States of Matter - Result Question 93

####93. A hydrocarbon contains 10.5g of carbon per gram of hydrogen. 1L of the vapour of the hydrocarbon at 127C and 1atm pressure weighs 2.8g. Find the molecular formula of the hydrocarbon.

(1980, 3M)

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Solution:

  1. First we determine empirical formula as
C H
Weight 10.5 1
Mole 10.512=0.875 1
Simple ratio 1 1/0.875=1.14
Whole no. 7 8

Empirical formula =C7H8

From gas equation : pV=(wM)RT

M=wRTpV=2.8×0.082×4001×1=91.8492

Molar mass (M) is same as empirical formula weight.

Molecular formula = Empirical formula =C7H8



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