Chemistry 9701/52 — October/November 2020

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

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Topics Planning · Analysis, Conclusions and Evaluation

Q1PlanningAnalysis, Conclusions and EvaluationFree sample

Calcium hydroxide, $\text{Ca(OH)}_2$ , is slightly soluble in water. Aqueous calcium hydroxide is also known as limewater. A student measures the concentration of saturated aqueous calcium hydroxide using the following method.

step 1 Add a spatula measure of solid calcium hydroxide to $250\text{ cm}^3$ of distilled water in a conical flask. Stopper the flask and shake the flask thoroughly.

step 2 Leave the flask stoppered for 24 hours.

step 3 Filter the aqueous calcium hydroxide into a clean, dry beaker.

step 4 Transfer $25.00\text{ cm}^3$ of the aqueous calcium hydroxide to a clean conical flask.

step 5 Carry out a titration with $0.100\text{ mol dm}^{-3}$ hydrochloric acid.

step 6 Record the volume of hydrochloric acid required to react with the aqueous calcium hydroxide.

(a)

(i)

Suggest why it is necessary for the flask to remain stoppered in step 2.

(ii)

Suggest why the student leaves the solution for 24 hours in step 2.

(b)

(i)

The student finds that the mean titre is $10.65\text{ cm}^3$ .
The equation for the reaction is shown.

\text{Ca(OH)}_2(\text{aq}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CaCl}_2(\text{aq}) + 2\text{H}_2\text{O}(\text{l})

Calculate the number of moles of calcium hydroxide that reacted.

\text{moles of } \text{Ca(OH)}_2 = \text{.............................. mol}

(ii)

Calculate the concentration of the aqueous calcium hydroxide in $\text{g dm}^{-3}$ .
[ $A_r$ : Ca, 40.1; O, 16.0; H, 1.0]

\text{concentration of } \text{Ca(OH)}_2(\text{aq}) = \text{.............................. } \text{g dm}^{-3}

(iii)

In a chemistry data book, the student found the value for the concentration of a saturated solution of calcium hydroxide at $293\text{ K}$ to be higher than the value calculated in (b)(ii).

Give two reasons why the data book value is higher than the value calculated in (b)(ii).
(Assume the student recorded all volumes correctly, carried out the titration accurately and recorded concordant titres.)

(c)

A second student carried out the experiment but did not filter their solution correctly in step 3, leaving some undissolved calcium hydroxide suspended in the solution.

Predict and explain how this would affect the titre obtained in their experiment.

(d)

Describe how to prepare exactly $250.0\text{ cm}^3$ of $0.100\text{ mol dm}^{-3}$ hydrochloric acid from $1.00\text{ mol dm}^{-3}$ hydrochloric acid. Include the names and capacities of any suitable apparatus used.

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2 more questions

Q2Planning · Analysis, Conclusions and Evaluation14M
Q3Planning · Analysis, Conclusions and Evaluation6M

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