What are the molecular formula of the compound?

Question

an unknown liquid is composed of 5.57%H, 28.01% CL, and 66.42%C. The molecular mass found by mass spectrometry is 126.58 amu. What are the  molecular formula of the compound?

Answer 1

The first thing you want to do is find the ratios of the number of atoms of each element.  To do that, divide each mass percentage by the atomic mass of the element:

 

H:  5.57% ÷ 1.01 = 5.51

Cl:  28.01% ÷ 35.45 = 0.79

C:  66.42% ÷ 12.01 = 5.53

 

H and C are almost the same, so their ratio is 1:1.  5.53 / .79 = 7, so the ratio of C and H to Cl is 7:1

 

So we can start with a formula of C₇H₇Cl

 

Now find the total molecular mass of C₇H₇Cl:

 

H: 1.01 * 7 = 7.07

Cl: 35.35 * 1 = 35.45

C: 12.01 * 7 = 84.07

 

TOTAL:  126.59

 

The total is almost exactly the same as the molecular mass of the substance, so the formula C₇H₇Cl is correct.