Calculating Molecular Formulas Chapter 7 3 Empirical formula

Calculating Molecular Formulas Chapter 7 -3

• Empirical formula vs Molecular formula • Empirical Formula: a formula which has been reduced to the lowest terms. • Molecular formula: a formula of a compound in which the subscripts give the actual number of each element in the formula

Molecular Formulas • Empirical formula vs Molecular formula • How to calculate molecular formulas • Ethyne and benzene have the same empirical formula CH but have a different molecular formula • Ethyne = C 2 H 2 Benzene = C 6 H 6 • These two compounds have different molar masses. • The molecular formula can be determine if you know the empirical formula and the molar mass of the compound.

Here are four formulas being used as examples: Molecular Formula Empirical Formula H 2 O C 2 H 4 O 2 CH 2 O C 6 H 12 O 6 CH 2 O Notice two things: 1. The molecular formula and the empirical formula can be identical. 2. You scale up from the empirical formula to the molecular formula by a whole number factor.

Sample Problem • Steps to calculate molecular formula • Calculate the molecular formula of a compound whose molar mass is 60. 0 g/mol and the empirical formula is CH 4 N. 1. Calculate the molar mass of the empirical formula. C = 1 x 12 = 12 H=4 x 1 = 4 N = 1 x 14 = 14 Molar mass CH 4 N = 30 g/mol

2 Find the ratio using molar masses Molar mass compound ( given) Ratio = Molar mass empirical formula 60 g/mol Ratio = = 30 g/mol 2

3 Determine Molecular Formula Empirical formula x Ratio CH 4 N x 2 = C 2 H 8 N 2 Molecular Formula