Question

A 3.00 g sample of unknown hydrocarbon is prepared for combustion analysis. after the hydrocarbon undergoes complete combustion, 8.80 g of co2 and 5.40 g of h2o are produced. what is the empirical formula of the unknown?

Answer 1

Answer:

CH₃.

Explanation:

• The complete combustion of hydrocarbon in an excess of oxygen produces CO₂ and H₂O according to the general reaction equation:

CₐHₓ + O₂(excess) → b CO₂ + c H₂O,

Where a is the no. of C atoms in the unknown hydrocarbon.

x is the no. of H atoms in the unknown hydrocarbon.

b is the no. of moles of CO₂ produced from the combustion of unknown hydrocarbon.

c is no. of moles of H₂O produced from the combustion of unknown hydrocarbon.

• We can calculate b and c using the relation:

n = mass / molar mass.

• n of CO₂ = (8.80 g) / (44.0 g/mol) = 0.2 mol.

Every molecule of CO₂ contains 1 C atom, so no. of moles of C atoms in CO₂ and thus in the original hydrocarbon is 0.2 mol.

• n of H₂O = (5.40 g) / (18.0 g/mol) = 0.3 mol.

Every molecule of H₂O contains 2 H atoms, so no. of moles of H atoms in H₂O and thus in the original hydrocarbon is (2 x 0.3) 0.6 mol.

• The ratio of C to H in the unknown hydrocarbon is (0.2): (2 x 0.3) = 0.2: 0.6.

• Dividing by the lowest no. of moles (0.2) that of C.

∴ The ratio of C to H in the unknown hydrocarbon is 1: 3.

So, the empirical formula of the unknown hydrocarbon is CH₃.