Chemistry 9701/33 — May/June 2012

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 purity of a sample of calcium carbonate.

This experiment involves three steps.
In step one, you will react the impure calcium carbonate with an excess of acid.
In step two, you will carry out a titration to find the amount of acid you used in step one.
In step three, you will carry out a second titration to find how much (excess) acid remained following the reaction in step one.
Finally, you will use the values found in the three steps to calculate the percentage purity of the calcium carbonate.
Assume the impurity in the calcium carbonate will not react with acid or alkali.

FA 1 is $0.100\text{ mol dm}^{-3}$ sodium hydroxide, $\text{NaOH}$ .
FA 2 is approximately $1\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .
FA 3 is FA 2 diluted by a factor of 10, approximately $0.1\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .
FA 4 is a sample of impure calcium carbonate, $\text{CaCO}_3$ .
methyl orange indicator

Read through the whole method before starting any practical work.

(a)

Method

Step 1

Fill the burette labelled FA 2 with FA 2.
Run $50.00\text{ cm}^3$ of FA 2 into a $250\text{ cm}^3$ beaker.
Weigh the tube containing the impure calcium carbonate, FA 4.
Tip the contents of the tube, in small portions, into the acid to avoid acid spray. Stir the mixture and leave the stirring rod in the beaker.
Reweigh the tube containing any residue.
Record the weighings and the mass of FA 4 added in a suitable form below.

Step 2

Fill the burette labelled FA 1 with FA 1.
Pipette $25.0\text{ cm}^3$ of FA 3 into a conical flask.
Add 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 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 1 added in each accurate titration.

Step 3

Stir the mixture from Step 1 again to ensure that all the solid has dissolved.
Transfer the solution to the $250\text{ cm}^3$ graduated (volumetric) flask labelled FA 5.
Rinse the beaker twice with a little distilled water and add the washings to the graduated flask.
Make the solution up to $250\text{ cm}^3$ with distilled water. Ensure that the contents of the flask are thoroughly mixed.
Transfer $25.0\text{ cm}^3$ of this solution, FA 5, into a second conical flask using a second pipette.
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 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 1 added in each accurate titration.

12M

(b)

Calculations

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

The equation for the reaction between sodium hydroxide and hydrochloric acid is shown below.

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

(i)

From your accurate titration results in Step 2, 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 3 required ...................... $\text{cm}^3$ of FA 1.

(ii)

Use your answer from (i) to calculate the number of moles of sodium hydroxide, FA 1, required to react with $25.0\text{ cm}^3$ of FA 3 in Step 2.

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

(iii)

Use your answer from (ii) to determine the number of moles of hydrochloric acid in $25.0\text{ cm}^3$ of FA 3.

moles of $\text{HCl}$ in $25.0\text{ cm}^3$ of FA 3 = ......................... mol

(iv)

FA 3 was produced by diluting FA 2. Calculate the number of moles of hydrochloric acid in $50.00\text{ cm}^3$ of FA 2.

moles of $\text{HCl}$ in $50.00\text{ cm}^3$ of FA 2 = ......................... mol

(c)

(i)

From your accurate titration results in Step 3, 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 5 required ...................... $\text{cm}^3$ of FA 1.

(ii)

Use your answer from (i) to calculate the number of moles of sodium hydroxide, FA 1, required to react with $25.0\text{ cm}^3$ of FA 5 in Step 3.

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

(iii)

Use your answer from (ii) to determine the number of moles of hydrochloric acid in $25.0\text{ cm}^3$ of FA 5.

moles of $\text{HCl}$ in $25.0\text{ cm}^3$ of FA 5 = ......................... mol

(iv)

Use your answer from (iii) to determine the number of moles of hydrochloric acid in $250\text{ cm}^3$ of FA 5.

moles of $\text{HCl}$ in $250\text{ cm}^3$ of FA 5 = ......................... mol

(d)

(i)

Write an equation for the reaction between calcium carbonate and hydrochloric acid.

(ii)

Calculate the number of moles of hydrochloric acid that reacted with calcium carbonate in FA 4 using the following expression.

\text{moles of HCl} = \textbf{(b)(iv)} - \textbf{(c)(iv)}

= ........................ mol

(iii)

Use your answers from (i) and (ii) to calculate the mass of $\text{CaCO}_3$ in FA 4.
$[A_r\text{: C, 12.0; O, 16.0; Ca, 40.1}]$
(If you were unable to answer (d)(ii), you may assume that the number of moles of hydrochloric acid that reacted with calcium carbonate was $0.0351\text{ mol}$ .)

mass of $\text{CaCO}_3$ in FA 4 = .......................... g

(iv)

Calculate the percentage purity by mass of the calcium carbonate in FA 4.

The percentage purity by mass of calcium carbonate is ........................... %

(e)

(i)

What is the maximum error in a single burette reading?

maximum error in a burette reading = ................... $\text{cm}^3$

(ii)

Calculate the maximum percentage error for one of your accurate titres in Step 3.
Show your working.

maximum percentage error = ........................ %

(f)

A student decided to use a larger mass of FA 4. State and explain whether this alteration will improve the accuracy of the percentage purity obtained.

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