MARK SCHEME – AS CHEMISTRY – 7404-1 – JUNE 2022.

AS CHEMISTRY JUNE 2021 Question Marking guidance Additional Comments/Guidelines Mark 01.1 the outer electron is in a higher (energy) level / there is an increase in shielding / the atoms get larger / more shells There is a weaker attraction between the nucleus and the outer electron. Mark independently 1 1 (2 x AO1) Question Marking guidance Additional Comments/Guidelines Mark 01.2 Mg2+(g) → Mg3+(g) + e– Mg2+(g) + e– → Mg3+(g) + 2 e– 1 (AO2) Question Marking guidance Additional Comments/Guidelines Mark 01.3 The electron is removed from 2p sub-shell / 2nd energy level / lower energy level / sub-shell that is closer to the nucleus (Electron being removed is) less shielded (than 3s) 1 1 (2 x AO2) Question Marking guidance Additional Comments/Guidelines Mark 02.1 M1 measure the mass of the weighing boat (or similar) and solid M2 Add the solid to a beaker (or other suitable container) and then reweigh the weighing boat (and subtract to find the mass of solid added.) OR M1 Place weighing boat on a balance and zero the balance M2 Add the solid to a beaker (or other suitable container), wash out weighing boat and transfer washing to the beaker. M1 place (an empty) beaker on balance and zero M2 add the solid to the beaker and record the mass OR M1 place (an empty) beaker on balance and measure its mass M2 add the solid to the beaker and subtract mass of empty beaker from the total mass 1 1 (2 x AO1) Question Marking guidance Additional Comments/Guidelines Mark 02.2 M1 M2 M3 Mr citric acid = 192.0 Amount of citric acid = Mass / Mr = 0.834 / 192 = 0.0043438 (mol) Concentration = moles / volume = 0.0043438 / 0.5 = 0.00869 (mol dm–3) M2 conseq on M1 M3 conseq on M2 Alternative Method M1 Concentration (g/dm3) = 0.834 / 0.50 = 1.668 M2 Mr citric acid = 192.0 M3 Concentration (mol/dm3) = M1/M2 = 0.00869 1 1 1 (3 x AO2) Question Marking guidance Additional Comments/Guidelines Mark 02.3 This question is marked using levels of response. Refer to the Mark Scheme Instructions for Examiners for guidance on how to mark this question. Level 3: Three stages are covered and the explanation of each stage is generally correct and virtually complete. Answer is well structured with no repetition or irrelevant points. Accurate and clear expression of ideas with no errors in use of technical terms. 5-6 Level 2: Three stages are covered but the explanation of each stage may be incomplete or may contain inaccuracies OR two stages are covered and the explanations are generally correct and virtually complete. Answer shows some attempt at structure. Ideas are expressed with reasonable clarity with, perhaps, some repetition or some irrelevant points. Some minor errors in use of technical terms. 3-4 Level 1: Two stages are covered but the explanation of each stage may be incomplete or may contain inaccuracies, OR only one stage is covered but the explanation is generally correct and virtually complete. Answer includes isolated statements but these are not presented in a logical order or show some confusion. Answer may contain valid points which are not clearly linked to an argument structure. Errors in the use of technical terms. 1-2 Level 0 Insufficient correct chemistry to gain a mark. 0 Use best t Stage 1 a. b. c. Stage 2 a. b. c. Stage 3 a. b. c. Stage 4 a. b. c. hree of these four stages Problem – using a measuring cylinder Explanation – large uncertainty / not accurate enough Improvement – use a (volumetric) pipette (Not dropping pipette) Problem – too much indicator Explanation – may react and affect the endpoint reading Improvement – use a smaller volume (2-6 drops) Problem – rinsing the burette with distilled or deionised water Explanation – will slightly dilute the alkali solution Improvement – rinse the burette with alkali solution Problem – adding alkali solution until the indicator “just” changes colour Explanation – acid may not have fully reacted (as mixture not swirled) Improvement – add alkali solution until a permanent colour change is seen. 6 (3 x AO1, 3 x AO3) Question Marking guidance Additional Comments/Guidelines Mark 02.4 Calculates the titres for each of 1,2,3 as 1 2 3 22.95 23.10 22.90 Averages concordant titres: (22.95 + 22.90) ÷ 2 = 22.93 cm3 Allow 22.9(25) cm3 1 1 (2 x AO1) Question Marking guidance Additional Comments/Guidelines Mark 02.5 (0.15 / 22.95) × 100 = 0.65% 0.15 / (Their Run 1) × 100 1 (AO1) Question Marking guidance Additional Comments/Guidelines Mark 03.1 AsF5 KrF2 Diagram of shape M1 M2 Bond angle(s) M3: 90 and 120 KrF2 must show lone pairs (either as lobes or crosses/dots) and must be linear. Ignore any lone pairs on fluorine. 3 (3 x AO1)