Empirical and Molecular Formula

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