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WAEC Chemistry 2011 Paper

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Question : 31

Total: 46

On evaporation to dryness, 250 cm3 of saturated solution of salt X with relative molar mass 101 gave 50.5g of the. What is the soluility of the salt?

1.0 mol dm^-3
2.0 mol dm^-3
4.0 moldm^-3
5.0 moldm^-3

Solution:

Concept:

Solubility is the amount of solute, in moles, dissolved in

1.0\,\mathrm{dm^3}

of saturated solution.

Explanation:

250\,\mathrm{cm^3}

of solution contains

50.5\,\mathrm{g}

of salt X.

Mass of salt X in

1000\,\mathrm{cm^3}

(

1\,\mathrm{dm^3}

) is:

\frac{50.5 \times 1000}{250} = 202\,\mathrm{g}

Moles of X in

1\,\mathrm{dm^3}

=

\frac{\text{mass}}{\text{molar mass}}

=

\frac{202}{101} = 2.0\,\mathrm{mol}

Thus, solubility =

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

.

Answer:

B.

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

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