How many molecules are contained in a 7.20-g sample of dimethylmercury?

The formula for dimethyl mercury is. HgC2H6 = (2x12) + (6x1) + (1x200.6) = 230.6

So the molar mass of dimethyl mercury is 230.6 g/mol.

Number of moles in 4.2g of dimethymercury = 4.2 / 230.6 = 0.0182 moles.

1 moles of dimethymercury contains 6.02 * 10^23

Hence 0.0182 moles contains X

X = 0.0182 * 6.02 * 10^23 = 0.10952 * 10^23 = 1.0952 * 10^22.