Question

A sample of a compound contains 32.0 g C and 8.0 g H. Its molar mass is 30.0 g/mol. What is the compound’s molecular formula

Answer 1

The compound's molecular formula is C2H6. This is obtained by:
  
            mass             moles               divided by smallest moles
C          32g         32/12 = 2.67                                 1
H           8g           8/1.01 = 7.92                        approx. 3

Next, divide both terms by the smallest number of moles, 2.67. This gives 1 and 3. So the empirical formula is CH3 which has a molar mass of 15g/mol. Given the molar mass of the molecular formula as 30g/mole, we can calculate the factor by which to multiply the subscripts of CH3.

X = molar mass of molecular formula / molar mass of empirical formula = 30/15
X=2

So (CH3)2 is C2H6.