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WAEC Chemistry 2021 Paper

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Question : 31

Total: 50

Student X titrated

25\,\mathrm{cm}^3

\mathrm{Na}_2\,\mathrm{CO}_2

0.1\,\mathrm{mol}\,\mathrm{dm}^{-3}\,\mathrm{HCl}

, using methyl orange as indicator. Student

Y

carried out the same exercise but used phenolphthalein as indicator. Which of the following statements about the titration is true?

hydrogen chloride gas was released in the reactions in both titrations
the titer values obtained from the titration are equal
the tirer value obtained by X is twice that of Y
the titer value obtained by Y is twice that of X

Solution:

Concept:

The indicator used decides how much

\mathrm{HCl}

is consumed in the titration of sodium carbonate.

Explanation:

Student Y used phenolphthalein, which stops at the bicarbonate stage:

\mathrm{Na_2CO_3 + HCl \rightarrow NaHCO_3 + NaCl}

Here, 1 mole of

\mathrm{Na_2CO_3}

reacts with 1 mole of

\mathrm{HCl}

.

Student X used methyl orange, which allows complete neutralisation up to carbon dioxide:

\mathrm{NaHCO_3 + HCl \rightarrow NaCl + CO_2 + H_2O}

Taking both steps together, 1 mole of

\mathrm{Na_2CO_3}

consumes 2 moles of

\mathrm{HCl}

.

Since both students used the same volume and concentration of sodium carbonate, the volume of

\mathrm{HCl}

used by X is double that used by Y.

Answer:

C. the titer value obtained by X is twice that of Y

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