Chemistry 5070/32 — May/June 2014

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

Q121MObservations and MeasurementsExperimental ContextsAnalysis, Conclusions and EvaluationFree sample

Reactions between alkalis and acids are exothermic. The change in temperature when aqueous sodium hydroxide is added to dilute hydrochloric acid of known concentration can be used to determine the concentration of the alkali.

P is $1.50\text{ mol / dm}^3$ hydrochloric acid.
Q is aqueous sodium hydroxide.

(a)

(i) Put P into a burette and use it to measure $10\text{ cm}^3$ of P into a plastic cup. Measure the temperature of P to the nearest $0.5\ ^\circ\text{C}$ and record the value in column D of the table.
(ii) Using a measuring cylinder, measure $40\text{ cm}^3$ of Q as accurately as possible. Pour this volume of Q into the plastic cup containing P. Stir, using the thermometer, and measure the highest temperature reached. Record the value in column E of the table.
(iii) Empty the plastic cup and rinse it with water.
(iv) Repeat the procedure described in (i) to (iii) but using the different volumes of P and Q given in columns B and C of the table for experiments 2 to 7.
(v) For each experiment, calculate the temperature rise and record the value in column F.

A	B	C	D	E	F
experiment number	volume of P / $\text{cm}^3$	volume of Q / $\text{cm}^3$	initial temperature of P / $^\circ\text{C}$	highest temperature of mixture / $^\circ\text{C}$	temperature rise / $^\circ\text{C}$
1	10	40
2	15	35
3	20	30
4	25	25
5	30	20
6	35	15
7	40	10

14M

(b)

Plot a graph of temperature rise (column F) against volume of P (column B) on the grid opposite. Using these points, draw two intersecting straight lines.

Fig. 1.1

(c)

From the graph, read the volume of P where the two lines cross.

volume of P = ______ $\text{cm}^3$

(d)

Calculate the number of moles of hydrochloric acid present in the volume of P you gave as an answer to (c).

moles of hydrochloric acid = ______

(e)

Deduce the number of moles of sodium hydroxide which react with the number of moles of hydrochloric acid you gave as an answer to (d).

\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}

moles of sodium hydroxide = ______

(f)

Calculate the concentration, in $\text{mol / dm}^3$ , of the aqueous sodium hydroxide, Q.

concentration of Q = ______ $\text{mol / dm}^3$

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