a chem question (20分)

Question

on complete combustion,150 cm3 of gaseous hydrocarbon forms 450cm3 of
carbon dioxide and 0.362 g of water.Assuming all volumes were measured at
S.T.P.,find the molecular formula of the hydrocarbon.

Answer 1

Denote the molecular formula of the hydrocarbon as CxHy­.
CxHy + (x + y/4)O2 → xCO2 + (y/2)H2O
Molar volume of a gas at s.t.p. = 22400 cm3 mol-1
Molar mass of H2O = 2(1) + 16 = 18 g mol-1

No. of moles of CxHy = vol/(molar vol) = 150/22400 = 0.0067 mol
No. of moles of CO2 = vol/(molar vol) = 450/22400 = 0.0201 mol
No. of moles of H2O = mass/(molar mass) = 0.362/18 = 0.0201 mol

Mole ratio CxHy : CO2 = 1 : x = 0.0067 : 0.0201
Hence, x = 3
Mole ratio CxHy : H2O = 1 : y/2 = 0.0067 : 0.0201
Hence, y = 6

Ans: The molecular formula of the hydrocarbon is C3H6.