How do you find molecular formula of a compound?

Question

Wyatt Atkins · Accepted Answer

The empirical formula and the molecular mass are the sources of the molecular formula.

The most straightforward whole number ratio between the components of the compound is the empirical formula.

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

AN EXAMPLE

What is the molecular formula of a compound that has a molecular mass of 180.18 u and contains 40.00% carbon, 6.72% hydrogen, and 53.28% oxygen?

Resolution:

A. Find the formula for empirical data.

The compound has 40.00 g C, 6.72 g H, and 53.28 g O in 100.0 g.

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

C:H:O = 3.33:6.67:3.33 = 1.00:2.00:1 ≈ 1:2:1 are the ratios.

CH3O is the empirical formula.

B. Determine the mass empirical formula.

u = 30.03 u, where CH₂O = (12.01 + 2.016 + 16.00).

C: Determine the molecular formula's empirical formula unit count.

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

D: Determine the chemical formula.

Formula for molecules: (CH₂O)₆ = C₆H₁₂O₆

Mateo Aguilar · Answer

undefined A compound's molecular formula can be obtained by first determining its molar mass and empirical formula. Then, you can find the ratio between the two by dividing the compound's molar mass by the empirical formula's molar mass, and finally multiplying the empirical formula's subscripts by this ratio.