Lecture #5

# Lecture #5 - Chemistry 2A-c w2009(Ch3-5)31 III Composition...

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Chemistry 2A Chemistry 2A - c w2009 c w2009 (Ch3 (Ch3 - 5)31 5)31 Example : Given that a compound contains 44.9% K, 18.4% S, and 36.7% O by weight, calculate the empirical formula. Solution : # mol K = 44.9 g/(39.1 g/mol)=1.148 mol # mol S = 18.4 g/(32.066 g/mol)=0.574 mol # mol O = 36.7 g/(15.999 g/mol)=2.294 mol The empirical formula is K 2 SO 4 . III. % Composition and chemical formula Example : Given that a compound has the following % composition: 78.16% B and 21.84% H, calculate the empirical formula. If the molecular weight of this compound is 27.67 amu, what is the molecular formula? Solution : # mol B = 78.16 g/(10.811 g/mol)=7.23 mol # mol H = 21.84 g/(1.008 g/mol)=21.67 mol Empirical formula BH 3 ; formula weight = (10.811 + 3x1.008) amu =13.835 amu Molecular formula = B 2 H 6

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