Chemistry 9701/35 — May/June 2023

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

questions

marks

120

minutes

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

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationQualitative AnalysisFree sample

Magnesium is a reactive metal which corrodes when left in air. Magnesium reacts with acid to release hydrogen.

You will determine the percentage purity of a sample of magnesium by reacting it with excess hydrochloric acid and measuring the volume of hydrogen formed.

\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}

FA 1 is hydrochloric acid, $\text{HCl}$ .
FA 2 is magnesium, $\text{Mg}$ .

(a)

Method

Weigh the container with FA 2. Record the mass.
Fill the tub with water to a depth of approximately $5\text{ cm}$ .
Fill the $250\text{ cm}^3$ measuring cylinder completely with water. Holding 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.
Use the $50\text{ cm}^3$ measuring cylinder to transfer $50.0\text{ cm}^3$ of 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 $250\text{ cm}^3$ measuring cylinder.
Remove the bung from the neck of the flask. Add all the FA 2 to the acid 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.
Weigh the empty container. Record the mass.
Calculate and record the mass of FA 2 that is added to the acid.

Start Question 2 or Question 3 while the gas is being collected.

When the reaction stops producing gas, record the final volume of gas collected.

volume of gas = ..................... $\text{cm}^3$

(b)

(i)

Calculate the amount, in mol, of hydrogen collected in the measuring cylinder at room conditions.

amount of $\text{H}_2$ = ................................. mol

(ii)

Use your answer to (b)(i) to deduce the amount, in mol, of magnesium that reacted in your experiment.

amount of $\text{Mg}$ = ...................................... mol

Hence calculate the percentage purity of the magnesium.

purity of $\text{Mg}$ = ......................................... %

(c)

A student carries out this practical procedure but uses magnesium powder rather than magnesium ribbon. State the effect this would have on the percentage purity the student calculates. Explain your answer.

(d)

Another student investigates the reaction of a metal carbonate with hydrochloric acid by measuring the change in mass during the reaction. The reaction is carried out in a beaker on the pan of a balance.

(i)

Explain why the mass displayed on the balance decreases during the reaction.

(ii)

Explain why using a balance to monitor the reaction between magnesium and hydrochloric acid is not accurate.

(iii)

Give the ionic equation for a solid carbonate, $\text{CO}_3^{2-}\text{(s)}$ , reacting with hydrochloric acid. Include state symbols.

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation12M
Q3Qualitative Analysis · Analysis, Conclusions and Evaluation16M

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