Chemistry mcq?

Equation for the reaction :
NaOH + HCl → NaCl + H₂O

The pH of the resulting solution is 2.0 :
[H⁺] in the resulting solution = 10⁻²·⁰ = 0.01 M
As HCl is a strong acid, excess [HCl] in the resulting solution = 0.01 M

Volume of the resulting solution = (50 + 50) cm³ = 100 cm³ = 0.1 dm³
No. of moles of excess HCl = (0.01 mol/dm³) × (0.1 dm³) = 0.001 mol

Initial no. of moles HCl = (0.1 mol/dm³) × (50/1000 dm³) = 0.005 mol
No. of moles of HCl reacted with NaOH = (0.005 - 0.001) = 0.004 mol

According to the equation, mole ratio NaOH : HCl = 1 : 1
No. of moles of NaOH in 50 cm³ of the sample solution = 0.004 mol
No. of moles of NaOH in 1 dm³ of the sample solution = (0.004 mol) × (1000/50) = 0.08 mol
No. of moles of NaOH in the sample = 0.08 mol

Molar mass of NaOH = (23.0 + 16.0 + 1.0) g/mol = 40.0 g/mol
Mass of NaOH in the sample = (40.0 g/mol) × (0.08 mol) = 0.32 g

The percentage purity of the NaOH sample = (0.32/4.00) × 100 = 80