NYJC 2020 Prelim 9729 P4 Answers

1 NYJC J2 Prelim Exam 2020 Paper 4 Suggested Mark Scheme 1(c) Results 1 2 initial burette reading / cm3 0.00 10.25 final burette reading / cm3 10.25 20.50 volume of FA 1 added / cm3 10.25 10.25 [2]  correct headers, units and results recorded in a single table  results recorded to 2dp and no 50.00 in the table and results recorded under the correct header  correct evaluation of volume of FA 1 added  3 pts – 2m, 2pts – 1m (d) volume of FA 1 = 10.25 10.25 2  = 10.25 cm3 volume of FA 1 = 10.25 …………………….. cm3 [1] Student obtains appropriate “average”, to 2 d.p., from any experiments with uncorrected end-point titre values within 0.10 cm3. Do not award this mark if the titres used are not identified either in the table (by, for example, a tick) or in a calculation. (e) (i) Volume of FA 1 = 10.25 × 50.0 10.0 (or × 5) = 51.25 cm3 volume of FA 1 = 51.25……………….. cm3 [1] (ii) Equilibrium amount of C2H5CO2H(aq) = amount of NaOH that reacts with the acid remaining in 50 cm3 of the aqueous layer = 51.25 0.0501000  = 0.002562  0.00256 mol [1] C2H5CO2H(aq) Ý C2H5CO2H(org) initial amount / mol 50 0.200 0.010001000 (1) 0 change in amount / mol  0.007437 (3) + 0.007437 (4) equilibrium amount / mol 0.002562 (2) 0.007437  0.00744 [1] Equilibrium amount of C2H5CO2H(aq) = 0.00256……………………. mol [1] Equilibrium amount of C2H5CO2H(org) = 0.00744……………………. mol [1]

2 (iii) 0.007437401000 0.002562501000 cK  = 3.628  3.63 Kc = 3.63……………………. [2] finding conc. – 1m, substituting the values into the Kc expression correctly – 1m (f) Increases [1] Solubility of propanoic acid in 2 -methylpropane decreases [1] [because energy released when propanoic acid forms instantaneous dipole-induced dipole forces of attraction with 2- methylpropane is not really enough to overcome the hydrogen bond between the propanoic acid molecules]. So more FA 1 is required to neutralise the prop anoic acid that remains in the aqueous phase. 2(a) Results [7] 1 mark: headers and units 1 mark: All temperature recorded to 1 decimal place and all vol umes recorded to whole number or 1 decimal place 1 mark: volumes of water added is correct 1 mark: correctly calculate average initial temperature and T for all expts to 1 decimal place. 1 mark: Selected four additional volumes of FA 3 and the selected volumes must be sequential and well-spaced i.e. 5 cm3 or more from any other volume. 1 mark: Student selects one FA 3 volume less than 35 cm3, except 0 cm3 and 30 cm3 1 mark: Student selects three FA 3 volumes more than 35 cm3 and less than or equal 60 cm3 Experiment 1 2 3 4 5 6 Volume of FA 4 (or H2SO4) / cm3 25 25 25 25 25 25 Volume of water (or H2O) / cm3 65 45 55 35 25 15 Volume of FA 3 (or NaOH) / cm3 10 30 20 40 50 60 Initial temperature of FA 4 and water / C 30.0 30.0 30.0 30.0 30.0 30.0 Initial temperature of FA 3 (or NaOH) / C 30.0 30.0 30.0