A-LevelChemistryAnalytical techniques9701/33 · Feb/Mar 2021 · Q212 Marks

In this experiment you will determine the relative formula mass of the same metal hydrogencarbonate, MHCO3, by thermal decomposition. Then you will compare the result obtained with your answer from 1(c)(iv). FA 4 is another sample of the metal hydrogencarbonate, MHCO3.

(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 one minute. • Heat strongly, with the lid off, for a further four minutes. • Replace the lid and leave the crucible to cool for at least five minutes. During each cooling period, you may wish to 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 two minutes. • Replace the lid and leave the crucible to cool for at least five minutes. • When the crucible has cooled, reweigh the crucible with its lid and contents. Record the mass. • Calculate and record the mass of residue obtained. • This residue is FA 5. Keep FA 5 for use in 2(b)(i). Results I II III IV V [5][5]
(b)(i)Pour a 1 cm depth of dilute hydrochloric acid into a test-tube. Add a spatula measure of residue FA 5 to the acid. Record all your observations and identify any gas formed. [2][2]
(b)(ii)Use your observations in (b)(i) to identify the anion in FA 5. Assume all the MHCO3 has decomposed. Anion in FA 5 is .................................... [1][1]
(b)(iii)Steam is produced when the metal hydrogencarbonate, FA 4, is thermally decomposed. Use your answer in (b)(ii) to complete the equation for the thermal decomposition of MHCO3. Include state symbols. .....MHCO3(s) → .....CO2(g) + .................................... + .................................... [1][1]
(b)(iv)The number of moles of carbon dioxide given off during the thermal decomposition is given by the formula below. moles of CO2 = mass lost during heating (Mr of CO2 + Mr of H2O) Calculate the number of moles of carbon dioxide given off. moles CO2 = .................................... mol [1][1]
(b)(v)Calculate the relative formula mass, Mr, of MHCO3. Show how you obtained your answer using your data from Question 2. Mr of MHCO3 = .................................... [1][1]
(b)(vi)You have obtained two values for the Mr of MHCO3; one in 1(c)(iv) and another in 2(b)(v). State which value is likely to be more accurate. Explain your answer in terms of the practical procedures used. The Mr obtained in Question .................................... is more accurate. reason .......................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................... [1][1]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2021). The mark or mark type is shown in brackets.

[1] Table of data: 6 correct headings shown • Mass of crucible (and lid) • Mass of crucible (lid) and FA 4 (or 'contents before heating') • Mass of crucible (lid) and contents/residue after heating (1) • Mass of crucible (lid) and contents/residue after heating (2) • Mass of FA 4 (used) • Mass of FA 5 / residue obtained (may be two readings) Reject FA 4 in 3rd and 4th readings. Lid can be omitted (provided this is done consistently for all readings).
[1] Readings recorded appropriately • Unit given for all data quoted. All four weighings needed. Units: /g or (g) or in gram(me)s in general mass heading or after every entry/heading All weighings recorded to same number of decimal places (one or more). • Third and fourth weighings are within 0.04 g of each other (or both equal if a one decimal place balance was used) Reject if any readings are impossible (e.g. 4th weighing lower than 1st or 4th greater than 2nd)
[1] Correct subtractions for masses Mass of FA 4 used and residue / FA 5 obtained are correctly shown.
[1] Award IV only if 0.05 < d ≤ 0.10
[1] Award IV and V if d < 0.05
[2] 2 or 3 bullets = 1 mark 4 bullets = 2 marks • fizzing/bubbling / effervescence • gas turns lime water milky / white ppt / cloudy white Reject 'cloudy' alone • gas is identified as carbon dioxide Reject if an incorrect positive gas test is observed • reaction is rapid / vigorous or colourless solution forms / (all) solid dissolves
[1] CO32-/carbonate
[1] 2MHCO3(s) → CO2(g) + H2O(g or l) + M2CO3(s) Allow use of the symbol of Group 1 metal identified in question 1
[1] Correctly calculates no of moles of CO2 n(CO2) = mass loss during heating/62 Answer must be given correct to 2–4 sig figs Allow ecf from (incorrect) subtraction mass shown in list (FA 4 – residue / FA 5)
[1] Correct use of (iv) to calculate Mr Mr = mass of FA 4 used / 2 x answer (iv) Answer to 2 – 4 sf but do not penalise sf more than once in Q2. Ecf on incorrect mole ratio in equation
[1] Any one answer from the following: • 2 is more accurate since fewer readings required. • 2 is more accurate since there is less cumulative error / fewer processes involved Reject less total error • 1 is more accurate because 2 was not heated to constant mass. Candidate must refer to their results Reject if mass readings after heating are within 0.02 of each other • 1 is more accurate since titres were consistent/concordant. Ignore comments about burettes/pipettes being more accurate