Test Index

WAEC Chemistry 2014 Paper

© africaexams.com

Question : 16

Total: 45

If 0.20 mol dm^-3 NaOH_(aq) was evaporated to yield 5.0g of solid NaOH, calculate the volume of NaOH _(aq) used. (Na = 23.0,O = 16:0; H = 1.00)

600cm-³
625cm³
1000³
1600 cm ³

Solution:

Concept:

Molarity relates moles of solute to the volume of solution.

Explanation:

Molar mass of

\mathrm{NaOH} = 23.0 + 16.0 + 1.00 = 40.0\ \mathrm{g\,mol^{-1}}

.

Moles of solid

\mathrm{NaOH} = \frac{5.0\ \mathrm{g}}{40.0\ \mathrm{g\,mol^{-1}}} = 0.125\ \mathrm{mol}

.

The solution concentration is

0.20\ \mathrm{mol\,dm^{-3}}

, meaning

0.20\ \mathrm{mol}

is present in

1000\ \mathrm{cm^3}

.

For

0.125\ \mathrm{mol}

, the volume used is

V = \frac{0.125}{0.20} \times 1000\ \mathrm{cm^3}

.

V = 625\ \mathrm{cm^3}

.

Answer:

Option B:

625\ \mathrm{cm^3}

.

© africaexams.com

Go to Question:

Prev Question

Next Question