Question

Combustion of 8.732 mg of an unknown organic compound gave 16.432 mg of CO2 and 2.840 mg of H2O. (a) find the wt% of C and H in the substance. (b) Find the smallest reasonable integer mole ration of C:H in the compound.

Answer 1

mass CO₂ = 16.432 mg

moles CO₂ = (mass CO₂) / (molar mass CO₂)

moles CO₂ = (16.432 mg) / (44.01 g/mol)

moles CO₂ = 0.373 mmol

moles Carbon in compound = moles CO₂

moles Carbon in compound = 0.373 mmol

mass carbon in compound = (moles Carbon in compound) * (molar mass carbon)

mass carbon in compound = (0.373 mmol) * (12.01 g/mol)

mass carbon in compound = 4.484 mg

wt % Carbon = (mass carbon / mass compound) * 100

wt % Carbon = (4.484 mg / 8.732 mg) * 100

wt % Carbon = 51.353 %

mass H₂O = 2.840 mg

moles H₂O = (mass H₂O) / (molar mass H₂O)

moles H₂O = (2.840 mg) / (18.0 g/mol)

moles H₂O = 0.158 mmol

moles H = 2 * (moles H₂O)

moles H = 2 * (0.158 mmol)

moles H = 0.315 mmol

mass H = (moles H) * (molar mass H)

mass H = (0.315 mmol) * (1.008 g/mol)

mass H = 0.318 mg

wt % Hydrogen = (mass H / mass compound) * 100

wt % Hydrogen = (0.318 mg / 8.732 mg) * 100

wt % Hydrogen = 3.640 %

(b) moles Carbon in compound = 0.373 mmol

moles Hydrogen in compound = 0.315 mmol

mole ratio C : H = (moles Carbon in compound) / (moles Hydrogen in compound)

mole ratio C : H = (0.373 mmol) / (0.315 mmol)

mole ratio C : H = 1.2

mole ratio C : H = 6 : 5