The amount of urea required to make a 500 mL solution of 20% (w/w) co…

The correct answer is Option (3) → 100 g

A mass percentage (w/w) gives the mass of solute in 100 g of solution:

    \( \displaystyle \text{mass \% of solute} = \frac{\text{mass of solute}}{\text{mass of solution}} \times 100 \)

So a 20% (w/w) urea solution has 20 g of urea in every 100 g of solution. The quantity of solution is given as 500 mL; taking the density of this aqueous solution as about 1 g mL⁻¹, its mass is about 500 g:

    \( \displaystyle \text{mass of urea} = \frac{20}{100} \times 500\,\mathrm{g} = 100\,\mathrm{g} \)

So 100 g of urea is needed.

Shortcut: 500 g of solution is five lots of 100 g, so it holds \( 5 \times 20 = 100 \) g of urea.

Note: It seems that there is a mistake in the original NTA paper, which gives a volume (500 mL) of solution for a 20% (w/w) concentration; a mass percentage needs the mass of the solution.