[Boxxxed]

The combustion of 55.0 mg of a substance containing only C and H yields 185.9 mg of CO2 and 38.1 mg of H2O. An independent but imprecise method indicates the molecular mass is between 78 and 80 amu.

Determine both the empirical and molecular formulae of this substance.

I can calculate moles of C and H using the ratios and masses of CO2 and H2O. From moles of C and H I can determine the formulae. What do I need the 55.0 mg for?

[opti384]

You'll need it to determine the molecular formula from the empirical formula.

[Boxxxed]

I've calculated the empirical formula to be C2H.

The molar mass of C2H is 25 g/mole.

Do I just divide 80 amu/25 g/mol to get approximately 3.

Thus molecular formula is C6H3?

[Boxxxed]

Reworked,

there are equal moles of H and C so empirical formula is CH which would give the molecular formula of C6H6.

[Borek]

Much better.