How do you work out the molecular formula of a hydrocarbon?

There will be the same number of moles of C and H on both sides of the equation, so all the Carbon in your original hydrocarbon is now present in CO2, and all H is in the H2O.

Hydrogen Calculate moles of H in H2O molar mass H2O = 16.00 + (2 x 1.008) = 18.016 g/mol molar mass H = 1.008 g/mol

moles = mass / molar mass moles H2O = 0.002560 g / 18.016 g/mol = 1.421 x 10^-4 moles of H2O

In ever H2O there are 2 moles of H therefore moles of H in H2O, and thus in your original hydrocarbon = 2 x 1.421 x 10^-4 = 2.842 x 10^-4 moles of H

Carbon Calculate moles C in CO2 molar mass CO2 = (16.00 x 2) + 12.01 = 44.01 g/mol moles CO2 = mass / molar mass = 0.012507 g / 44.01 g/mol = 2.842 x 10^-4 moles of CO2 Every CO2 molecule has 1 Carbon, therefore moles of C in CO2 and thus in the original hydrocarbon = 2.842 x 10^-4 moles of C

Now work out the ratio of C : H C : H = 2.842 x 10^-4 : 2.842 x 10^-4 To get it into whole number divide both numbers in the ratio by the lowest number (both here are the same, but this will not always be the case C : H = (2.842 x 10^-4 / 2.842 x 10^-4) : (2.842 x 10^-4 / 2.842 x 10^-4) = 1:1

So empirical formula is CH

The molar mass of CH = 12.01 + 1.008 = 13.018 g/mol

To work out the molecular mass divide the molar mass of the actual compound by the molar mass of the empirical formula, this will tell you how may time the empirical formula “repeats in the molecular formula.

40 / 13.018 = 3.07

molecular formula = C3H3