The amount of urea required to make a 500 mL solution of 0.25 molarity is
(Molar mass of Urea = \( 60\, g\, mol^{-1} \))
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 7.5 g
Molarity is the number of moles of solute in one litre of solution:
\( \displaystyle M = \frac{n}{V\,(\mathrm{L})} \quad \Rightarrow \quad n = M \times V \)
The volume must be in litres: 500 mL = 0.5 L.
\( \displaystyle n = 0.25\,\mathrm{mol\,L^{-1}} \times 0.5\,\mathrm{L} = 0.125\,\mathrm{mol} \)
\( \displaystyle \text{mass} = n \times M_{urea} = 0.125\,\mathrm{mol} \times 60\,\mathrm{g\,mol^{-1}} = 7.5\,\mathrm{g} \)
So 7.5 g of urea is needed.
Shortcut: One litre of 0.25 M urea holds \( 0.25 \times 60 = 15 \) g; half a litre holds half of that, 7.5 g.