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9701/36

Chemistry 9701/36 — October/November 2020

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

3
questions
40
marks
120
minutes

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

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

Many salts occur in a hydrated form such as hydrated potassium carbonate, K2CO3∙xH2O\text{K}_2\text{CO}_3\bullet x\text{H}_2\text{O}, where xx is an integer. You will determine the formula of a sample of hydrated potassium carbonate by adding it to an excess of hydrochloric acid and collecting the gas produced.

K2CO3∙xH2O(s)+2HCl(aq)→2KCl(aq)+(x+1)H2O(l)+CO2(g)\text{K}_2\text{CO}_3\bullet x\text{H}_2\text{O(s)} + 2\text{HCl(aq)} \rightarrow 2\text{KCl(aq)} + (x+1)\text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}

FB 1 is hydrated potassium carbonate, K2CO3∙xH2O\text{K}_2\text{CO}_3\bullet x\text{H}_2\text{O}.
FB 2 is 0.50 mol dm−30.50\text{ mol dm}^{-3} hydrochloric acid, HCl\text{HCl}.

(a)

Method

  • Fill the tub with water to a depth of approximately 5 cm5\text{ cm}.
  • Fill the 250 cm3250\text{ cm}^3 measuring cylinder completely with water. Hold a 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.
  • Use the 50 cm350\text{ cm}^3 measuring cylinder to transfer 50.0 cm350.0\text{ cm}^3 of FB 2 into the flask labelled X.
  • Check that the bung fits tightly in 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 FB 1 and record the mass.
  • Remove the bung from the neck of the flask. Tip all of FB 1 into the acid in the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents. Swirl the flask occasionally until no more gas is produced. Replace the flask in the clamp after each swirl.
  • Measure and record the final volume of gas in the measuring cylinder.
  • Weigh the container and any residual FB 1 and record the mass.
  • Calculate and record the mass of FB 1 added.

Results

3M
(b)

Calculations

(i)

Calculate the number of moles of carbon dioxide collected in the measuring cylinder.
Assume 1 mol1\text{ mol} of gas occupies 24.0 dm324.0\text{ dm}^3.

1M
(ii)

Use your answer to (b)(i) and the information on page 2 to calculate the relative formula mass, MrM_r, of FB 1.

1M
(iii)

Calculate the value of xx in the formula of the hydrated potassium carbonate, K2CO3∙xH2O\text{K}_2\text{CO}_3\bullet x\text{H}_2\text{O}.
Show your working.

2M
(c)

One of the errors associated with this method is caused by the solubility of carbon dioxide in water.

Suggest two modifications which could reduce this error.

modification 1:

modification 2:

2M

The rest of this paper

2 more questions
  • Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation14M
  • Q3Qualitative Analysis · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Presentation of Data and Observations17M
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