Question

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.

Answer 1

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%