7-4: Determining Chemical Formulas -Define empirical formula, and explain how the term applies to ionic and molecular compounds -Determine an empirical formula from either a percentage or a mass composition -Explain the relationship between the empirical formula and the molecular formula of a given compound -Determine a molecular formula from an empirical formula

 Empirical formula: consists of the symbols for the elements combined in a compound, with subscripts showing the smallest whole-number mole ratio of the different atoms in the compound  For an ionic compound: the formula unit is usually the compound’s empirical formula  For a molecular compound: the empirical formula does not necessarily indicate the actual numbers of atoms present in each molecule  Ex: Gas diborane BH 3 B 2 H 6 empirical formula molecular formula

Calculation of Empirical Formulas Percentage composition  mass composition  composition in moles  smallest whole-number mole ratio of atoms

Sample Problem

A)A compound is found to contain 63.42% iron and 36.48% sulfur. Find its empirical formula.  FeS B)Determine the empirical formula of the compound that contains 17.15% carbon, 1.44% hydrogen, and 81.41% fluorine.  CHF 3 You Try!

Sample Problem

A)Analysis of a 20.0 g of a compound containing only calcium and bromine indicates that 4.00 g of calcium are present. What is the empirical formula of the compound formed?  CaBr 2 B)A g sample of an unidentified compound contains g sodium, g chromium, and g oxygen. What is the compound’s empirical formula?  Na 2 Cr 2 O 7 You Try!

Calculation of Molecular Formulas

Sample Problem

A)Determine the molecular formula of the compound with an empirical formula of CH and a formula mass of amu. C6H6C6H6 B)A sample of a compound with a formula mass of amu is found to consist of 0.44 g H and 6.92 g O. Find its molecular formula. H2O2H2O2 You Try!

1.Determine the empirical formula of a compound found to contain 52.11% carbon, 13.14% hydrogen, and 34.75% oxygen. 2.What is the molecular formula of the molecule that has an empirical formula of CH 2 O and a molar mass of g/mol? 3.What is the empirical formula of a molecule containing 65.5% carbon, 5.5% hydrogen, and 29.0% oxygen? 4.If the molar mass of the compound in problem 3 is 110 g/mol, what is the molecular formula? 5.What is the empirical formula of a molecule containing 18.7% lithium, 16.3% carbon, and 65.0% oxygen? 6.If the molar mass of the compound in problem 5 is 73.8 g/mol, what is the molecular formula? Practice Problems