Chemistry 9701/34 — October/November 2016

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

questions

marks

120

minutes

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

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

You will find the relative atomic mass, $A_r$ , of magnesium by measuring the volume of hydrogen produced when a known mass of metal reacts with an excess of acid.

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

FB 1 is $1.00\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .
FB 2 is magnesium, $\text{Mg}$ .

(a)

Method

Read through the whole method before starting any practical work.

Fill the tub with water to a depth of about $5\text{ cm}$ .
Weigh the magnesium, FB 2, and note its mass below. If you are using a balance reading to 1 decimal place and the reading with the magnesium is zero, you should record this value.
Fill the burette to about the $20\text{ cm}^3$ mark with hydrochloric acid, FB 1.
Add distilled water to reach the $0\text{ cm}^3$ mark on the burette.
Bend the magnesium strip into a U-shape.
Place the magnesium in the burette so that it is above the liquid and friction holds it in position. Use a glass rod to push the magnesium about $2\text{ cm}$ into the burette.
Hold a piece of paper towel over the open end of the burette, invert the burette and immediately place it in the tub of water. Remove the paper towel and clamp the burette as shown in the diagram.
The liquid level should now be on the scale of the burette. If it is not, open the tap for a moment to allow the level to drop.

Fig. 1

Record the initial reading on the burette. Remember that the scale is now upside down.
Leave the apparatus so that the acid from the burette diffuses around the magnesium and reacts.
You should start Question 2 or Question 3 while waiting for the reaction to complete.
When all the magnesium has reacted, note and record the final reading on the burette.
Calculate the volume of hydrogen produced.

Results

(b)

Calculations

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

(i)

Calculate the number of moles of hydrogen produced.
(Assume that 1 mole of gas occupies $24.0\text{ dm}^3$ under these conditions.)

\text{moles of H}_2 = \text{............................ mol}

(ii)

Use your answer to (i) and the mass of magnesium used to calculate the $A_r$ of magnesium.
(If you used a balance reading to 1 decimal place, you should assume that the mass of magnesium was $0.04\text{ g}$ correct to 2 decimal places.)

\text{A}_r\text{ of Mg} = \text{............................}

(c)

(i)

Calculate the percentage error in the mass and volume readings in this experiment.

(ii)

Suggest a change that could be made to reduce the greater error calculated in (i).

(d)

What would be the effect on the value of the $A_r$ of magnesium calculated if the temperature of the room was much lower than that for your experiment? Explain your answer.

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Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation15M
Q3Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Qualitative Analysis15M

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