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.
<em>So, the empirical formula of the unknown hydrocarbon is CH₃.</em>
