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Chemistry

A gaseous hydrocarbon contains 82.76% of carbon. Given that it's vapours density is 29, find it's molecular formula.

[C = 12, H = 1]

Stoichiometry

ICSE 2016

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Answer

Element% compositionAt. wt.Relative no. of atomsSimplest ratio
Carbon82.761282.7612\dfrac{82.76}{12} = 6.896.896.89\dfrac{6.89}{6.89} = 1
Hydrogen17.24117.241\dfrac{17.24}{1} = 17.2417.246.89\dfrac{17.24}{6.89} = 2.5

Simplest ratio of whole numbers = C : H = 1 : 2.5 = 2 : 5

Hence, empirical formula is C2H5

Empirical formula weight = 2(12) + 5(1) = 29

V.D. = 29

Molecular weight = 2 x V.D. = 2 x 29

n=Molecular weightEmpirical formula weight=2×2929=2\text{n} = \dfrac{\text{Molecular weight}}{\text{Empirical formula weight}} \\[0.5em] = \dfrac{2 \times 29}{29} =2

∴ Molecular formula = n[E.F.] = 2[C2H5] = C4H10

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