Benzene is a hydrocarbon, a compound of carbon and hydrogen only. It is found to contain 7.74% H by mass. Its molar mass is 78.10 gmol^-1. Determine its empirical and molecular formula. Help?

The is ##(CH)_n## and the is ##C_6H_6##.

Since we know that a hydrocarbon only contains carbon and hydrogen, we can use the percent of hydrogen given to determine how much carbon the compound contains

##%"carbon" = 100%-7.74% = 92.26%##

The next step in determining the empirical formula is to divide each element's percentage by its

##"For C": (92.26%)/12.0 = 7.69##

##"For H": (7.74%)/(1.00) = 7.74##

You then divide each of these two numbers by the smallest one to get the ratios of the two in the molecule:

##"For C": 7.69/7.69 = 1##

##"For H": 7.74/7.69 ~= 1##

This is your empirical formula: ##(CH)_n## - the ratio between carbon and hydrogen atoms is ##1:1##. We now have to determine how many of each the molecule contains.

This is done by using benzene' molar mass to determine the value of ##n##:

##78.10 = (1 * 12.0 + 1 * 1.00) * n = 13.0 * n##

##n = 78.10/13.0 = 6##

Therefore, your molecular formula is ##C_6H_6##.