Discussion
The molarity of an aqueous solution containing 0.4g of NaOH (molar mass=40g/mol) in 250mL of a solution is
Answer: A
The molarity is 0.04 M.
Molarity is moles of solute per litre of solution.
\[ \begin{aligned} n &= \frac{0.4\ \text{g}}{40\ \text{g mol}^{-1}} = 0.01\ \text{mol} \\ M &= \frac{0.01\ \text{mol}}{0.250\ \text{L}} = 0.04\ \text{M} \end{aligned} \]
The other values come from slips with the volume: putting 0.01 mol in 0.025 L (a misplaced decimal) gives 0.40 M, and halving or doubling the 0.25 L gives 0.08 M or 0.02 M.
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