Question 2
What mass of magnesium chloride is needed for 500 cm³ of a 0.5 mol dm⁻³ solution? (Mg = 24, Cl = 35.5)
The right answer is the one marked above.
47.5 g. The molar mass is 95 g mol⁻¹. The amount needed is 0.5 mol dm⁻³ × 0.5 dm³ = 0.25 mol, and 95 × 0.25 = 47.5 g.
95 g would be the mass for a full cubic decimetre at 1 mol dm⁻³. Both the concentration and the volume are halved here, so the mass is a quarter of that.