Chemistry Homework

Question

The combustion of 135 mg of a hydrocarbon produces 440 mg of CO₂ and 135 mg H₂O. The molar mass of the hydrocarbon is 270 g/mol. Determine the molecular formula of this compound.

Answer 1

Convert mols of CO2 to moles of carbon (C)

Convert mols of H2O to moles of hydrogen (H)

Determine empirical formula

Determine molecular formula

moles C: 440 mg CO2 x 1 g/1000 mg x 1 mol CO2/44 g x 1 mol C/ mol CO2 = 0.01 moles C

moles H: 135 mg H2O x 1 g/1000 mg x 1 mol H2O/18 g x 2 mols H/mol H2O = 0.015 mols H

Convert these to whole numbers by dividing both by 0.01 and then multiplying by 2...

moles C = 0.01 / 0.01 = 1 x 2 = 2 mols C

moles H = 0.015 / 0.01 = 1.5 x 2 = 3 mols H

Empirical formula = C₂H₃

molar mass of empirical formaul = 2x12 + 3x1 = 24 + 3 = 27 g/mol

Molecular formula:

270 g/mol / 27 g/ mol = 10

Molecular formula = C₂₀H₃₀