AQA AS Specimen Paper 1 Q6 (with explained solutions) |

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AQA AS Specimen Paper 1 Q6

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12 6 This question is about reactions of calcium compounds. . 1 0 6 A pure solid is thought to be calcium hydroxide. The solid can be identified from its relative formula mass. The relative formula mass can be determined experimentally by reacting a measured mass of the pure solid with an excess of hydrochloric acid. The equation for this reaction is Ca(OH)2 + 2HCl CaCl2 + 2H2O The unreacted acid can then be determined by titration with a standard sodium hydroxide solution. You are provided with 50.0 cm3 of 0.200 mol dm3 hydrochloric acid. Outline, giving brief practical details, how you would conduct an experiment to calculate accurately the relative formula mass of the solid using this method. [8 marks] Barcode Typesetter code<br />
13 . 2 0 6 A 3.56 g sample of calcium chloride was dissolved in water and reacted with an excess of sulfuric acid to form a precipitate of calcium sulfate. The percentage yield of calcium sulfate was 83.4%. Calculate the mass of calcium sulfate formed. Give your answer to an appropriate number of significant figures. [3 marks] Mass of calcium sulfate formed = Turn over for the next question Barcode Typesetter code Turn over <br />

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MARK SCHEMEAS ChemistrySpecimen paper 1 Marking guidance Mark AO Comments Stage 1: appreciation that the acid must be in excess and calculation of amount of solid that permits this. Statement that there must be an excess of acid Moles of acid = 50.00.200/1000 = 1.00102 mol 2 mol of acid react with 1 mol of calcium hydroxide therefore moles of solid weighed out must be less than half the moles of acid = 0.51.00102 = 5.00103 mol Mass of solid must be < 5.0010374.1 = < 0.371 g Stage 2: Experimental method. Measure out 50 cm3 of acid using a pipette and add the weighed amount of solid in a conical flask Titrate against 0.100 (or 0.200) mol dm3 NaOH added from a burette and record the volume (v) when an added indicator changes colour Stage 3: How to calculate Mr from the experimental data. Moles of calcium hydroxide = 5.00103(v/2conc NaOH)/1000 = z mol Mr = mass of solid / z Extended response Maximum of 7 marks for answers which do not show a sustained line of reasoning which is coherent, relevant, substantiated and logically structured. 1 1 1 1 1 1 AO2d AO3 2a AO3 2b AO3 2a AO3 2b AO3 2b AO3 2a AO3 2a 10 of 13 Question 06.1<br />
MARK SCHEMEAS ChemistrySpecimen paper 1 Moles of calcium chloride = 3.56 / 111.1 = 3.204102 Moles of calcium sulfate = 3.20410283.4/100 = 2.672102 Mass of calcium sulfate = 2.672102136.2 = 3.6398 = 3.64 (g) 1 1 1 AO2h AO2h AO2h Answer must be to 3 significant figures 11 of 13 06.2<br />

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