Suppose a chemist has chosen ${10}^{20}$as the number of particles in a mole. Then the molecular mass of oxygen is

Molecular mass of oxygen is 32 g. means 32g oxygen contain 23 2 6.023 10 O  molecules.

$32\mathrm{g} \mathrm{of} {\mathrm{O}}_2 \mathrm{\_\_\_\_\_\_} 6.023 \times {10}^{23}{\mathrm{O}}_2 \mathrm{molecules}$

$\mathrm{MM} \mathrm{of} \hspace{0.17em}{\mathrm{O}}_2\mathrm{x}-----{10}^{20}{\mathrm{O}}_2 \mathrm{molecules}$

$\mathrm{x}\times 6.023\times {10}^{23}=32\times {10}^{20 }$

$\mathrm{x}\approx \frac{32\times {10}^{20}}{6.023\times {10}^{23}}$

$\mathrm{x}\approx 5.32\times {10}^{-3}\mathrm{g}$

Hence mass of ${10}^{20}$ molecules of ${\mathrm{O}}_2$ is $5.32\times {10}^{-3}\mathrm{g}.$