$10$ g of a mixture of $BaO$ and $CaO$ requires $100c{m}^3$ of $2.5$M $HCl$ to react completely. The percentage of calcium oxide in the mixture is approximately?

The correct option is B $55.1$

Gram equivalent in 2.5 M HCl$=100\times 2.5/1000$

$=0.25$

and this is must be equal to the sum of the both gram eq. (BaO + CaO)

$2HCl+BaO\to BaC{l}_2+H_{2}O$ ; x/2 moles of HCl react with BaO

$2HCl+CaO\to CaC{l}_2+H_2$ ; x/2 moles of HCl reacts with CaO

Thus;

$\frac{2x}{MM\left(BaO\right)}+\frac{2x}{MM\left(CaO\right)}=0.25$

$\frac{2x}{153}+\frac{2x}{56.07}=0.25$

$x=5.208$

So $\%$ of $CaO$ $=\frac{x\times 100}{10}=52.08$ or 52.1