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WAEC Chemistry 2021 Paper

Question : 32

Total: 50

100\ \mathrm{cm}^{3}

[\mathrm{KOH}=56]

Solution:

Concept:

Molar concentration is found by dividing the number of moles of solute by the volume of solution in

\mathrm{dm}^3

Explanation:

The molar mass of potassium hydroxide is

M(\mathrm{KOH}) = 56\ \mathrm{g\ mol}^{-1}

Mass of

\mathrm{KOH}

dissolved

= 0.28\ \mathrm{g}

Number of moles

= \frac{0.28}{56} = 0.005\ \mathrm{mol}

Volume of solution

= 100\ \mathrm{cm}^3 = 0.1\ \mathrm{dm}^3

Concentration

= \frac{\text{moles}}{\text{volume in } \mathrm{dm}^3} = \frac{0.005}{0.1} = 0.05\ \mathrm{mol\ dm}^{-3}

Answer:

0.05\ \mathrm{mol\ dm}^{-3}

Therefore, the correct option is B.

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