The number of moles and molecules in 64grams of ${\text{O}}_2$  and the mass of one molecule of ${\text{O}}_2$ are respectively

$\text{No}\text{.of}\text{moles}\text{in 64 gram of}{\text{O}}_2,n=\frac{64\text{g}}{32\text{g}}\times 1\text{mol}=2\text{mol}$

$2\text{mol}\times \frac{6.023\times {10}^{23}\text{molecules}}{1\text{mol}}=12.046\times {10}^{23}\text{molecules}$

$\text{Mass}\text{of}\text{one}{\text{O}}_2\text{molecule}=32\text{a}\text{.m}\text{.u}\times \frac{1.66\times {10}^{-24}\text{g}}{1\text{a}\text{.m}\text{.u}}=5.313\times {10}^{-23}\text{g}$