A 0.518 g sample of limestone is dissolved in HCl and then the calcium is precipitated as ${\mathrm{CaC}}_2{\mathrm{O}}_4$. After filtering and washing the precipitate, it requires 40.0 mL of 0.250 N ${\mathrm{KMnO}}_4$ solution acidified with ${\mathrm{H}}_2{\mathrm{SO}}_4$ to titrate it as. The percentage of CaO in the sample is

${\mathrm{MnO}}_4^{-} + {\mathrm{H}}^{+} + {\mathrm{C}}_2{\mathrm{O}}_4^{-2} \stackrel{ }{\to } {\mathrm{Mn}}^{2+} +{\mathrm{CO}}_2 + 2{\mathrm{H}}_2\mathrm{O}$