Chemistry 9701/35 — May/June 2011

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

questions

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

You are to determine the percentage by mass of water in the borax crystals.

Borax reacts with hydrochloric acid according to the equation.

\text{Na}_2\text{B}_4\text{O}_7(\text{aq}) + 2\text{HCl}(\text{aq}) + 5\text{H}_2\text{O}(\text{l}) \rightarrow 2\text{NaCl}(\text{aq}) + 4\text{H}_3\text{BO}_3(\text{aq})

FA 1 is an aqueous solution containing $38.10\text{ g dm}^{-3}$ of borax crystals. Borax has the formula, $\text{Na}_2\text{B}_4\text{O}_7\cdot x\text{H}_2\text{O}$ .
FA 2 is $1.00\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .

You are also provided with an indicator suitable for the titration of a strong acid and a weak base.

The indicator provided is ..............................................................

(a)

Method

Dilution

Fill the burette with FA 2.
Run between $44.50\text{ cm}^3$ and $45.50\text{ cm}^3$ of FA 2 from the burette into the $250\text{ cm}^3$ graduated (volumetric) flask, labelled FA 3.
Make the solution up to the mark with distilled water.
Shake the flask to mix the solution of FA 3.

In the space below record your burette readings and the volume of FA 2 added to the graduated flask.

Titration

Fill a second burette with FA 3, the diluted hydrochloric acid.
Pipette $25.0\text{ cm}^3$ of FA 1 into a conical flask.
Add to the flask a few drops of the indicator provided.
Titrate the borax in the flask with FA 3 until the appropriate colour change is observed for the end-point.

You should perform a rough titration.
In the space below record your burette readings for this rough titration.

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

Carry out as many accurate titrations as you think necessary to obtain consistent results.
Make certain 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 FA 3 added in each accurate titration.

(b)

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

$25.0\text{ cm}^3$ of FA 1 required .................... $\text{cm}^3$ of FA 3.

(c)

Calculations

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

(i)

Calculate the concentration of hydrochloric acid, in $\text{mol dm}^{-3}$ , in the diluted solution, FA 3.

The concentration of $\text{HCl}$ in FA 3 was .................... $\text{mol dm}^{-3}$ .

(ii)

Calculate how many moles of $\text{HCl}$ were present in the volume of FA 3 calculated in (b).

.................... $\text{mol of HCl}$

(iii)

Calculate how many moles of $\text{Na}_2\text{B}_4\text{O}_7$ reacted with the $\text{HCl}$ in (ii).

\text{Na}_2\text{B}_4\text{O}_7(\text{aq}) + 2\text{HCl}(\text{aq}) + 5\text{H}_2\text{O}(\text{l}) \rightarrow 2\text{NaCl}(\text{aq}) + 4\text{H}_3\text{BO}_3(\text{aq})

The $\text{HCl}$ run from the burette reacted with .................... $\text{mol of } \text{Na}_2\text{B}_4\text{O}_7$ .

(iv)

Calculate the concentration, in $\text{g dm}^{-3}$ , of $\text{Na}_2\text{B}_4\text{O}_7$ in FA 1.
[ $A_r$ : B, 10.8; O, 16.0; Na, 23.0]

The concentration of $\text{Na}_2\text{B}_4\text{O}_7$ in FA 1 is .................... $\text{g dm}^{-3}$ .

(v)

Use your answer to (iv) and the information at the start of question 1 to calculate the percentage by mass of water in the borax crystals, $\text{Na}_2\text{B}_4\text{O}_7\cdot x\text{H}_2\text{O}$ .
[ $A_r$ : H, 1.0; B, 10.8; O, 16.0; Na, 23.0]

Borax crystals contain .................... % water.

(d)

The maximum error for a $25\text{ cm}^3$ pipette commonly used in schools is $\pm0.06\text{ cm}^3$ .
The maximum error in any single burette reading is $\pm0.05\text{ cm}^3$ .

Calculate the maximum percentage error in each of the following.

(i)

The volume of FA 1 pipetted into the conical flask.

maximum percentage error in the pipetted volume = .................... %.

(ii)

The titre volume calculated in (b).

maximum percentage error in titre volume = .................... %.

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation10M
Q3Qualitative Analysis · Presentation of Data and Observations · Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation15M

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