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24 April, 21:42

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?

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  1. 24 April, 22:09
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    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.
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