OCR A AS Jun 2016 Paper 1 Q22 (with explained solutions) |

Skip to content

OCR A AS Jun 2016 Paper 1 Q22

Answers available below

22 This question is about several salts. 14 (a) A hydrated salt, compound A, is analysed and has the following percentage composition by mass: Cr, 19.51%; Cl, 39.96%; H, 4.51%; O, 36.02%. Calculate the formula of compound A, showing clearly the water of crystallisation. Show your working. formula of compound A =[3] (b) A student carries out an experiment to determine the amount of water of crystallisation in the formula of another hydrated salt. The student intends to remove the water by heating the hydrated salt. A diagram of the apparatus used by the student is shown below. crucible pipeclay triangle (cid:129) (cid:129) (cid:129) The student adds the hydrated salt to the crucible and weighs the crucible and contents. The student heats the crucible and contents and allows them to cool. The student weighs the crucible and residue. The students results are shown below. Mass of crucible + hydrated salt / g Mass of crucible + residue after heating / g 16.84 16.26OCR 2016<br />
(i) The maximum error in each mass measurement using the balance is 0.005 g. Calculate the percentage error in the mass of water removed. 15 percentage error =% [1] (ii) Suggest one modification that the student could make to their method to reduce the percentage error in the mass of water removed.[1] (iii) The student is not sure that all the water of crystallisation has been removed. How could the student modify the experiment to be confident that all the water of crystallisation has been removed?[1]OCR 2016 Turn over<br />
16 (c) A student prepares a solution of sodium sulfate, Na2SO4, by adding 6.25102 mol dm3 sulfuric acid, H2SO4, from a burette to 25.0 cm3 of 0.124 mol dm3 NaOH in a conical flask. 2NaOH(aq) + H2SO4(aq)Na2SO4(aq) + 2H2O(l) Calculate the minimum volume of the H2SO4 that the student would need to completely react with the NaOH present. volume of H2SO4 =cm3 [3] (d) Salts can also be prepared in redox reactions of metals with acids. A student prepares a solution of aluminium sulfate by reacting aluminium with dilute sulfuric acid. 2Al(s) + 3H2SO4(aq)Al2(SO4)3(aq) + 3H2(g) Using oxidation numbers, show which element has been oxidised and which has been reduced in this reaction. State the changes in oxidation numbers, including all signs. element oxidisedoxidation number change: fromtoelement reducedoxidation number change: fromto[2]OCR 2016<br />

Show answer

H032/01 Question 22 (a) Answer Initial ratios 35.5 ; H, 4.51 52.0 ; Cl, 39.96 1.0 ; O, 36.02 Cr, 19.51 16.0 OR Cr, 0.375; Cl,1.126; H,4.51; O, 2.25Whole number ratios Cr, 1; Cl, 3; H, 12; O, 6Formula with water of crystallisation CrCl36H2OMarks AO element AO1.2 AO1.2 AO2.2 (b) (i) 20.005 0.58100 = 1.72%(b) (ii) Use balance weighing to 3/more decimal places OR Use a larger mass/amountAO2.8 AO3.3 Mark Scheme June 2016 Guidance NOTE: If only the correct answer of CrCl36H2O is seen with no working, award 1 mark only IF there is no whole number ratio, ALLOW empirical formula: CrCl3H12O6 ALLOW ECF from incorrect whole number ratio, provided ONLY Cl incorrect AND 6H2O, e.g. CrCl26H2O ALLOW 2% OR 1.7% up to calculator value of 1.724137931 ALLOW more precise/more accurate/ more sensitive/higher resolution/smaller division IGNORE less error/smaller interval balance IGNORE any reference to lid on crucible (water cant escape) IGNORE weigh straight after heating IGNORE idea of repeating the experiment/ taking an average/ getting concordant results /larger sample size, etc.<br />
H032/01 Question Answer (b) (iii) Heat to constant massMarks AO element AO3.4 Mark Scheme June 2016 Guidance ALLOW response that implies heating to constant mass, e.g. Heat again until the mass does not change IGNORE heat for longer Needs link to constant mass ALLOW ECF from n(NaOH) ALLOW ECF from n(H2SO4)MAX 1 mark if no + sign for oxidation number ALLOW 3+ ALLOW 1+ ALLOW H2 for hydrogen ALLOW 1 mark for all oxidation numbers correct, but oxidised and reduced the wrong way around IGNORE numbers around equation i.e. treat as rough working 6.25102 1000 (c) (d) 1000 = 3.1(0)103 (mol)FIRST CHECK ANSWER ON THE ANSWER LINE If answer = 24.8 (cm3) award 3 marks ----------------------------------------------------------------- n(NaOH) = 0.12425.0 n(H2SO4) = 3.10103 V(H2SO4) = 1.55103Element oxidised: Element reduced: hydrogen/H/H+ +1 to 06.25102 = 24.8 (cm3)= 1.55103 (mol)1000 aluminium/Al 0 to +3Total 14 10 AO2.8 3 AO1.1 AO1.2<br />

Question 21 22 23 24 25 26

About us | Need Help? | Feedback

Proudly powered by WordPress | Theme: Sydney by aThemes.