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9701/31

Chemistry 9701/31 — May/June 2019

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 · Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation · Qualitative Analysis

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

Metal carbonates react with acid to produce carbon dioxide. You will determine the identity of a Group 2 metal M in a carbonate of formula MCO3\text{MCO}_3 by reacting the carbonate with excess dilute hydrochloric acid and measuring the volume of carbon dioxide produced.

MCO3(s)+2HCl(aq)→MCl2(aq)+CO2(g)+H2O(l)\text{MCO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{MCl}_2(\text{aq}) + \text{CO}_2(\text{g}) + \text{H}_2\text{O}(\text{l})

FA 1 is 50 cm350\text{ cm}^3 of 4.00 mol dm−34.00\text{ mol dm}^{-3} hydrochloric acid, HCl\text{HCl}.
FA 2 is the metal carbonate, MCO3\text{MCO}_3.

(a)

Method

  • Fill the tub with water to a depth of approximately 5cm.
  • Fill the 250 cm3250\text{ cm}^3 measuring cylinder completely with water. Hold a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub.
  • Remove the paper towel and clamp the inverted measuring cylinder so the open end is in the water just above the base of the tub.
  • Add all the FA 1 into the flask labelled X.
  • Check that the bung fits tightly into the neck of flask X, clamp flask X and place the end of the delivery tube into the inverted 250 cm3250\text{ cm}^3 measuring cylinder.
  • Weigh the container with FA 2 and record the mass.
  • Remove the bung from the neck of the flask. Tip the FA 2, from the container, into the acid in the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents.
  • Replace the flask in the clamp. Leave for several minutes, swirling the flask occasionally.

You may wish to start Question 2 while the gas is being evolved.

  • When no more gas is collected, measure and record the final volume of gas in the measuring cylinder.
  • Weigh the container, with any residual FA 2, and record the mass.
  • Calculate and record the mass of FA 2 added to flask X.

Results

3M
(b)

Calculations

(i)

Calculate the number of moles of carbon dioxide collected in the measuring cylinder.

[Assume that 1 mol of gas occupies 24.0 dm324.0\text{ dm}^3 under these conditions.]

1M
(ii)

Use your answer to (b)(i) and the information on page 2 to calculate the relative atomic mass, ArA_r, of M.

3M
(iii)

Use your answer to (b)(ii) to identify M.

1M
(c)
(i)

A student suggested that, using the same apparatus, the accuracy of the experiment would be increased if approximately 2g of MCO3\text{MCO}_3 were used to react with the excess hydrochloric acid.

State and explain whether the student was correct.

1M
(ii)

Another student suggested that the experiment would be more accurate if the carbon dioxide was collected in a gas syringe rather than over water.

State and explain whether the student was correct.

1M

The rest of this paper

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