Calculating Empirical Formulas

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Empirical and Molecular Formulas
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Presentation transcript:

Calculating Empirical Formulas Calculate the empirical formula of a compound from its composition data. Determine the mole ratio of each element from the mass to determine the formula of radium oxide, Ra?O?. A 1.640 g sample of radium (Ra) metal was heated to produce 1.755 g of radium oxide (Ra?O?). What is the empirical formula? 1.755 of Ra?O? -1.640 g Ra = 0.115 g O.

Calculating Empirical Formulas The molar mass of radium is 226.03 g/mol and the molar mass of oxygen is 16.00 g/mol. 1 mol Ra 226.03 g Ra 1.640 g Ra × = 0.00726 mol Ra 1 mol O 16.00 g O 0.115 g O × = 0.00719 mol O Ra0.00726O0.00719. Simplify the mole ratio by dividing by the smallest number. Ra1.01O1.00 = RaO is the empirical formula.

Empirical Formulas from Percent Composition Use percent composition data to calculate empirical formulas. Assume that you have 100 grams of sample. Benzene is 92.2% carbon and 7.83% hydrogen, what is the empirical formula. If we assume 100 grams of sample, we have 92.2 g carbon and 7.83 g hydrogen.

Empirical Formulas from Percent Composition Calculate the moles of each element: 1 mol C 12.01 g C 92.2 g C × = 7.68 mol C 1 mol H 1.01 g H 7.83 g H × = 7.75 mol H The ratio of elements in benzene is C7.68H7.75. Divide by the smallest number to get the formula. 7.68 C = C1.00H1.01 = CH 7.75 H

Molecular Formulas n = 6 and the molecular formula is C6H6. = CH (CH)n The empirical formula for benzene is CH. This represents the ratio of C to H atoms of benzene. The actual molecular formula is some multiple of the empirical formula, (CH)n. Benzene has a molar mass of 78 g/mol. Find n to find the molecular formula. n = 6 and the molecular formula is C6H6. = CH (CH)n 78 g/mol 13 g/mol