We known that no. of moles =Vlitre × Molarity & No. of millimoles =Vml× Molarity so millimoles of NaOH=250×0.5=125 Millimoles of HCl=500×1=500 Now reaction is
NaOH
+
HCl
→
NaCl
+
H2O
t=0
125
500
0
0
t=t
0
375
125
125
so millimoles of HCl left =375 Moles of HCl=375×10−3 No. of HCl molecules =6.022×1023×375×10−3 =225.8×1021 ≃226×1021=226