Question

A liquid extracted from petroleum contains 92.3% carbon, the rest being hydrogen. Its molar mass is around 80 g/mol. Determine the empirical formula and molecular formula of the liquid.

Answer 1

Assume we have 100 grams of the liquid.
This gives us:
92.3 grams of Carbon 
7.7 grams of hydrogen
So those are the approximate amounts of each element we have in 100 grams of the liquid. Now, we have to find out how many moles of hydrogen and carbon are in 100 grams of the liquid, using the molar masses of carbon (12.01 grams / mole), and the molar mass of hydrogen (1.01 grams / mole)
If we divide the number of grams by the molar mass, we get the number of moles of each element:
#moles of carbon = (92.3 grams)/(12.01 grams/mole) = 7.69 moles of carbon
#moles of hydrogen = (7.7 grams)/(1.01 grams/mole) = 7.62 moles of hydrogen
To find the ratio of carbon moles to hydrogen moles, we divide the number of moles of carbon by the number of moles of hydrogen:
(7.69 moles of carbon)/(7.62 moles of hydrogen) = around 1, since they have to have a whole number ratio.
We know that the empirical formula of the substance is 1 carbon and 1 hydrogen, so the empirical formula is CH. 
To find the molar mass of the empirical formula, it is a sum of their atom masses:
(12.01 grams/mole) + (1.01 grams/mole) = 13.02 grams/mole
We know that the actual compound is around 80 grams per mole, so we can just divide the actual compound's molar mass of 80 grams per mole by our empirical formula's molar mass of 13.02 grams per mole to get our ratio:
(80 grams/mole)/(13.02 grams/mole) = 6.1, which we need to round to the nearest whole number, meaning that the ratio between the empirical formula and molecular formula is 6:1, so we have to multiply each component in our empirical formula (CH) by six. This means our molecular formula is C6H6.