroxide... [1] Copper(II) hydroxide... [1] Sodium sulfate... [1] Write a balanced molecular equation, including state symbols, for the reaction between copper(ii) sulfate and sodium hydroxide in aqueous solution to give a precipitate of copper(ii)hydroxide and a solution of sodium sulfate. c) (i) Equations I and II relate to the Haber process and contact process, respectively. Both are incorrect. In each case state the fault (or faults) and then write down the correct equation. Equation I N(g) + 3H(g) ¾ NH 3 (g) Fault(s)... [1] Correct equation... [1] Equation II SO 2 (g) + O 2 (g) ¾ SO 3 (g) Fault(s)... [1] Correct equation... [1] The equations in c) (i) are each written with a ¾ sign instead of an arrow. What is the reason for this? d) Iron and HCl react together according to the following basic equation: Fe + 2HC1 FeCl 2 + H 2 Depending on the reaction conditions the product may be (i) solid, anhydrous iron(ii) chloride, an aqueous solution of iron(ii) chloride, or (iii) crystals of iron(ii) chloride-4-water. Select suitable state symbols and make any other necessary modifications to the basic equation to represent the formation of each of these products. (i) Passing dry hydrogen chloride gas over heated iron to give anhydrous iron(ii) chloride. Dissolving iron in hydrochloric acid to give a solution of iron(ii) chloride. (iii) Dissolving iron in hydrochloric acid and then crystallising the solution to give iron(ii) chloride-4-water. TOTAL / 17
TOPIC 2 Questionsheet 2 IONIC EQUATIONS Note State symbols should be included in all the ionic equations in this Questionsheet. a) An extract from a textbook reads as follows: An ionic equation focuses attention on the essential chemistry, because it shows only those ions or molecules which take part in the reaction. Spectator ions do not appear. (i) What are spectator ions? What is wrong with the following ionic equation? Cl 2 (aq) + 2K + (aq) + 2Br - (aq) Br 2 (aq) + 2K + (aq) + 2C1 - (aq) (iii) Rewrite the ionic equation in a) correctly. b) (i) Write a balanced molecular equation (with state symbols) for calcium carbonate dissolving in hydrochloric acid to give an aqueous solution of calcium chloride, water and carbon dioxide. Rewrite the equation in b) (i) as an ionic equation. c) (i) Write an ionic equation for sodium chloride and silver nitrate reacting together in aqueous solution to give a precipitate of silver chloride and a solution of sodium nitrate. What difference (if any) would there be in the ionic equation if sodium chloride were to be replaced by potassium chloride? d) (i) Write ionic equations for the following reactions. I Hydrochloric acid and sodium hydroxide in aqueous solution. II Nitric acid and potassium hydroxide in aqueous solution. In the light of your answers to d) (i), suggest why the enthalpy change (heat given out) is approximately the same for both reactions. e) The reaction between potassium manganate(vii) and iron(ii) sulfate in the presence of dilute sulfuric acid is represented by the following ionic equation: MnO 4- (aq) + 5Fe 2+ (aq) + 8H + (aq) Mn 2+ (aq) + 5Fe 3+ (aq) + 4H 2 O(1) Use the appropriate spectator ions to expand the ionic equation into a molecular equation. TOTAL / 15
a) Define the chemical term mole. TOPIC 2 Questionsheet 3 MOLES b) Calculate the mass of each of the following: (i) 1 mol of atomic nitrogen. 1 mol of molecular nitrogen. c) How many moles are there in each of the following? (i) 0.48 g magnesium. 100 kg calcium hydroxide. (iii) 2 tonnes Sulfur. d) Calculate the percentage by mass of tin in tin(iv) chloride, SnCl 4. e) 2 mol of a compound has a mass of 117.0 g. What is its relative molecular mass? f) What is the total number of moles of particles (ions, molecules) present in 0.35 mol of copper(ii) Sulfate-5-water, CuSO 4.5H 2 O? TOTAL / 14
TOPIC 2 Questionsheet 4 EMPIRICAL FORMULAE a) Explain the meaning of the term empirical formula. b) A compound contains 29.1 % of sodium, 40.5 % of Sulfur and 30.4 % of oxygen. Calculate its empirical formula. c) 10.00 g of an oxide of lead, when reduced, produced 9.07 g of metal. Determine the empirical formula of the oxide. d) 4.00 g of sodium are burnt in excess oxygen, to produce 6.78 g of a yellow solid E. Find the empirical formula of E. e) Compound F contains 39.74 % carbon, 7.28 % hydrogen and 52.98 % bromine. Show that the empirical formula is C 5 H 11 Br. TOTAL / 16
TOPIC 2 Questionsheet 5 MOLECULAR FORMULAE a) (i) Explain the meaning of the term molecular formula. If the empirical formula of a compound is known, what additional information is required in order to determine its molecular formula? (iii) Calculate the percentages by mass of phosphorus, oxygen and chlorine in the compound POCl 3. b) A hydrocarbon D is known to have a formula C n H (2n+2). It is found to have a relative molecular mass of 156. Deduce its molecular formula. c) A salt of silver comprises silver, carbon and oxygen only. It contains 7.89 % carbon and 21.06 % oxygen by mass and has a relative formula mass of 304. Determine the empirical and molecular formulae of this salt. d) The empirical formula of a diprotic acid is CHO 2. 0.125 g of the acid was neutralised by a solution containing 0.111 g of sodium hydroxide. Calculate its molecular formula.... [5] TOTAL / 17
TOPIC 2 Questionsheet 6 USES OF FORMULAE a) Calculate the mass of combined strontium in a 125 g tube of toothpaste, which contains 10 % by mass of strontium chloride-6-water, SrCl 2.6H 2 O. b) What mass of tetraethyllead (TEL), Pb(C 2 H 5 ) 4, would have to be added to 1000 dm 3 of petrol to give a lead content of 0.1 g dm -3? c) How much sodium fluoride should be added to 10 tonnes of drinking water to provide a fluoride ion concentration of 1 part per million? d) A motorist can be prosecuted if the ethanol content of his/her blood is greater that 80 mg per 100 ml of blood. How many moles of ethanol would there be in 1 dm 3 of the blood of a motorist who was at the limit? TOTAL / 16
TOPIC 2 Questionsheet 7 YIELD CALCULATIONS a) (i) What mass of aluminium could theoretically be obtained by the electrolysis of 1 tonne of aluminium oxide, Al 2 O 3? In a particular plant, the yield obtained is 90 %. What mass of aluminium oxide would be needed to produce 1 tonne of aluminium? b) In the preparation of nitrobenzene, C 6 H 5 NO 2, one mole of product is obtained theoretically from one mole of benzene, C 6 H 6. In one particular experiment, a student obtained 43.0 g of nitrobenzene from 40.0 g of benzene. (i) What mass of nitrobenzene should, in theory, have been produced? Calculate the percentage yield obtained in this experiment. (iii) Give two reasons why the yield may have been less than that predicted. c) An aqueous solution of sodium hydroxide, containing 35.0 g NaOH, was reacted with dilute Sulfuric acid containing 49.0 g H 2 SO 4. The resulting solution was evaporated to dryness and strongly heated to produce anhydrous sodium Sulfate. (i) Write the overall equation for the reaction. Calculate the number of moles of both sodium hydroxide and sulfuric acid and, by referring to your equation, state which of these substances is deficient. Moles of NaOH... [1] Moles of H 2 SO 4... [1] Deficient reactant... [1] (iii) Calculate the theoretical yield of anhydrous sodium sulfate. TOTAL / 16
TOPIC 2 Questionsheet 8 CALCULATION OF REACTING MASSES a) Calculate the mass of sodium hydrogencarbonate that must be heated to obtain 100 g of sodium carbonate:... 2NaHCO Na CO + H O + CO 3 2 3 2 2 b) Copper could be recovered from copper(ii) sulfate solution by a displacement reaction with aluminium. (i) Write the equation for this reaction. What is the minimum mass of aluminium that would be required to recover 1 kg of copper? c) During steel making, sulfur is removed from molten iron by adding magnesium: Mg + S... MgS If a 100 tonne batch of molten iron contains 0.25% sulfur, how many kilograms of magnesium are needed to remove it? d) Haematite is an ore of iron, containing 30 % by mass of iron(iii) oxide, Fe 2 O 3. In a particular blast furnace, 95 % of the iron is extracted. What mass of haematite is required to produce 10 tonnes of pure iron? TOTAL / 17
TOPIC 2 Questionsheet 9 CALCULATIONS INVOLVING GASES For all these questions assume that 1 mole of any gas occupies 24 dm 3 at room temperature and pressure a) Calculate the volume of hydrogen produced at room temperature and pressure by 3.25 g of zinc metal when reacted with excess acid. Zn(s) + H 2 SO 4 (aq) ZnSO 4 (aq) + H 2 (g) b) 0.160 g of calcium carbide (CaC 2 ) was treated with excess water. Calculate the volume of ethyne (C 2 H 2 ) gas produced at room temperature and pressure. CaC 2 (s) + 2H 2 O(l) C 2 H 2 (g) + Ca(OH) 2 (s) c) 0.480 g of aluminium carbide (Al 4 C 3 ) was treated with excess water. Calculate the volume of methane (CH 4 ) gas produced at room temperature and pressure. Al 4 C 3 (s) + 12H 2 O(l) 3CH 4 (g) + 4Al(OH) 3 (s) d) Lithium hydroxide is used to absorb carbon dioxide in space suits. Calculate the volume of carbon dioxide measured at room temperature and pressure absorbed by 1.00 g of lithium hydroxide. 2LiOH(s) + CO 2 (g) Li 2 CO 3 (s) + H 2 O(l) TOTAL (Continued...) / 13
TOPIC TOPIC 2 Questionsheet 2 9 Continued 10 CALCULATIONS INVOLVING GASES e) Calculate the volume of carbon dioxide measured at room temperature and pressure absorbed by 1.00 g of calcium hydroxide. Ca(OH) 2 (s) + CO 2 (g) CaCO 3 (s) + H 2 O(l) f) Some drain cleaners use aluminium powder and sodium hydroxide. When placed in water the aluminium powder reacts with the sodium hydroxide as follows to give hydrogen and heat. 2Al(s) + 2NaOH(aq) + 6H 2 O(l) 2NaAl(OH) 4 (aq) + 3H 2 (g) Calculate the volume of hydrogen produced at room temperature and pressure from 1.00 g of aluminium. g) Sodium azide (NaN 3 ) on heating decomposes to nitrogen gas as follows: 2NaN 3 (s) 2Na(s) + 3N 2 (g) Calculate the volume of gases produced from 100 g of sodium azide at room temperature and pressure. This is the basis of an air bag system for cars, in which sodium azide is mixed with an oxidiser of iron oxide and silicon dioxide. TOTAL / 21
TOPIC 2 Questionsheet 11 10 CALCULATIONS INVOLVING THE IDEAL GAS EQUATION NOTE In this Questionsheet, assume that the ideal gas constant R = 8.31 J K -1 mol -1 QUESTION 1 0.688 g of a hydrocarbon occupied 472cm 3 at 98 kpa and 67 0 C. a) Calculate the (i) mol of hydrocarbon in 0.688 g relative molecular mass of the hydrocarbon. b) (i) Suggest the molecular formula and name of the hydrocarbon Give an equation for the complete combustion of the hydrocarbon (iii) Calculate the volume of oxygen needed to completely react with 0.21 g of the hydrocarbon under the same conditions. TOTAL / 13
TOPIC TOPIC 2 Questionsheet 2 10 Continued 12 CALCULATIONS INVOLVING THE IDEAL GAS EQUATION NOTE In this Questionsheet, assume that the ideal gas constant R = 8.31 J K -1 mol -1 QUESTION 2 1500cm 3 dry chlorine reacted with 1.40g iron at 30 0 C and 99.5 kpa. Following the complete reaction of the iron, 552cm 3 chlorine remained. a) Calculate: (i) the volume of chlorine reacted with the iron the volume of one mole of gas under these conditions (iii) the mol chlorine reacted with the iron (iv) the mol iron reacted b) Show how many mol of chlorine would react with one mole of iron in this reaction c) From your answer to b) suggest an equation for the reaction d) Suggest why the chlorine was dry TOTAL / 13
TOPIC TOPIC 2 Questionsheet 2 10 Continued 13 CALCULATIONS INVOLVING THE IDEAL GAS EQUATION NOTE In this Questionsheet, assume that the ideal gas constant R = 8.31 J K -1 mol -1 QUESTION 3 1.08 g of a Group 2 metal completely reacted with an excess of hydrochloric acid and 1120 cm 3 hydrogen formed at 96.5 kpa and 16 o C a) Give an ionic equation for the reaction using M as the symbol of the metal. b) Calculate the volume of one mole of gas under these conditions c) Calculate the mol of hydrogen formed d) Calculate the relative atomic mass of the metal and suggest the identity e) If the experiment had been carried out at 24 o C and 100 kpa using 0.50 g calcium and 231 cm 3 hydrogen formed, calculate the percentage of the metal that was unreacted TOTAL / 15
TOPIC 2 Questionsheet 14 11 EXPERIMENT TO FIND THE FORMULA OF A METAL OXIDE Metal oxide Hydrogen Gases out Heat Porcelain boat A porcelain boat was weighed empty and then filled with a metal oxide and reweighed. The apparatus was set up as in the diagram and the oxide heated in a stream of hydrogen until there was no further loss in mass of the boat and its contents. This removes the oxygen, leaving the metal. Complete the table below and then calculate the formula of the oxide a) Results for an oxide of copper Mass of boat and metal oxide before heating = 11.25 g Mass of boat empty = 9.90 g Mass of metal oxide = [1] Mass of boat and metal oxide after heating = 11.10 g Mass of boat empty = 9.90 g Mass of metal = [1] Calculation (use atomic masses: Cu = 64, O=16) Complete the table below and then calculate the formula of the oxide b) Results for an oxide of iron Mass of boat and metal oxide before heating = 9.29 g Mass of boat empty = 9.00 g Mass of metal oxide = [1] Mass of boat and metal oxide after heating = 9.21 g Mass of boat empty = 9.00 g Mass of metal = [1] Calculation (use atomic masses: Fe = 56, O=16) TOTAL / 12
TOPIC 2 Questionsheet 15 12 EXPERIMENT TO DETERMINE THE FORMULA OF AN HYDRATED SALT In an experiment to determine the formula of hydrated salts, a sample of hydrated barium chloride was heated in a previously weighed crucible until a constant mass was reached. The results are given below. Results Mass of crucible + lid + barium chloride crystals = 17.44 g Mass of crucible + lid empty = 15.00 g Mass of hydrated barium chloride crystals = [1] Mass of crucible + lid + barium chloride after heating = 17.08 g Mass of crucible + lid empty = 15.00 g Mass of anhydrous barium chloride = [1] Mass of hydrated barium chloride crystals = [1] Mass of anhydrous barium chloride = [1] Mass of water of crystallisation = [1] Complete the table above and then determine the value of n in the formula BaCl 2 nh 2 O Show all the steps in your calculation with full explanation. TOTAL / 9
TOPIC 2 Questionsheet 16 13 EXPERIMENT TO DETERMINE OF THE RELATIVE MOLECULAR MASS OF A GAS BY DIRECT WEIGHING Wooden block Cap In an experiment the following method was followed. 1. Weigh the 20 cm 3 syringe empty (with vacuum), using a wooden block to hold back the plunger. 2. Flush out the syringe with the gas twice, before filling to the mark with the gas and using the wooden block as the guide. 3. Weigh the syringe + gas + wooden block. 4. Repeat steps 2 and 3 with other gases. 5. Repeat step 1 and average the two results. 6. Put the results in the table below. Gas Mass of gas Average mass Mass of gas Mass of 24 dm 3 Calculated M r from A r + syringe of syringe empty of the gas of constituent atoms O 2 29.618 g 29.592 g CO 2 29.635 g 29.592 g N 2 O 29.628 g 29.592 g SO 2 29.644 g 29.592 g C 4 H 10 29.639 g 29.592 g CH 4 29.606 g 29.592 g C 2 H 2 29.614 g 29.592 g a) Complete the above table. [4] b) Plot a graph of Mass of 24 dm 3 of the gas against M r [2] c) Is there a correlation between the mass of 24 dm 3 of gas and the M r value? d) Comment on the reliability of the results. e) What are the main errors in the experiment? TOTAL / 12
TOPIC 2 Questionsheet 17 14 USING THE IDEAL GAS EQUATION EXPERIMENT TO DETERMINE THE RELATIVE MOLECULAR MASS OF A GAS Steam Rubber cap Hypodermic syringe Steam Thermometer In an experiment to determine the relative molecular mass of trichloromethane, the following method carried out. Trichloromethane is a volatile liquid that boils at 62 C. 1. Set up the apparatus as shown in the diagram, but without the hypodermic syringe. Put 5 cm 3 of air into the syringe and wait until the readings are steady. 2. Rinse the hypodermic syringe with the test liquid, then fill the hypodermic syringe with the test liquid. Make sure that all the air is expelled from the hypodermic syringe, and then seal the end with silicone rubber. 3. Weigh the hypodermic syringe, then inject about 0.2 cm 3 of liquid into the graduated gas syringe and immediately reweigh the syringe. 4. Record the value of the reading on the scale of the graduated syringe when it is steady. Also note the temperature and the atmospheric pressure. Results Mass of hypodermic syringe and liquid before injection = 11.28 g Mass of hypodermic syringe and liquid after injection = 11.12 g Temperature of vapour = 100 C Atmospheric pressure = 105 kpa Volume of syringe at start with air only = 5 cm 3 Volume of syringe with air and vapour = 39 cm 3 (The gas constant R = 8.31 JK -1 mol -1 ) a) Calculate the relative molecular mass of trichloromethane (CHCl 3 ) using the results of the experiment.... [5] b) Calculate a value from relative atomic masses.... [1] c) What are the likely significant errors in the experiment? TOTAL / 10