Chemistry 9701/34 — May/June 2020

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

FB 1 is a solution of hydrated sodium carbonate, $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ , where $x$ is an integer. You will determine the value of $x$ in this compound by a titration method.

You will add FB 1 to a known volume and concentration of hydrochloric acid, FB 2. The hydrochloric acid is in excess. You will then titrate the remaining acid with aqueous sodium hydroxide, FB 3.

FB 1 is a solution containing $37.5\text{ g dm}^{-3}$ hydrated sodium carbonate, $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .
FB 2 is $0.200\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .
FB 3 is $0.100\text{ mol dm}^{-3}$ sodium hydroxide, $\text{NaOH}$ .
bromophenol blue indicator

(a)

Method

Fill the burette with FB 3.
For each titration:
- Use the $25\text{ cm}^3$ pipette to place $25.0\text{ cm}^3$ of FB 2 into a conical flask.
- Use the $10\text{ cm}^3$ pipette to place $10.0\text{ cm}^3$ of FB 1 into the same conical flask.
Add a few drops of bromophenol blue indicator.
Titrate the contents of the conical flask with FB 3 from the burette.
Perform a rough titration and record your burette readings in the space below.

The rough titre is .............................. $\text{cm}^3$ .

Carry out as many accurate titrations as you think necessary to obtain consistent results.
Make sure any recorded results show the precision of your practical work.
Record, in a suitable form below, all of your burette readings and the volume of FB 3 added in each accurate titration.

(b)

From your titration results, obtain a suitable value to be used in your calculations. Show clearly how you obtained this value.

volume of FB 3 used in titration = .............................. $\text{cm}^3$

(c)

Calculations

(i)

Give your answers to (c)(ii), (c)(iii) and (c)(iv) to an appropriate number of significant figures.

(ii)

Calculate the number of moles of sodium hydroxide present in the volume of FB 3 calculated in (b).

moles of $\text{NaOH}$ = .............................. mol

This number of moles of sodium hydroxide neutralises the remaining hydrochloric acid. Deduce the number of moles of remaining hydrochloric acid.

moles of remaining $\text{HCl}$ = .............................. mol

(iii)

Calculate the initial number of moles of hydrochloric acid in each $25.0\text{ cm}^3$ sample of FB 2 pipetted into the conical flask.

initial moles of $\text{HCl}$ in each sample of FB 2 = .............................. mol

You have calculated:

the number of moles of remaining $\text{HCl}$
the initial number of moles of $\text{HCl}$ in each sample of FB 2.

Calculate the number of moles of hydrochloric acid neutralised by the $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ in FB 1 in each titration.

number of moles of $\text{HCl}$ neutralised by the $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ = .............................. mol

(iv)

Complete the equation. Include state symbols.

\text{...... } \text{Na}_2\text{CO}_3(\text{aq}) + \text{...... } \text{HCl}(\text{......}) \rightarrow \text{.......................} + \text{.......................} + \text{.......................}

Deduce the number of moles of $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ present in each $10.0\text{ cm}^3$ sample of FB 1.

number of moles of $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ present = .............................. mol

(v)

FB 1 contains $37.5\text{ g dm}^{-3}$ $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .

Use your answer to (c)(iv) to calculate the relative formula mass, $M_r$ , of $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ . Show your working.

relative formula mass of $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ = ..............................

(vi)

Determine the value of $x$ in the formula $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .

$x$ = ..............................

(d)

A student suggested a different method.

To $250\text{ cm}^3$ of FB 2, add $100\text{ cm}^3$ of FB 1.
Pipette $25.0\text{ cm}^3$ of this mixture of solutions into a conical flask.
Titrate this mixture of solutions with FB 3.
Repeat titrations until concordant results are achieved.

Comment on one disadvantage or one advantage of using this method rather than the method you used in (a).

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation11M
Q3Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Qualitative Analysis13M

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