Discussion
What is the volume of methanol needed for making 2 L of 0.4 M solution? (Density of methanol = 0.64 kgL-1 and molar mass = 32 g mol-1)
Answer: C
The volume needed is 40 mL.
Moles of methanol in 2 L of 0.4 M solution, then mass, then volume from the given density (0.64 kg L-1 = 0.64 g mL-1):
\[ \begin{aligned} n &= 0.4 \times 2 = 0.8\ \text{mol} \\ m &= 0.8 \times 32 = 25.6\ \text{g} \\ V &= \frac{25.6}{0.64} = 40\ \text{mL} \end{aligned} \]
20 mL and 80 mL halve or double the moles (using 1 L or 4 L), and 10 mL or 4 mL come from mixing up kilograms and grams in the density.
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