Chemistry 5070 · O Level · Acid–base titrations

Acid–base titrations — practice question

Milk of Magnesia is a liquid medicine that is used to relieve indigestion. Milk of Magnesia contains magnesium hydroxide, $\text{Mg(OH)}_2$. A student carries out a sequence of titrations to find the percentage by mass of magnesium hydroxide in Milk of Magnesia. Diagrams of some of the apparatus used by the student are shown.
(a)[3]

Name the three pieces of apparatus shown.

(b(i))[3]

Enter the figures into the results table. Complete the results table for titrations $1$, $3$ and $4$.

(b(ii))[1]

Tick (✓) the best titration results in the table. Use these best titration results to calculate the average volume of $0.600\,\text{mol dm}^{-3}$ hydrochloric acid used.

(b(iii))[1]

Suggest why the volume of distilled water the student adds to apparatus A does not affect the results of the titrations.

(c(i))[1]

Calculate the number of moles of $0.600\,\text{mol dm}^{-3}$ hydrochloric acid used by this second student.

(c(ii))[1]

Calculate the number of moles of magnesium hydroxide present in $5.0\,\text{cm}^3$ of the Milk of Magnesia.

(c(iii))[2]

Calculate the mass of magnesium hydroxide in $5.0\,\text{cm}^3$ of the Milk of Magnesia. [$A_r$: Mg, $24$; O, $16$; H, $1$]

(c(iv))[1]

The density of the Milk of Magnesia is $2.34\,\text{g cm}^{-3}$. $\text{density in g/cm}^3 = \frac{\text{mass in g}}{\text{volume in cm}^3}$. Calculate the mass of $5.0\,\text{cm}^3$ of the Milk of Magnesia.

(c(v))[1]

Calculate the percentage by mass of magnesium hydroxide in the Milk of Magnesia.

Worked solution & mark scheme

This 14-mark question has a full step-by-step worked solution and mark scheme. One marking point: A refers to conical flask / Erlenmeyer flask

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