$\text { so millimoles of } {NaOH}=250 \times 0.5=125$

$\text { Millimoles of } {HCl}=500 \times 1=500$

$\text { Now reaction is }$

${NaOH}+{HCL} \rightarrow {NaCl}+{H}_{2} {O}$

${t}=0 \quad 125\quad 500 \quad 0 \quad 0$

${t}=0 \quad 0 375 \quad 125 \quad 125$

$\text { so millimoles of } {HCl} \text { left }=375$

$\text { Moles of } {HCl}=375 \times 10^{-3}$

$\text { No. of } {HCl} \text { molecules }=6.022 \times 10^{23} \times 375 \times 10^{-3}$

$=225.8 \times 10^{21}$

$\approx 226 \times 10^{21}=226$