Chemistry 5070 · O Level · Acid–base titrations

Acid–base titrations — practice question

A student explores the temperature changes that occur when an acid neutralises aqueous sodium hydroxide. The ionic equation for the reaction is shown: $\mathrm{H^+ + OH^- \rightarrow H_2O}$. This reaction releases heat. The temperature change is used to work out the concentration of hydrogen ions in an acid. X is $1.60\,\mathrm{mol/dm^3}$ sodium hydroxide solution. Y is an acid with concentration $2.00\,\mathrm{mol/dm^3}$. The student: step 1 fills the burette with Y; step 2 places a plastic cup inside a beaker; step 3 uses a volumetric pipette to add $25.0\,\mathrm{cm^3}$ of X to the plastic cup; step 4 measures the starting temperature of X and writes it in Table 2.1; step 5 adds $5.0\,\mathrm{cm^3}$ of Y from the burette to the plastic cup; step 6 stirs the mixture for 30 seconds and records the mixture temperature in Table 2.1; step 7 repeats steps 5 and 6 until a total of $40.0\,\mathrm{cm^3}$ of Y has been added. Fig. 2.1 shows two thermometer readings in ^{\circ}C during the investigation.
(a)[4]

Finish Table 2.1. You should: copy the student’s results for $5.0\,\mathrm{cm^3}$ and $30.0\,\mathrm{cm^3}$ from Fig. 2.1; subtract the starting temperature of X from each recorded temperature to obtain the temperature change. Every temperature and temperature change should be given to +/- $0.5\,^{\circ}\mathrm{C}$.

(b)[5]

Plot a graph of the temperature change (y-axis) against the volume of Y (x-axis) on Fig. 2.2. Draw a line of best fit through the points where the temperature change is increasing. Draw a line of best fit through the points where the temperature change is decreasing. Extend both lines so that they cross.

(c)[1]

Find the point where the two lines cross on the graph. Work out the volume of Y at this point.

(d)[1]

X is $1.60\,\mathrm{mol/dm^3}$ sodium hydroxide solution. Sodium hydroxide has formula NaOH. Calculate the amount of hydroxide ions, $\mathrm{OH^-}$, in $25.0\,\mathrm{cm^3}$ of X.

(e)[1]

The volume of Y in (c) is the volume required to neutralise $25.0\,\mathrm{cm^3}$ of X completely. Use your answers to (c) and (d) to calculate the concentration of hydrogen ions, $\mathrm{H^+}$, in Y. $\mathrm{H^+ + OH^- \rightarrow H_2O}$.

(f)[1]

Suggest why Y is added to X using a burette rather than a measuring cylinder.

(g)[1]

Suggest why the temperatures are recorded after 30 seconds instead of immediately after mixing.

Worked solution & mark scheme

This 14-mark question has a full step-by-step worked solution and mark scheme. One marking point: “M1 temperature for 5 ^{\circ}C” …

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