Empirical Formula and Reacting Masses Revision - Higher

Transcription

1 Empirical Formula and Reacting Masses Revision - Higher Q. Perfumes contain a mixture of chemicals. The main ingredients of perfumes are a solvent and a mixture of fragrances. (a) A sample of the solvent used in one perfume contained 0.60 g of carbon, 0.5 g of hydrogen and 0.40 g of oxygen. Relative atomic masses: H = l; C = 2; O = 6. Calculate the empirical (simplest) formula of the solvent. You must show all of your working to gain full marks for this question. (4) (b) Solvent molecules evaporate easily. Explain why substances made of simple molecules evaporate easily. (2) Page

2 (c) Most companies claim that their perfumes have been tested on skin. A study was made of the tests they used. The study found that each company used different tests. The perfumes were tested in the companies own laboratories and not by independent scientists. Some companies did not give any information about the tests that they had used. (i) Suggest why companies test their perfumes on skin. () (ii) Did the study show that the tests made by the different companies were valid and reliable? Explain your answer. (2) (Total 9 marks) Q2. Cosmetic powders were widely used in ancient Egypt. Cosmetic powders that may have been used in face paints have been analysed. These powders contained compounds that are rare in nature. The compounds must have been made by the ancient Egyptians using chemical reactions. Page 2

3 One of these compounds is called phosgenite. Analysis of this compound shows that it contains: 76.0% lead (Pb) 3.0% chlorine (Cl) 2.2% carbon (C) 8.8% oxygen (O) Calculate the empirical formula of this compound. To gain full marks you must show all your working. Relative atomic masses: C = 2 ; O = 6 ; Cl = 35.5 ; Pb = (Total 4 marks) Q3. Limestone is a useful mineral. Every day, large amounts of limestone are heated in limekilns to produce lime. Lime is used in the manufacture of iron, cement and glass and for neutralising acidic soils. CaCO 3 CaO + CO 2 (i) The decomposition of limestone is a reversible reaction. Explain what this means. (2) Page 3

4 (ii) Calculate the mass of lime, CaO, that would be produced from 250 tonnes of limestone, CaCO 3. Relative atomic masses: C 2; O 6; Ca 40. Mass of lime =... tonnes (3) (Total 5 marks) Q4. Ammonium nitrate is an important fertiliser. It is made by reacting nitric acid with the alkali ammonia. (i) State the type of reaction taking place. () (ii) The equation for this reaction is: NH 3 + HNO 3 NH 4NO 3 Calculate the number of tonnes of ammonium nitrate that can be made from 68 tonnes of ammonia. (Relative atomic masses: H =, N = 4, O = 6) (3) (Total 4 marks) Q5. This cake recipe is taken from a cookery book. Soda Cake Mix the flour and butter and add the sugar, currants and flavouring. Then add the beaten egg. Add a little milk with a teaspoonful of baking soda (sodium hydrogencarbonate) and mix it in well. Bake in a moderate oven for about 30 minutes. Page 4

5 When sodium hydrogencarbonate is heated in an oven, it forms carbon dioxide gas. 2 NaHCO 3 Na 2CO 3 + H 2O + CO 2 A teaspoonful of baking soda contains a mass of g of sodium hydrogencarbonate. Calculate the mass of carbon dioxide that could be made from g of sodium hydrogencarbonate. Show clearly how you work out your final answer. Relative atomic masses: H = ; C = 2; O = 6; Na = M. (a) C H O Mass of carbon dioxide =... g (Total 3 marks) = 0.05 = 0.5 = C 2H 6O mark for dividing the correct amount or multiples of correct amount by A r mark for proportions mark for whole number ratio accept any multiple mark for correctly written simplest formula correct formula without working gets only 2 marks correct formula gains full marks provided steps and 2 are correct. ecf can be allowed from step 2 to 3 or step 3 to 4 formula can be in any order eg OH 6C 2 (b) intermolecular forces / bonds are weak or (covalent) bonds are weak = 0 Page 5

6 forces between molecules or bonds between molecules () (attractive) forces are weak = are weak () if no marks awarded, allow low boiling point or small M r for mark (c) (i) to check the safety of the perfume (owtte) accept references to possible harmful / dangerous effects of perfume or possible reactions on skin eg to show it does not damage skin / cause cancer etc. allow to see what it smells like on the skin allow so the company do not have to test on animals (ii) any two from: idea from text linked with an explanation the company claim to have tested the product: but we cannot be certain they have or how thorough they are or how accurately reported companies did not disclose how they did their tests: so they could not be checked or so they could not be shown to be reliable / valid or so they could not be repeated or converse eg companies should disclose how they did their tests so that results can be checked etc. companies may not have repeated their tests: so they may not be reliable companies do their own tests: so they may be biased or so they may not be truthful about their results or so they may not be reliable or converse eg independent tests should be done so as to ensure there is no bias etc. the companies are using different tests: so the results cannot be compared or so results will be different or so results will not be fair / valid / reliable or converse eg companies should do the same tests so that the results will be fair etc. companies would not give false information because of damage to reputation or it might lead to litigation 2 [9] Page 6

7 M2. Pb Cl C O mark for dividing one mass by A r allow upside down ratio to lose this mark only = = = 0.83 = 0.55 mark for one correct proportion accept to one d.p. or rounded up to d.p. mark for all four correct proportions correctly rounded or Pb 2Cl 2CO 3 mark for correctly written formula or correct whole number ratio correct formula without working gets only mark. e.c.f. can be allowed from incorrect proportions to formula or ratio [4] M3. (i) a reaction in which the products can be changed back to reactants accept a reaction that can go forwards or backwards under certain conditions (ii) M r CaCO 3 = 00 M r CaO = 56 mass of CaO = 40 (tonnes) mark consequentially [5] Page 7

8 M4. (i) neutralisation/acid base reaction for mark (ii) 7 (tonnes) give 80 (tonnes) (even if only in working) for mark each 320 (tonnes) or alternative method) 3 marks for correct answer (if 7 and 80 not given allow mark for correct answer using their figures) 3 [4] M5. 68g 44g g g 2.88g (2.9g) care with rounding or Mr values 84 and 44 () moles hydrogen carb = () mass of CO 2 = = 2.9g answer 2.88 to 2.9 gets 3 marks answer of 3 gets 2 marks () [3] Page 8