\( 250 \, \text{mL} \) of \( 0.5 \, \text{M} \) \( \mathrm{NaOH} \) was added to \( 500 \, \text{mL} \) of \( 1 \, \text{M} \) \( \mathrm{HCl} \). The number of unreacted \( \mathrm{HCl} \) molecules in the solution after complete reaction is ______ \( \times 10^{21} \). (Nearest integer). ( \( N_A = 6.022 \times 10^{23} \) ).
