Discussion

What is the mass of the substance 'X' required to prepare 250 mL of 0.5 molar aqueous solution? (Molar mass of 'X 'is 40 g \(mol^{-1}\) )

Answer: C

The mass required is 5 g.

Moles needed = molarity \(\times\) volume in litres:

\[ \begin{aligned} n &= 0.5 \times 0.250 = 0.125\ \text{mol} \\ m &= 0.125 \times 40 = 5\ \text{g} \end{aligned} \]

20 g would make a full litre, 10 g ignores the 0.5 M, and 2 g, 4 g and 0.5 g do not follow from the data.

A quick check: 0.5 M means 20 g of X per litre (half of 40 g), and 250 mL is a quarter of a litre, giving 5 g.

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