Empirical and Molecular Formulas April 2016

Empirical vs Molecular Is the simplest (i.e. reduced) whole number ratio of atoms in a compound All formulas for ionic compounds are empirical Example: NaCl, not Na2Cl2 Not all correct formulas for covalent compounds are empirical Example: H2O2 Is the same or a multiple of the empirical formula It shows the actual number of atoms in a compound Example: N2O4 is the molecular formula for dinitrogen tetraoxide and it is double its empirical formula NO2

Complete the chart below: Empirical formula Molecular formula CO2 H2O2 C3H6 C2H4O2 SnO

To determine the empirical formula of a compound: You need to know the number of moles of each element present in the compound. Example: What is the empirical formula for the compound that contains: 1 mol S, 4 mol O and 2 mol Na? b) 4 mol C and 10 mol H c) 1 mol Cl and 2.5 mol O d) 0.4 mol C and 1.2 mol H

Empirical formulas can be determined from: A) Mass of elements in the compound Example 1: A sample of caffeine contains 24.13 g of carbon 2.51 g of hydrogen 14.09 g of nitrogen 8.02 g of oxygen

Example 2: A sample of a compound contains: 56. 4g phosphorus 43 Example 2: A sample of a compound contains: 56.4g phosphorus 43.6g oxygen

Empirical formulas can be determined from: B) Percent composition Example 3: A compound has the following % composition 21.6% sodium 33.3% chlorine 45.1% oxygen

Complete p. 102 #1 and 2 showing full solutions in the workbook.

Determining Molecular Formulas You need the empirical formula and the molar mass Example 1: The empirical formula of a compound is CH2 and its molar mass is 84.16 g/mol. What is its molecular formula? Example 2: The empirical formula for caffeine is C4H5N2O and its molar mass is 194.20 g/mol. What is its molecular formula?

Example 3: A compound has the following % composition 63. 13% carbon 8 Example 3: A compound has the following % composition 63.13% carbon 8.831% hydrogen 28.04% oxygen a) What is its empirical formula? b) If the molar mass of the compound is 114.14 g/mol, what is the molecular formula?

Example 4: A compound has the following % composition 85.63% carbon 14.37% hydrogen and a molar mass of 56.10 g/mol, what is the molecular formula?

Complete p. 104 #2-4 showing full solutions in the workbook.