Mass percentage (w / w) =  \(\frac{\text { Mass of the solute }}{\text { Mass of the solution }} \times 100\)
\(10=\frac{\text { Mass of the urea }}{500} \times 100\)
Mass of urea = 50 g

Mass of the solute = 25 g
Mass of the solvent = 100 g
\(\begin{aligned} \text { Mass Percentage } \end{aligned}=\frac{\text { Mass of the solute }}{\text { Mass of the solution }} \times 100\)
\(\begin{aligned} \text { Mass Percentage } \end{aligned}=\frac{\text { Mass of the solute }}{\begin{array}{l} \text { Mass of the solute }+ \text { Mass of the solvent } \end{array}} \times 100\)
\(=\frac {25}{25+100}\times\)100
= \(\frac{25}{125}\times\)100
= 20%

At 30°C, 36 g of sodium chloride is dissolved in 100 g of water.
\(\therefore\) Mass of sodium chloride that would be need for 100 g of water = 36 g
\(\therefore\) Mass of sodium chloride dissolved in 50 g of water = \(\frac {36 \times 50}{100}\)
=18 g

Mass of the solute = 1.5 g
Mass of the solvent = 15 g
Solubility of the solute = \(\frac{\text { Mass of the solute }}{\text { Mass of the solvent }} \times 100\) 
Solubility of the solute = \(\frac{1.5}{15}\times 100\) 
= 10 g

(i) 'A' is copper sulphate pentahydrate (CuSO₄.5H₂O).
(ii) 'B' is Anhydrous copper sulphate (CuSO₄)