Chemistry 9701/34 — May/June 2015

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 ObservationAnalysis, Conclusions and EvaluationPresentation of Data and ObservationsFree sample

The concentration of aqueous ammonia used in qualitative analysis is $2\text{ mol dm}^{-3}$ but it is supplied in a much more concentrated form. This is referred to as '.880 ammonia'. You are to determine the concentration of '.880 ammonia' by titration of a solution of ammonia, FB 1, with hydrochloric acid of known concentration. The equation for the reaction is given below.

\text{NH}_3(\text{aq}) + \text{HCl}(\text{aq}) \rightarrow \text{NH}_4\text{Cl}(\text{aq})

FB 1 is a dilute solution of ammonia, $\text{NH}_3(\text{aq})$ . It was prepared by measuring out $5.91\text{ cm}^3$ of the '.880 ammonia' and then adding distilled water until the solution had a volume of $1\text{ dm}^3$ .

FB 2 is $0.100\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}(\text{aq})$ .

methyl orange indicator

(a)

Method

Fill the burette with FB 2.
Use the pipette to transfer $25.0\text{ cm}^3$ of FB 1 into a conical flask.
Add a few drops of methyl orange indicator.
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 achieve 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 FB 2 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 obtained this value.

$25.0\text{ cm}^3$ of FB 1 required ...................... $\text{cm}^3$ of FB 2.

(c)

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 hydrochloric acid present in the volume of FB 2 calculated in (b).

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

(ii)

Use your answer to (i) to determine the number of moles of ammonia present in $25.0\text{ cm}^3$ of FB 1, pipetted into the conical flask.

moles of $\text{NH}_3$ = .......................... mol

(iii)

Use your answer to (ii) to calculate the concentration, in $\text{mol dm}^{-3}$ , of the diluted ammonia, FB 1.

concentration of $\text{NH}_3$ (diluted) in FB 1 = ......................... $\text{mol dm}^{-3}$

(iv)

Use your answer to (iii) and the information on page 2 to calculate the concentration, in $\text{mol dm}^{-3}$ , of '.880 ammonia'.

concentration of '.880 ammonia' = ......................... $\text{mol dm}^{-3}$

(d)

A student analysed a different sample of concentrated ammonia and determined the concentration to be $15.0\text{ mol dm}^{-3}$ . Calculate the percentage difference in concentration of the '.880 ammonia' you have determined compared with that of the student.

(If you have been unable to complete the calculation, assume the concentration of '.880 ammonia' was $9.35\text{ mol dm}^{-3}$ . This is not the correct value.)

percentage difference in concentration = ............................. %

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation14M
Q3Qualitative Analysis · Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation14M

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