2022 MI H2 Chem Prelim Paper 4 - Mark Scheme

2022 PU3 H2 Chemistry EOY Paper 4 Mark Scheme Question Answer Marks 1 (a) All masses consistently recorded to 2 (or 3) decimal places. and correct mass headers Mass of bottle + FA 1 Mass of bottle + residual of FA 1 Mass of FA 1 Mass of empty bottle Mass of bottle + FA 1 Mass of FA 1 *Inform students to be always reminded to weigh residual mass M1 Mass of FA 1 used correctly calculated Mass of FA 1 used 5.00 ±0.05 M2 (b)(i) A table including the appropriate header and units for: • Initial burette readings • Final burette readings • volume added M3 All accurate burette readings are recorded to the nearest 0.05 cm3. Do not award this mark if: 50.00cm3 is used as an initial burette reading M4 Accuracy Titre/mass ratio: 4.05 ± 0.50 M5 (b)(ii) Correct calculation of the average of two consistent readings within 0.10 cm3 difference M6 (c)(i) amount of I2 = 0.5 (ans from b(ii)/1000) x 0.10 = A mol M7 (ii) amount of Cu2+ in 25.0 cm3 = A × 2 = B mol M8 (iii) amount of CuSO4.nH2O in 250 cm3 = 10 × B = C mol M9 (iv) Mr = mass of FA 1 / C = D Must be 1dp, will affect M27 M10 n = (D -159.6) / 18.0 n must be an integer M11 (d) Due to the slow release of I3- from the starch-I3- complex, the end point would have exceeded the equivalence point, resulting in a larger titre value. This leads to larger amount of CuSO4.nH2O hence the Mr would have been smaller than the actual value. OR The I2 is trapped by the starch complex and hence resulting in less I3- a smaller titre value This leads to a smaller amount of CuSO4 and hence Mr will be larger M12 M13

2 Question Answer Marks 2(a) Table with correct headers and units • volumes of FA 5, FA 6 and deionised water • time • 1/t • lg (1/t) • lg (VFA 5) M14 Record all data with correct precision ● volumes to 0.5 cm3 ● t to nearest second ● calculated values to 3 sf M15 Complete set of volume of FA 5 and time readings for 5 experiments and all values of t increase as volume of FA 5 decreases. Data for experiments 1 and 2 must be included M16 Choose 3 other well-spaced values for the volume of FA 5. All volumes must differ by at least 5 cm3 M17 Correctly calculate 1/t, lg (1/t) and lg (VFA 5) values for all stated experiments. M18 (b)(i) Axes correct way round, with correct labels and appropriate scale. Scale must be chosen so that plotted points occupy at least half the graph grid in both x and y directions Both accepted, must be in logical numerical sense M19 All plotted points correct to within ± ½ small square M20 A best fit line is drawn with anomalous points excluded M21 (b)(ii) Correct calculation of gradient of the graph with working shown. The m value is equal to the gradient of the line Must show accurate coordinates falling on the line or triangle or best both M22 (c) ● Correct calculation of lg(1/15) = –1.18 ● Correct reading of lg( VFA 5) val