A 0.500 g sample of impure KOH is treated with 25.00 mL of 0.200 M HCl. After the reaction between KOH and HCl is complete, the excess acid requires 10.50 mL of 0.150 M NaOH for neutralization. What is the percent by mass of KOH in the original sample? (Molar mass of KOH = 56.11 g mol⁻¹)
a) 56 \%
b) 38 \%
c) 45 \%
d) 28 \%