To calculate the number of molecules in a 7.85-g sample of dimethylmercury, we need to determine the number of moles of dimethylmercury in the sample and then multiply that by Avogadro's number (6.022 × 10²³ molecules/mol).

1. Calculate the number of moles of dimethylmercury in 7.85 g.

The molar mass of dimethylmercury (CH₃)₂Hg is approximately 230.63 g/mol.

Number of moles of dimethylmercury = Mass / Molar mass

= 7.85 g / 230.63 g/mol

= 0.034 mol

2. Multiply the number of moles by Avogadro's number.

Number of molecules = Number of moles × Avogadro's number

= 0.034 mol × 6.022 × 10²³ molecules/mol

= 2.06 × 10²² molecules

Therefore, there are approximately 2.06 × 10²² molecules in a 7.85-g sample of dimethylmercury.