A 0.518 g sample of limestone is dissolved in HCl and then the calcium is precipitated as CaC2O4. After filtering and washing the precipitate, it requires 40 mL of 0.25 N KMnO4 solution acidified with H2SO4 to titrated it as,
MnO4−+H++C2O42−→CO2+Mn2++2H2O
The percentage of CaO in the sample is: