Chemistry 9701/33 — May/June 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 · Presentation of Data and Observations · Qualitative Analysis · Analysis, Conclusions and Evaluation

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsFree sample

You will determine the concentration of a solution of hydrochloric acid by diluting it and then titrating the diluted solution against an alkali.

\text{HCl(aq)} + \text{NaOH(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}

FA 1 was made by dissolving $1.06\text{ g}$ of sodium hydroxide, $\text{NaOH}$ , in distilled water to make $250\text{ cm}^3$ of solution.
FA 2 is hydrochloric acid, $\text{HCl}$ .
bromophenol blue indicator

(a)

Method

Pipette $25.0\text{ cm}^3$ of FA 2 into the $250\text{ cm}^3$ volumetric flask. Keep remaining FA 2 for use in Question 2.
Add distilled water to make $250\text{ cm}^3$ of solution and shake the flask thoroughly. Label this solution FA 3.
Fill the burette with FA 3.
Use the second pipette to transfer $25.0\text{ cm}^3$ of FA 1 into a conical flask.
Add about 10 drops of bromophenol blue.
Perform a rough titration and record your burette readings in the space below. The end point is reached when the solution becomes a permanent yellow colour.

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 for the volume of FA 3 to be used in your calculations. Show clearly how you 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, in $\text{mol dm}^{-3}$ , of sodium hydroxide in FA 1.
Use the data in the Periodic Table on page 12.

concentration of $\text{NaOH}$ in FA 1 = ......................... $\text{mol dm}^{-3}$

(ii)

Calculate the number of moles of sodium hydroxide present in $25.0\text{ cm}^3$ of FA 1.

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

(iii)

Deduce the number of moles of hydrochloric acid present in the volume of FA 3 you have calculated in (b).

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

(iv)

Calculate the concentration, in $\text{mol dm}^{-3}$ , of hydrochloric acid in FA 2.

concentration of $\text{HCl}$ in FA 2 = ......................... $\text{mol dm}^{-3}$

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Qualitative Analysis · Analysis, Conclusions and Evaluation11M
Q3Manipulation, Measurement and Observation · Qualitative Analysis · Analysis, Conclusions and Evaluation16M

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