18) E

Reaction equation:

NaOH + HCl  ->  NaCl + H₂O

Volume of HCl = 50 cm³ = 0.05 dm³
Concentration of HCl = 0.5
Moles of HCl = 0.05 x 0.5 = 0.025 mol

Molar ratio of NaOH : HCl = 1 : 1
Moles of NaOH = 0.025 mol
Mr of NaOH = 23 + 16 + 1 = 40

Moles = Mass ÷ Mr
Mass = Moles x Mr
Mass = 0.025 x 40 = 1 g

Mass of sample = 1.20 g

Percentage purity = (1 ÷ 1.2) x 100 = 83.3%