Question

A compound is composed of 85.6% Carbon and the rest is Hydrogen. The molecular mass of the compound is 42.078g/mol. What is the molecular formula for the compound?

Answer 1

The molecular formula for the compound is  $C_{3} H_{6}$

Explanation:

As with all of these problems, we assume 100 g of an unknown compound.

And thus, we determine the elemental composition by the given percentages.

Moles of carbon = 85.64 / 12.011

                            = 7.13 mol.

Moles of hydrogen = 14.36 / 1.00794

                               = 14.25 mol.

There are 2 moles of hydrogen per mole of carbon. And thus the empirical formula is CH$_{2}$.

And molecular formula = n × (empirical formula)

Thus, 42.08 = n × (12.011 + 2 × 1.00794)

And thus n = 3, and molecular formula = $C_{3} H_{6}$