Chemistry

A gaseous hydrocarbon weights 0.70 g. and contains 0.60 g. of carbon. Find the molecular formula of the compound if it's molecular weight is 70. [C = 12, H = 1]

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H = 0.70 - 0.60 = 0.1 g

Element	% composition	At. wt.	Relative no. of atoms	Simplest ratio
Carbon	$\dfrac{0.60}{0.70}$ x 100 = 85.71	12	$\dfrac{85.71}{12}$ = 7.142	$\dfrac{ 7.142}{7.142}$ = 1
Hydrogen	$\dfrac{0.1}{0.70}$ x 100 = 14.28	1	$\dfrac{14.28}{1}$ = 14.28	$\dfrac{14.28}{7.142}$ = 2

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

Hence, empirical formula is CH₂

Empirical formula weight = 12 + 2(1) = 14

Molecular weight = 70

$\text{n} = \dfrac{\text{Molecular weight}}{\text{Empirical formula weight}} \\[0.5em] = \dfrac{70}{14} = 5$

Molecular formula = n[E.F.] = 5[CH₂] = C₅H₁₀

Hence, Molecular formula = C₅H₁₀

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