The number of oxygen atoms present in 50g of calcium carbonate is

Molecular formula of calcium carbonate is ${\mathrm{CaCO}}_3$

No. of oxygen atoms present in one molecule ${\mathrm{CaCO}}_3$=3

$100\mathrm{g} \mathrm{of} {\mathrm{CaCO}}_3\to 3\times 6.23\times {10}^{23} \text{'}\mathrm{O}\text{'} \mathrm{atoms}$

50g of ${\mathrm{CaCO}}_3$$\to$?

$\mathrm{x}\times 100=3\times 6.23\times {10}^{23}$

$\mathrm{x}=\frac{3\times 6.023\times {10}^{23} \text{'}\mathrm{O}\text{'} \mathrm{atoms}}{100\mathrm{g}}\times 50\mathrm{g}=9.035\times {10}^{23} \text{'}\mathrm{O}\text{'} \mathrm{atoms}$