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9701/33

Chemistry 9701/33 — October/November 2021

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

3
questions
40
marks
120
minutes

Topics Presentation of Data and Observations · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Qualitative Analysis

Q1Presentation of Data and ObservationsManipulation, Measurement and ObservationAnalysis, Conclusions and EvaluationFree sample

Quantitative analysis

Read through the whole method before starting any practical work. Where appropriate, prepare a table for your results in the space provided.

Show your working and appropriate significant figures in the final answer to each step of your calculations.

Group 1 metal carbonates have the formula M2CO3\text{M}_2\text{CO}_3. The identity of the metal ion, M+\text{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(l)+CO2(g)\text{M}_2\text{CO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow 2\text{MCl}(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})

FA 1 is a Group 1 metal carbonate, M2CO3\text{M}_2\text{CO}_3.
FA 2 is 2.0 mol dm−32.0\text{ mol dm}^{-3} hydrochloric acid, HCl\text{HCl}.

(a)

Method

  • Use the 25 cm325\text{ cm}^3 measuring cylinder to transfer 25.0 cm325.0\text{ cm}^3 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

4M
(b)

Calculations

(i)

Calculate the number of moles of carbon dioxide given off in the experiment.

1M
(ii)

Calculate the relative formula mass, MrM_r, of M2CO3\text{M}_2\text{CO}_3.

1M
(iii)

Identify the Group 1 cation, M+\text{M}^+, in FA 1.

Show your working.

1M
(c)

One source of error in this experiment is the solubility of carbon dioxide in water.

(i)

Suggest one modification, to the method in (a), to reduce the solubility of carbon dioxide in the solution in the flask.

1M
(ii)

An assumption made in the method in (a) is that the acid is in excess.

Show by calculation that this assumption is true.

2M

The rest of this paper

2 more questions
  • Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation14M
  • Q3Qualitative Analysis16M
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