Empirical and Molecular Formulas Honors Chemistry Mrs. Coyle

Empirical Formula A chemical formula that represents the simplest whole number ratio of atoms in a compound.

Problem # 1 Determine the empirical formula of C4H10 . Answer: C2H5

Molecular Formula The exact formula of a molecule. (Empirical Formula)n n= Molar Mass Empirical Formula Mass

Problem # 2 Determine the empirical and molecular formulas of a compound that has a percent composition by mass of 43.64% phosphorous and 56.36% oxygen and a formula mass of 283.88 g.

Problem # 2 Solution Assume 100g of the compound. #mol P= 43.64 g P x 1 mol P=1.409 mol P 30.97g P #mol O= 56.36g O x 1 mol O= 3.523 mol O 16.00 g O

Problem # 2 Solution Cont’d Find the mole ratio of the larger to the smaller number of moles. 3.523 mol O = 2.5 mol O 1.409 mol P 1 mol P Simplest whole number ratio= 5 mol O 2 mol P So, empirical formula: P2O5

Problem # 2 Solution Cont’d Molecular formula calculation: n=Molar Mass______ = 283.88 g = 2 Empirical Formula Mass 141.94 g So, Molecular Formula = (P2O5)2 P4O10