Edexcel Jan 2014 (IAL) Paper 3 Q3 (with explained solutions) |

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Edexcel Jan 2014 (IAL) Paper 3 Q3

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3 The mass of magnesium hydroxide, Mg(OH)2, in an indigestion tablet was determined as follows: Step 1 The tablet was crushed and dissolved in exactly 40.00 cm3 of dilute hydrochloric acid (an excess). Step 2 The amount of hydrochloric acid remaining was measured by titration with 0.250 mol dm3 sodium hydroxide solution. 22.80 cm3 of this sodium hydroxide solution was required. HCl(aq) + NaOH(aq) (cid:314) NaCl(aq) + H2O(l) (a) (i) A student suggested using Universal Indicator for the titration. Why would this indicator be unsuitable? (1)(ii) Suggest a suitable indicator and give its colours in acidic and alkaline solutions. (2) IndicatorColour in acidColour in alkali(b) (i) Calculate the number of moles of sodium hydroxide used in the titration. (ii) Hence state the number of moles of hydrochloric acid that react with the sodium hydroxide in (b)(i). (1) (1) *P42974RA0916* Turn over<br />
(iii) The amount of acid added to the tablet in Step1 was 2.00 (cid:117) 102 mol. Use this information and your answer to (b)(ii) to calculate the number of moles of hydrochloric acid that reacted with the tablet. (iv) The equation for the reaction of the magnesium hydroxide in the tablet with hydrochloric acid is shown below. Mg(OH)2(s) + 2HCl(aq) (cid:314) MgCl2(aq) + 2H2O(l) Calculate the mass of magnesium hydroxide in the tablet. Give your answer to three significant figures. The molar mass of magnesium hydroxide is 58.3 g mol1. (c) The volume of hydrochloric acid added to the tablet was 40.00 cm3. (i) Suggest a change in the procedure which would make the result of the experiment more reliable for each tablet which is analysed. (1) (2) (1)10 *P42974RA01016*<br />
(ii) The hydrochloric acid was measured using a burette. Each time the burette was read, the error was0.05 cm3. Calculate the total percentage error in measuring 40.00 cm3 of hydrochloric acid. (d) When an indigestion tablet reacts with hydrochloric acid in the stomach, it is important that the reaction is not too exothermic. The enthalpy change of this reaction can be determined by reacting magnesium hydroxide with an excess of hydrochloric acid in an insulated container and measuring the maximum temperature change. State two ways, other than improvements in insulation or use of more accurate measuring instruments, which would ensure that the measured temperature change was the maximum possible for the amounts of reactants used. (2) (2) 12(Total for Question 3 = 13 marks) *P42974RA01116* 11 Turn over<br />

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Acceptable answers Reject Mark No sharp colour change / colour change gradual / difficult to see end- point/ end-point not clear / end-point not obvious / no specific colour change at end-point / colour change hard to distinguish many different colours during the vertical section of a pH curve Colour changes slowly Does not give accurate results No significant colour change end-point not visible Colour does not change in vertical section of pH curve Forms various colours with acid and alkali Comments about suitability for weak/ strong acids Acceptable answers Reject Mark Litmus Orange for red Spelling is not a reasonable match for pronunciation Suitable acid-base indicator (1) correct colours (1) Likely answers: methyl orange (1) red in acid, yellow in alkali (1) Phenolphthalein (1) colourless in acid, pink / purple / red in alkali (1) ALLOW Bromothymol blue (1) Yellow in acid, blue in alkali (1) No TE on colours for litmus Second mark depends on first Question Number 3(a)(i) Question Number 3(a)(ii)<br />
Question Number 3(b)(i) Question Number 3(b)(ii) Question Number 3(b)(iii) Acceptable answers (22.80 x 0.250) = 1000 5.70 x 10-3/ 5.7 x 10-3 / 0.0057 Acceptable answers 5.70 x 10-3 / 5.7 x 10-3 / 0.0057 TE: Same as 3b(i) Reject 6 x 10-3 Reject Mark Mark Acceptable answers Reject Mark (2.00 x 10-25.70 x 10-3 ) =1.43 x 10-2 / 14.30 x 10-3 / 0.0143 TE 2.00 x 10-2answer from 3b(ii) IGNORE SF except 1 Acceptable answers Question Number 3(b)(iv) Mol Mg(OH)2 = answer to 3b(iii) /2 (1) =7.15 x 10-3 /0.00715 mass = 58.3 x number of mol =0.416845 =0.417 (g) (1) TE for second mark based on number of moles calculated for first mark. ALLOW use of 58 instead of 58.3 (giving 0.415(g)) Correct answer with no working scores 2 Numbers obtained without having done a subtraction Reject Mark Answers with more or less than 3 significant figures<br />
Acceptable answers Reject Mark Split sample into two / several portions (so that titration can be repeated) OR make solution to a standard volume (e.g. 100cm3) and take measured aliquots Just repeat the titration Justuse more concentrated acidor Use more acid. Use more accurate burettes / pipettes / balance Acceptable answers Reject Mark 100 x (2 x 0.05) = () 0.250/ 0.25% 40.00 Doubling error in each reading (1) Final answer (1) 0.125%/ 0.13% scores 1 mark 0.3/ 0.30% 0.12, 0.1 Acceptable answers Reject Mark Any two from Use (finely) powdered magnesium hydroxide Use moderately concentrated acid / acid which is not very dilute/ acid of higher concentration / keep excess acid to minimum possible volume. Ensure good mixing / stir mixture ALLOW Measure temperature before adding magnesium hydroxide and for some time after; plot temperatures against time and extrapolate (cooling) line (to where reagents are mixed, to allow for cooling) IGNORE Put a lid on the container/ other comments on insulation Break magnesium hydroxide into smaller pieces Use a more accurate thermometer Just plot temperatures against time and extrapolating Total for Question 3 = 13 marks Question Number 3(c)(i) Question Number 3(c)(ii) Question Number 3(d)<br />

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