A-LevelChemistryAnalytical techniques9701/33 · May/June 2022 · Q212 Marks

2 In this experiment you will identify the metal, M, in a metal carbonate, MCO3, by thermal decomposition. MCO3(s) → MO(s) + CO2(g) FA 4 is the metal carbonate, MCO3.

(a)Method • Weigh the empty crucible with its lid. Record the mass. • Transfer all the FA 4 from the container into the crucible. • Weigh the crucible, lid and FA 4. Record the mass. • Calculate and record the mass of FA 4 used. • Place the crucible and contents on a pipe-clay triangle. • Heat the crucible gently, with the lid on, for approximately 1 minute. • Heat strongly, with the lid off, for a further 4 minutes. • Replace the lid and leave the crucible to cool for at least 5 minutes. During the cooling period, you may wish to begin work on Question 3. • When the crucible has cooled, weigh the crucible with its lid and contents. • Record the mass. • Heat strongly, with the lid off, for a further 2 minutes. • Replace the lid and leave the crucible to cool for at least 5 minutes. • When the crucible has cooled, reweigh the crucible with its lid and contents. • Record the mass. • Calculate and record the total loss of mass and the mass of residue obtained. • This residue is FA 5. Keep FA 5 for use in 2(d). Results I II III IV V[5]
(b)(i)Calculations (i) Calculate the amount, in mol, of carbon dioxide given off in your experiment. amount of CO2 = ................................... mol[1]
(b)(ii)(ii) Calculate the relative formula mass, Mr, of MCO3. Mr of MCO3 = ...................................[1]
(b)(iii)(iii) From your results, deduce the identity of M. Show your reasoning. M is = ...................................[1]
(c)A student carries out the same procedure, using the same mass of solid. However, the student uses the basic carbonate, MCO3•M(OH)2, instead of the pure carbonate, MCO3. When the metal hydroxide part of the basic carbonate decomposes, metal oxide and steam are produced. The metal carbonate part decomposes in the usual way. State how the loss of mass from the student's solid compares with the loss of mass you obtained when you carried out your experiment. Explain your reasoning.[2]
(d)Use a spatula to transfer a small quantity of your cold residue, FA 5, into a test-tube. Add about a 1 cm depth of dilute hydrochloric acid to the FA 5 in the test-tube. Record what you observe. State whether or not the thermal decomposition of MCO3 is complete. Justify your answer based on your observations.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

[5] I Unambiguous headings and units for four weighings and entered in the space provided: • (mass of) crucible, lid (empty) • (mass of) crucible, lid and FA 4 (or 'contents before heating') • (mass of) crucible, lid and FA 5 / residue / contents after first heating • (mass of) crucible, lid and FA 5 / residue / contents after second heating Units: / g or (g) or gram(me)s or in grams II Readings are appropriately recorded: • all weighings recorded to same number of decimal places (two or more) • mass of FA 4 is within the range 0.80–1.80 g (from weighings) • fourth weighing is within +0.02 and –0.05 g of third weighing III Correct subtractions to give: • mass of FA 4 • mass of FA 5 • mass loss For assessment of accuracy marks: calculate supervisor's mass ratio (to 2 d.p.) = mass FA 4 / mass FA 5 (residue) Write this value in a ring on each script. Calculate the 25% and 10% ranges correct to 2 dp. Calculate the candidate's mass ratio (to 2 dp) = mass FA 4 / mass FA 5 (residue) Award accuracy marks as shown below. Award IV if δ is within 25% of supervisor AND ratio > 1.00 Award V if δ is within 10% of supervisor AND ratio > 1.00
[1] Correctly calculated amount of CO2 = mass loss / 44 mol AND answer to 2–4 significant figures
[1] Correctly uses Mr of FA 4 = mass of FA 4 used / (b)(i) AND answer to 2–4 sf
[1] Ar of metal = (b)(ii) – 60 AND Group 2 metal correctly deduced from Ar Be if Ar is in range 0 – 12.1: Mg for Ar 12.2 – 32.2: Ca for Ar 32.2 – 63.8: Sr for Ar 63.9 – 112.4: Ba for Ar 112.5 – 250 (Other metals forming white carbonates with formula MCO3 are acceptable provided the Ar is close to the value given in the periodic table.)
[2] M1: student's mass loss is lower AND Mr H2O < Mr CO2 (or values given) M2: using Mg as example % loss from Mg(OH)2 = 18/58.3 × 100 % loss from MgCO3 = 44/84.3 × 100 (which is greater than % for Mg(OH)2)
[2] EITHER M1: solid dissolves OR colourless solution formed M2: decomposition is complete AND reason: no gas / no CO2/ no fizzing produced (when acid added) OR M1: effervescence (owtte) OR gas / CO2 gives white ppt with limewater M2: decomposition is not complete AND reason: carbonate reacts / fizzes / gives out CO2 with acid / oxide does not fizz with acid