We are asked for the mass of NaOH in 5.00 dm³ of a 0.125 mol dm⁻³ solution. The molar mass is given as 40 g mol⁻¹.

First, find the amount (number of moles) of NaOH in the solution. Molar concentration (C) = amount (n) / volume (V in dm³). So n = C × V.

n = 0.125 mol dm⁻³ × 5.00 dm³ = 0.625 mol.

Now, convert moles to mass using the molar mass. Mass (m) = amount (n) × molar mass (M).

m = 0.625 mol × 40 g mol⁻¹ = 25.0 g.

The most tempting wrong option, B) 0.625 g, is the number of moles, not the mass.

So the correct answer is D) 25.0 g.

Textbook reference: Textbook of Practical Chemistry, pp. 110–113, 217