1. Starter  Finish the study guide from yesterday

2. Ch. 7B Using Chemical Formulas 7B.4 Determining Molecular Formulas

3. Molecular Formulas  the molecular formula is the actual formula for a compound  could be same as the empirical formula but doesn’t have to be same  to find molecular formula: need empirical formula need empirical formula need actual molar mass (or formula mass) need actual molar mass (or formula mass)  compare the molar mass of empirical formula to actual molar mass

4. Example 1  In the last example the empirical formula was found to be P 2 O 5. Experimentation shows that the molar mass is actually 283.89 g/mol. Find the molecular formula.

5. Example 1  Find molar mass of empirical formula 2(30.9738) + 5(15.9995) = 141.9446  Divide the actual molar mass by the empirical formula’s molar mass 283.89 / 141.9446 ≈ 2  Multiply this number by each subscript in empirical formula P 4 O 10

6. Example 2  Use the percent composition to write the empirical formula: 41.39 % C, 3.47 % H, and 55.14 % O / 3.443 = 1

7. Example 2  empirical formula: CHO  If the actual molecular mass is 116.07 g/mol, what is the molecular formula? Find the molar mass of CHO Find the molar mass of CHO 1(12.011) + 1(1.00794) + 1(15.9994) = 29.018 Compare it to the molecular mass Compare it to the molecular mass 116.07 / 29.018 = 4 Multiply that by subscripts Multiply that by subscripts 4(CHO) = C 4 H 4 O 4

8. Example 3  The empirical formula of a compound is C 2 H 5 and its formulas mass is 87 amu. What is the molecular formula?  empirical formula mass= 2(12.011) + 5(1.00794) = 29.0617 amu  87 / 29.0617 = 3  molecular formula = C 6 H 15