What mass of potassium hydroxide, KOH, is required to make 250cm3 of 0.5M solution?

250 cm³ = 0.25 dm³ (= 0.25 L)

For a solution, C=$\frac{n}{V}$ (concentration = number of moles/volume).

Rearranging, n=CV=0.50×0.25=0.125 mol

We need 0.125 mol of KOH and the molar mass of KOH is 56.1 g mol⁻¹, so 56.1×0.125=7.0 g.

We need 7.0 g of KOH.