The mass of NaOH required to make a 500 mL solution of 0.5 molarity is

The correct answer is Option (4) → 10 g

Molarity is the number of moles of solute in one litre of solution:

    \( \displaystyle M = \frac{n}{V\,(\text{in L})} \)

The volume must be in litres: 500 mL = 0.5 L.

    \( \displaystyle n = M \times V = 0.5\,\mathrm{mol\,L^{-1}} \times 0.5\,\mathrm{L} = 0.25\,\mathrm{mol} \)

The molar mass of NaOH is \( 23 + 16 + 1 = 40\,\mathrm{g\,mol^{-1}} \):

    \( \displaystyle m = n \times M_{\ce{NaOH}} = 0.25\,\mathrm{mol} \times 40\,\mathrm{g\,mol^{-1}} = 10\,\mathrm{g} \)

So 10 g of NaOH is needed.