How do you find the molecular formula of a compound?

With an example it can be explained.

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:

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.

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

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

Find the molecular formula.

Molecular formula = (CH₂O)₆ = C₆H₁₂O₆

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