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 = C2H2 Benzene = C6H6 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 H2O C2H4O2 CH2O C6H12O6 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 Calculate the molecular formula of a compound whose molar mass is 60.0 g/mol and the empirical formula is CH4N. 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 CH4N = 30 g/mol Steps to calculate molecular formula

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

Determine Molecular Formula Empirical formula x Ratio CH4N x 2 = C2H8N2 Molecular Formula