Chemistry 5070/31 — May/June 2010

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

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

Q118MExperimental ContextsObservations and MeasurementsAnalysis, Conclusions and EvaluationFree sample

An organic acid has the molecular formula $\text{C}_3\text{H}_4\text{O}_5$ .

You are required to find by experiment the number of moles of sodium hydroxide that react with 1 mole of this organic acid.

P is $0.300\text{ mol / dm}^3$ sodium hydroxide.
Q is an aqueous solution of the organic acid, $\text{C}_3\text{H}_4\text{O}_5$ , containing $18.0\text{ g / dm}^3$ .

(a)

Put Q into the burette.

Pipette a $25.0\text{ cm}^3$ (or $20.0\text{ cm}^3$ ) portion of P into a flask and titrate with Q, using the indicator provided.

Record your results in the table, repeating the titration as many times as you consider necessary to achieve consistent results.

Results

Burette readings

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

Summary

Tick (✓) the best titration results.

Using these results, the average volume of Q required was ______ $\text{cm}^3$ .

Volume of P used was ______ $\text{cm}^3$ .

12M

(b)

P is $0.300\text{ mol / dm}^3$ sodium hydroxide.

Calculate the number of moles of sodium hydroxide in the volume of P used.

moles of sodium hydroxide in the volume of P used = ______

(c)

Q is an aqueous solution of $\text{C}_3\text{H}_4\text{O}_5$ containing $18.0\text{ g / dm}^3$ .

Calculate the concentration, in $\text{mol / dm}^3$ , of $\text{C}_3\text{H}_4\text{O}_5$ in Q.
[The relative molecular mass of C3H4O5 is 120.]

concentration of $\text{C}_3\text{H}_4\text{O}_5$ in Q = ______ $\text{mol / dm}^3$

(d)

Calculate the number of moles of $\text{C}_3\text{H}_4\text{O}_5$ in the average volume of Q used in the titration.

moles of $\text{C}_3\text{H}_4\text{O}_5$ = ______

(e)

Using your answers from (b) and (d) calculate the number of moles of sodium hydroxide which react with 1 mole of $\text{C}_3\text{H}_4\text{O}_5$ .

moles of sodium hydroxide = ______

(f)

Using your answer to (e) write an equation for the reaction of the organic acid, $\text{C}_3\text{H}_4\text{O}_5$ , with sodium hydroxide.

______

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