Step I. Calcualtion of mass of oxalic acid
Normality of solution `= ("Mass of oxalic acid/Equivalent mass")/("Volume of solution in litres")`
Normality of solition `=0.2 N=0.2 "equiv. L"^(-1)`
Equivalent mass of acid `= ("Molar mass")/("Basicity")=(2xx1+2xx12+4xx16+2xx18)/(2)=(126)/(2)=63 "g equiv."^(-1)`
Volume of solution `= 100 mL = (100)/(1000)=0.1 L`
`:. ("0.2 equiv. L"^(-1))=("Mass of oxalic acid")/(("63 g equiv"^(-1))xx("0.1 L"))`
Mass of oxalic acid `= (0.1 "equiv. L"^(-1))xx("63 equiv."^(-1))xx(0.1 L)=1.26 g`
Step II. Calculation of no of molecules of oxalic acid
Molar mass of oxalic acid `= 126 g mol^(-1)`
Now, 126 g oxalic acid has molecules = Avogadro No. `= 6.022 xx10^(23)`
1.26 g of oxalic acid has molecules `= (6.022xx10^(23))/((126 g))xx(1.26g) = 6.022 xx 10^(21)`.