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JAMB Chemistry 2019 Paper

Question : 34

Total: 80

_{2}

_{3}

\to

_{2}

_{2}

Solution:

Concept:

The titration of

\mathrm{Na}_{2}\mathrm{CO}_{3}

with

\mathrm{HCl}

requires an indicator that changes colour in an acidic medium.

Explanation:

During the reaction,

\mathrm{Na}_{2}\mathrm{CO}_{3}

reacts completely with

\mathrm{HCl}

to form

\mathrm{NaCl}

\mathrm{H}_{2}\mathrm{O}

, and

\mathrm{CO}_{2}

At the equivalence point, the solution is slightly acidic because carbonic acid is formed and carbon dioxide escapes.

Methyl orange changes colour over the pH range

3.1

4.4

This makes it the best indicator for this titration.

Among the given options,

3

is the closest pH value to the endpoint range of methyl orange.

Answer:

C. 3

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