Chemistry 5070/31 — October/November 2021

Cambridge O-Level · Practical Test · worked solutions for every part, with the mark scheme

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Topics Observations and Measurements · Experimental Contexts · Analysis, Conclusions and Evaluation · Qualitative Analysis

Q120MObservations and MeasurementsExperimental ContextsAnalysis, Conclusions and EvaluationFree sample

Hydrated sodium carbonate is sometimes known as washing soda.

An aqueous solution of washing soda, P, is made by dissolving $13.73\text{ g}$ of washing soda crystals in water to make $200\text{ cm}^3$ of solution.

The mass of sodium carbonate, $\text{Na}_2\text{CO}_3$ , in this aqueous solution of washing soda is determined by titration with dilute nitric acid, $\text{HNO}_3\text{(aq)}$ .

\text{Na}_2\text{CO}_3 + 2\text{HNO}_3 \rightarrow 2\text{NaNO}_3 + \text{CO}_2 + \text{H}_2\text{O}

Methyl orange is used to determine the end-point of the titration.

Q is $0.400\text{ mol / dm}^3\text{ }\text{HNO}_3\text{(aq)}$ .

(a)

Fill the burette with Q.

Pipette $25.0\text{ cm}^3$ of P into a flask and titrate with Q using three drops of methyl orange as the indicator.

The end-point is the first appearance of a pink colour that remains for 30 seconds.

Record your results in the table.

Repeat the titration as many times as necessary to achieve consistent results.

Results

titration number	1	2
final burette reading / $\text{cm}^3$
initial burette reading / $\text{cm}^3$
volume of Q used / $\text{cm}^3$
best titration results ( $\checkmark$ )

Summary

Tick ( $\checkmark$ ) the best titration results.

Use the best titration results to calculate the average volume of Q used.

______ $\text{cm}^3$

12M

(b)

Q is $0.400\text{ mol / dm}^3\text{ }\text{HNO}_3\text{(aq)}$ .

Use your results from (a) to calculate the number of moles of $\text{HNO}_3$ in the average volume of Q used.

Give your answer to three significant figures.

______ $\text{mol}$

(c)

Use your answer from (b) to calculate the number of moles of $\text{Na}_2\text{CO}_3$ in $25.0\text{ cm}^3$ of P.

\text{Na}_2\text{CO}_3 + 2\text{HNO}_3 \rightarrow 2\text{NaNO}_3 + \text{CO}_2 + \text{H}_2\text{O}

______ $\text{mol}$

(d)

Use your answer from (c) to calculate the number of moles of $\text{Na}_2\text{CO}_3$ in $200\text{ cm}^3$ of P.

______ $\text{mol}$

(e)

Use your answer from (d) to calculate the mass of $\text{Na}_2\text{CO}_3$ in $200\text{ cm}^3$ of P.

[ $A_r$ : C, 12; O, 16; Na, 23]

______ $\text{g}$

(f)

The aqueous solution of washing soda, P, is made by dissolving $13.73\text{ g}$ of washing soda crystals in water to make $200\text{ cm}^3$ of solution.

Washing soda crystals contain water of crystallisation.

The formula is $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$

Use your answer from (e) to calculate the mass of water, $\text{H}_2\text{O}$ , in $13.73\text{ g}$ of washing soda crystals.

______ $\text{g}$

(g)

Use your answer from (f) to calculate the number of moles of water, $\text{H}_2\text{O}$ , in $13.73\text{ g}$ of washing soda crystals.

[ $M_r$ : $\text{H}_2\text{O}$ , 18]

______ $\text{mol}$

(h)

Use your answers from (d) and (g) to calculate the value of $x$ .

Give your answer to the nearest whole number.

Show your working.

$x =$ ______

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