S2O2−3→2SO2−4
Change in oxidation number of sulphur per molecule of S2O2−3=2×(6−2)=8
Change in oxidation number of Mn per molecule of MnO−4=7−4=3
No. of moles in 0.158 g of Na2S2O3=0.158158=1×10−3
No. of equivalents =8×10−3
Normality of 0.1 M KMnO4 solution =0.1×3=0.3
Let V mL of volume of KMnO4 be required; then
V1000×0.3=8×10−3
or V=80.3×10−3=103=26.7 mL.