Chemistry 5070 · O Level · Acid–base titrations

Acid–base titrations — practice question

Ethanedioic acid crystals are represented by the formula $H_2C_2O_4\cdot xH_2O$. A student uses titration to find the value of $x$.
(a)[1]

Determine the mass of the crystals used in the experiment.

(b)[2]

The crystals in the beaker are added to water. State two methods for making them dissolve as fast as possible.

(c)[1]

Suggest how the student can ensure that every drop of solution in the beaker ends up in the volumetric flask.

(d)[1]

Why is a pipette chosen rather than a measuring cylinder?

(e(i))[1]

Which colour change is seen in the conical flask when the titration reaches the end-point?

(e(ii))[4]

Use the diagrams to fill in the results table and work out the average volume of $0.0100\text{ mol dm}^{-3}$ potassium manganate(VII).

(f)[1]

Why must this student carry out three titrations rather than averaging the first two results?

(g)[1]

Calculate the amount, in moles, of potassium manganate(VII) present in the average volume of $0.0100\text{ mol dm}^{-3}$ potassium manganate(VII).

(h)[1]

Find the number of moles of $H_2C_2O_4\cdot xH_2O$ contained in $25.0\text{ cm}^3$ of $D$.

(i)[1]

Find the number of moles of $H_2C_2O_4\cdot xH_2O$ in $250\text{ cm}^3$ of $D$.

(j)[1]

Using your answers to (a) and (i), work out the relative molecular mass of $H_2C_2O_4\cdot xH_2O$.

(k)[2]

Using your answer to (j), calculate the value of $x$ in $H_2C_2O_4\cdot xH_2O$.

Worked solution & mark scheme

This 17-mark question has a full step-by-step worked solution and mark scheme. One marking point: Mass = 0.806 g

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