Chemistry 9701/34 — May/June 2014

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

You are to determine the percentage of calcium carbonate in a sample of crushed limestone. You will first react a known mass of the crushed limestone in a known amount of hydrochloric acid, $\text{HCl}(\text{aq})$ , to make a solution. You may assume that only the calcium carbonate present in the sample will react with the acid.

\text{CaCO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CaCl}_2(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})

The amount of acid that did not react with the carbonate is then found by a titration using sodium hydroxide. You may assume that no compounds present in the limestone will react with the sodium hydroxide.

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

FB 1 is crushed limestone, impure calcium carbonate.
FB 2 is $2.0\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .
FB 4 is $0.20\text{ mol dm}^{-3}$ sodium hydroxide, $\text{NaOH}$ .
methyl orange indicator

(a)

Method

Read through the method before starting any practical work.

Making the solution

Weigh the container with the limestone, FB 1, and record the mass below.
Tip all the solid FB 1 into a $250\text{ cm}^3$ beaker.
Reweigh the container and record the mass.
Fill the burette with FB 2.
Slowly run between $47.5$ and $48.5\text{ cm}^3$ of FB 2 into the beaker containing FB 1.
Record, in the space below, both your burette readings and the volume of FB 2 added.
Stir the mixture carefully until all the solid has reacted.
Transfer the contents of the beaker into the volumetric flask.
Rinse the beaker with distilled water and add it to the volumetric flask. Make the solution up to $250\text{ cm}^3$ with distilled water and mix thoroughly. This is solution FB 3.

Titration

Empty and rinse the burette with distilled water.
Fill the burette with FB 3 from the volumetric flask.
Pipette $25.0\text{ cm}^3$ of FB 4 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 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 FB 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 FB 4 required ................ $\text{cm}^3$ of FB 3.

(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 sodium hydroxide present in $25.0\text{ cm}^3$ of FB 4.

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

(ii)

Hence state the number of moles of hydrochloric acid present in the volume of FB 3 calculated in (b).

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

(iii)

Use your answer to (ii) to calculate the number of moles of hydrochloric acid present in $250\text{ cm}^3$ of FB 3.

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

(iv)

Calculate the number of moles of hydrochloric acid, FB 2, added to FB 1 in (a).

moles of $\text{HCl}$ added to FB 1 = ....................... mol

(v)

Use your answers to (iii) and (iv) to calculate the number of moles of hydrochloric acid that reacted with the calcium carbonate in FB 1.

moles of $\text{HCl}$ reacted with $\text{CaCO}_3$ = ....................... mol

(vi)

Calculate the number of moles of calcium carbonate present in your sample of FB 1.

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

(vii)

From your answer to (vi) and the mass of FB 1 used in (a), calculate the percentage by mass of calcium carbonate in the limestone.
[ $A_r$ : C, 12.0; O, 16.0; Ca, 40.1]

percentage of calcium carbonate = ................... %

(d)

(i)

The maximum error in a single burette reading is $\pm 0.05\text{ cm}^3$ .
Student X, carrying out this experiment, recorded that $48.50\text{ cm}^3$ of FB 2 was added to FB 1.
What are the smallest and largest possible volumes of FB 2 that were added?

smallest volume used = .................. $\text{cm}^3$

largest volume used = .................. $\text{cm}^3$

(ii)

Student Y used an identical mass of FB 1 but added $47.70\text{ cm}^3$ of FB 2. How would the value obtained in (b) by student X compare with the value obtained by student Y?
Explain your answer.

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2 more questions

Q2Presentation of Data and Observations · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation7M
Q3Qualitative Analysis · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation13M

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