A-LevelChemistryAtoms, molecules and stoichiometry9701/33 · Oct/Nov 2024 · Q110 Marks
When a metal carbonate reacts with a suitable acid, carbon dioxide is produced. You will determine the relative formula mass, M₁, of a metal carbonate by reacting it with excess hydrochloric acid and measuring the mass of gas produced. FA 1 is the metal carbonate. FA 2 is 2.00 moldm⁻³ hydrochloric acid, HCl.
- (a)Method • Use the measuring cylinder to transfer 30.0cm³ of FA 2 into a conical flask. • Weigh the conical flask containing FA 2. Record the mass. • Weigh the container with FA 1. Record the mass. • Tip all the FA 1 slowly into the conical flask. When the reaction slows, swirl the flask gently. • Weigh the container with any residual FA 1. Record the mass. • Calculate the mass of FA 1 added. Record the mass. • Leave the conical flask and its contents for 15 minutes. Swirl the flask occasionally during this time. During this period begin work on Question 2. • After 15 minutes weigh the flask and its contents. Record the mass.[3]
- (b)(i)Use your readings to calculate the mass of carbon dioxide produced. mass of CO2 = .........................g[1]
- (b)(ii)The ionic equation for the reaction of FA 1 with FA 2 is shown. CO3²⁻(aq) + 2H⁺(aq) → CO2(g) + H2O(l) Calculate the relative formula mass, M₁, of the metal carbonate FA 1. Show your working. M₁ of FA 1 =[2]
- (c)(i)A student carries out the experiment as described in (a), except that the acid used is 15°C colder than the acid you used. The student calculates the M₁ correctly from the readings obtained. State whether the value of the M₁ calculated by the student is higher or lower than the value you calculated in (b)(ii). Explain your answer.[2]
- (c)(ii)Another student suggests that the experiment will be more accurate if the conical flask is tilted carefully to an almost horizontal position while the reaction is taking place. Explain why the student is correct.[1]
- (d)State the uncertainty in a single balance reading. uncertainty = ±.........................g Give an expression that would enable you to calculate the percentage error in your weighing of FA 1.[1]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2024). The mark or mark type is shown in brackets.
- [1] 5 headings and units for data
- [1] All four balance readings recording consistently either to 2 or to 3 d.p. and Subtraction for mass of FA 1 correct
- [1] Calculate supervisor to 2 d.p. = mass of FA 1 / mass loss Write this, in a ring, on each candidate's script. Calculate the ratio for each candidate to 2 d.p. Upper and lower limits for the accuracy mark are the supervisor value plus or minus 15% and then rounded to 2 d.p. Award mark III if rounded candidate value is between rounded upper and lower limits (inclusive).
- [1] Correct calculation of mass loss (= mass of CO2) Mass loss = x + y − z (see 1(a) answer I) and answer given to 2–4 sig figs.
- [2] M1 Correct expression to calculate no of moles of CO2 produced n(CO2) = (b)(i) / 44 M2 Correct use of data to calculate Mr M = mass of FA 1 / moles of CO2 = mass of FA 1 × 44 / (b)(i) and answer given to 2–4 sig figs
- [2] M1 (At lower temp) more CO2 dissolves in the solution and less CO2 escapes / mass loss reduced OR The reaction is slower (at a lower temperature) and less CO2 produced / mass loss reduced M2 Lower amount / moles of CO2 (calculated) or lower amount / moles of (metal) carbonate (calculated) and higher Mr
- [1] (Tilting) allows air to replace / displace carbon dioxide in the flask or (Tilting) tips out the (denser) carbon dioxide gas (from the flask) or (Tilting) allows the acid to react with solid stuck to the walls (owtte) of the conical flask.
- [1] Correct expression Uncertainty U (for 2 d.p. balance) = 0.01 or 0.005 and % error in weighing FA 1 = 2 × U/mass of FA 1 (used) × 100
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