The molar mass of dimethylmercury (CH3)2Hg is:
2(12.01 g/mol) + 12 x 1.008 g/mol + 200.59 g/mol = 230.66 g/mol
The number of moles in a 5.20-g sample of dimethylmercury is:
$$5.20 \text{ g} \times \frac{1 \text{ mol}}{230.66 \text{ g}} = 0.02257 \text{ mol}$$
The number of molecules in a 5.20-g sample of dimethylmercury is:
$$0.02257 \text{ mol} \times 6.022 \times 10^{23} \text{ molecules/mol} = 1.357 \times 10^{23} \text{ molecules}$$
Therefore, there are 1.357 x 10^23 molecules contained in a 5.20-g sample of dimethylmercury.
