A-LevelChemistryAtoms, molecules and stoichiometry9701/33 · Oct/Nov 2021 · Q110 Marks
Group 1 metal carbonates have the formula M₂CO₃. The identity of the metal ion, M⁺, may be determined by a gravimetric method. The metal carbonate is reacted with excess acid and the mass of carbon dioxide given off is measured. M₂CO₃(s) + 2HCl(aq) → 2MCl(aq) + H₂O(I) + CO₂(g) FA 1 is a Group 1 metal carbonate, M₂CO₃. FA 2 is 2.0 mol dm⁻³ hydrochloric acid, HCl.
- (a)Method • Use the 25 cm³ measuring cylinder to transfer 25.0 cm³ of FA 2 into a conical flask. Weigh the flask with the acid and record the mass. • Weigh the container with FA 1 and record the mass. • Carefully tip all of FA 1 into the acid in the conical flask. Swirl the contents of the flask and leave the flask to stand. • Weigh the container with any residual FA 1. Record the mass. • Calculate and record the mass of FA 1 added to the conical flask. • Calculate and record the theoretical initial mass of flask + acid + FA 1. • Swirl the flask occasionally while leaving it to stand for approximately 5 minutes. During this step you may wish to start Question 2 or Question 3. • Weigh the flask and contents and record this mass. • Calculate and record the mass of carbon dioxide given off during the experiment. Results I II III IV [4][4]
- (b)(i)Calculate the number of moles of carbon dioxide given off in the experiment. moles of CO₂ = .............................................. mol [1][1]
- (b)(ii)Calculate the relative formula mass, Mr, of M₂CO₃. Mr of M₂CO₃ = .............................................. [1][1]
- (b)(iii)Identify the Group 1 cation, M⁺, in FA 1. Show your working. M⁺ is .............................................. [1][1]
- (c)(i)Suggest one modification, to the method in (a), to reduce the solubility of carbon dioxide in the solution in the flask. [1][1]
- (c)(ii)An assumption made in the method in (a) is that the acid is in excess. Show by calculation that this assumption is true. [2][2]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.
- [1] I Unambiguous headings AND correct units in the space provided • (mass) flask + acid / FA 2 • (mass) container + M2CO3/FA 1 • (mass) container (+ residue) • (mass) flask + contents (provided this label not used for theoretical mass) • (mass) FA 1 (added) • theoretical / total mass or flask + FA 2 / acid + FA 1/M2CO3 • (mass) CO2
- [1] II Four balance readings recorded to the same number of decimal places AND answer to at least 1 decimal place
- [1] III Three masses correctly calculated.
- [1] IV Examiner calculates ratio mass FA 1 / mass carbon dioxide to 2 dp for candidate and difference from supervisor ratio. The mark is awarded for a difference < 0.25.
- [1] Correctly calculates amount of CO2 = mass CO2 lost / 44 mol AND answer given to 2 – 4 significant figures
- [1] Correctly uses Mr = mass FA 1 / (b)(i) AND answer given to 2 – 4 sf (sf error is not penalised more than once in 1(b))
- [1] Correct expression Ar M+ = ((b)(ii) – 60) / 2 AND M+ is the closest group 1 cation 0.1 < Li+ < 14.9 15.0 < Na+ < 31.1 31.2 < K+ < 62.3 62.3 < Rb+ ≤ 109.2 109.2 < Cs+ < 250
- [1] Any one of: • saturate the acid with CO2 initially • pre-heat the acid • use more concentrated acid • shake / swirl the flask vigorously / more • add named inert solute to solution
- [2] M1: amount of acid added to flask, n(H+) = 0.05 mol correct working or answer OR 2 x (b)(i) M2: n(H+) > 2 × n(M2CO3) where n(M2CO3) = (b)(i) or mass FA 1 / (b)(ii) Other valid methods may be awarded.
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