Question

A sample of a compound contains 30.0 g C and 5.05 g H. Its molar mass is 78.12 g/mol. What is the compound’s molecular formula?

Answer 1

The relative atomic mass of C is 12 g. So, to calculate how many atoms of C there are, we will divide 30 g by 12 g:
30 g ÷ 12 g = 2.5

The relative atomic mass of H is 1 g. So, to calculate how many atoms of C there are, we will divide 5.05 g by 1 g:
5.05 g ÷ 1 g = 5.05 ≈ 5

Now, let's calculate the ratio of C and H atoms:
$\frac{H-atoms}{C-atoms} = \frac{5}{2.5} =2$

This means there are 2 H atoms more than C atoms and therefore, the compound’s molecular formula is CH₂.