Question

A compound of carbon and hydrogen contains 85.63% C and has a molar mass of 56.10 g/mol. What is its molecular formula?

Answer 1

The empirical formula of the compound is calculated as follows:

Given that  

It contains 85.63% carbon,  and 100-85.63=  14.37 % hydrogen  

now assume that the amount of compound is100 g thus there is 85.63 g C and  14.37 g H

NOW  divide these percentages by the molar mass of the elements to get a number of moles for each substance.  

NUMBER OF MOLES = AMOUNT IN G / MOLAR MASS

Carbon: 85.63/12 = 7.1  

Hydrogen: 14.37/1 = 14.3  

Now arrange these in a ratio as follows:  

C:H

7.1/7.1 :14.3/7.1

1:2

divide through by the smallest number

Therefore, the empirical formula is CH2

Then calculate the molar mass of that molecule:  

(1x12) + (2x1) = 14

Here the molar mass of compound is  56.10 g/mol

Since, 14 goes into 56.10 g/mol  4 times thus the  molecular formula is : C4H8