Empirical and Molecular Formula Review

Empirical and Molecular Formula

Determine the chemical formula from the percentage composition or the molar mass.

Follow the Rhyme to get it every time

Change percent(%) to mass (g)

Convert mass to moles (g --> mol)

Divide by the smallest

Multiply til whole.

Sample Problem

An unknown compound is analyzed and found to contain 24.3% carbon, 4.1% hydrogen, and 71.6% chlorine. If the molecular mass of the compound is 98.8g/mol, what is the compounds empirical formula and molecular formula?

Change percent to grams

24.3g C

4.1g H

71.6g Cl

Convert grams to moles

24.3g C x 1 mol/12.01g = 2.02

4.1g H x 1 mol/1.01g = 4.06

71.6g Cl x 1 mol/35.45g = 2.02

Divide by the smallest

24.3g C x 1 mol/12.01g = 2.02/2.02 = 1

4.1g H x 1 mol/1.01g = 4.06/2.02 = 2

71.6g Cl x 1 mol/35.45g = 2.02/2.02 = 1

Multiply til whole (if necessary)

24.3g C x 1 mol/12.01g = 2.02/2.02 = 1

4.1g H x 1 mol/1.01g = 4.06/2.02 = 2

71.6g Cl x 1 mol/35.45g = 2.02/2.02 = 1

C₁H₂Cl_{1 -->} CH₂Cl is the empirical formula (Ef)

Find the molar mass of the empirical formula

CH₂Cl

C - 1 x 12.01 amu = 12.01

H - 2 x 1.01 amu = 2.02

Cl- 1 x 35.45 amu = 35.45

__________________

49.48 amu

Divide the masses Mf/Ef

98.8 amu/49.48 amu = 2

Multiply the integer by the empirical to get the molecular formula

(CH₂Cl)2 = C₂H₄Cl_{2 is the molecular formula (Mf)}

Try this one on your own!

Naphthalene, commonly known as moth balls, is composed of 93.7% carbon and 6.3% hydrogen. The molecular mass is 128g/mol. Determine the empirical and molecular formulas for naphthalene.

Answers

Ef = C₅H₄

Mf = C₁₀H₈