A hydrocarbon (an organic compound containing only C and H) is found to be 7.74% H. If the molecular weight is known to be between 70 and 85 u, find the molecular formula.

The molecular weight is between 70 and 85. Let's accidentally pick some quantity in amid, like 78.

7.74% of 78 is how many 'u' there are of H:

78 u * 7.74% = 78 u *.0774 = 6.0372 u

The only hydrocarbon below that has 6 hydrogens is C6H6. We can even check this answer:

Total u = 6C + 6H

= 6 (12.01) + 6 (1.0079)

= 78.11 u

6 (1.0079) / 78.11 * 100 = 7.74%