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

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

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 g/mol. Find the molecular formula.

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

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

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

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( ) = amu  87 / = 3  molecular formula = C 6 H 15