Question

How do you find molecular formula of a compound?

Answer 1

The molecular formula comes from the empirical formula and the molecular mass.

The empirical formula is the simplest whole number ratio between the elements in the compound.

The molecular formula is the actual whole number ratio between the elements in the compound.

EXAMPLE:

A compound with a molecular mass of 180.18 u contains 40.00% carbon, 6.72% hydrogen, and 53.28% oxygen. What is its molecular formula?

Solution:

A. Determine the empirical formula.

In 100.0 g of the compound, we have 40.00 g C, 6.72 g H, and 53.28 g O.

Moles of C: 40.00 g C × `"1 mol C"/"12.01 g C"` = 3.33 mol C

Moles of H: 6.72 g H × `"1 mol H"/"1.008 g H"` = 6.67 mol H

Moles of O = 53.28 g O ×`"1 mol O"/"16.00 g"` = 3.33 mol O

Ratios: C:H:O = 3.33:6.67:3.33 = 1.00:2.00:1 ≈ 1:2:1

The empirical formula is CH₂O.

B. Calculate the empirical formula mass.

CH₂O = (12.01 + 2.016 + 16.00) u = 30.03 u

C: Find the number of empirical formula units in the molecular formula.

Number of empirical formula units = `"180.18 u"/"30.03 u"` = 6.001 ≈ 6

D: Find the molecular formula.

Molecular formula = (CH₂O)₆ = C₆H₁₂O₆