4 Empirical FormulaThe lowest whole number ratio of elements in a compound
5 Calculate empirical formula from percent composition: 1st - Begin by pretending to have 100 g sample & convert the % tograms.2nd - Convert grams to the moles by dividing by molar mass.3rd - Obtain whole number ratios of moles by:1) dividing each number by the smallest number2) multiplying to get rid of any fractions4th - Write the empirical formula using the number of moles assubscripts.
6 Example 1Calculate the empirical formula of a compound composed of % C, % H, and %N.Assume 100 g so38.67 g C / 12 g/mol C = mole C16.22 g H / 1.07g/mol H = mole H45.11 g N / 14 g/mol N = mole NDivide each by the smallest value
7 Example 1 cont. 3.220 mole C / 3.22 = 1 16.09 mole H / 3.22 = 5 3.22 mole N / = 1The empirical formula is CH5N
8 Example 2A compound is % P and % O. What is the empirical formula?43.6 g P /31 g/mol = 1.4 mole P /1.4 = 156.4 g O /16 g/mol = 3.5 mole O /1.4 = 2.5Divide by smallest numberMultiply the result to get rid of any fractions.P1O2.5= P2O52 X
9 Molecular Formulas Are whole number multiples of empirical formulas. Divide molar mass of compound by molar mass of empirical formula.Multiply subscripts in compound by the number from step 1.Ex. An empirical formula is CH5N.The molar mass is 93 g/mol.93 g/mol / 31 g/mol = 33 x CH5N = C3H15N3