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Question

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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Answer

H = 0.70 – 0.60 = 0.1 g

Element% compositionAt. wt.Relative no. of atomsSimplest ratio
Carbon(0.60/0.70) × 100 = 85.711285.71/12 = 7.1427.142/7.142 = 1
Hydrogen(0.1/0.70) × 100 = 14.28114.28/1 = 14.2814.28/7.142 = 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

n = Molecular\ weight\over Emperical\ Formula\ Weight

= 70\over 14 = 5

Molecular formula = (CH2)5 = C5H10

Hence, Molecular formula = C5H10

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