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ICSE Class 10 Chemistry Question 20 of 37

Mole Concept and Stoichiometry — Question 11

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Question 11

An organic compound contains 4.07% hydrogen, 71.65% chlorine and remaining carbon. Its molar mass is 98.96. Find its,

(a) Empirical formula

(b) Molecular formula

Answer
Element% compositionAt. wt.Relative no. of atomsSimplest ratio
Hydrogen4.0714.071\dfrac{4.07 }{1} = 4.074.072.01\dfrac{4.07 }{2.01} = 2
chlorine71.6535.571.6535.5\dfrac{ 71.65}{35.5} = 2.012.012.01\dfrac{2.01}{2.01 } = 1
Carbon24.281224.2812\dfrac{24.28 }{12} = 2.022.022.01\dfrac{ 2.02 }{2.01} = 1

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

Hence, empirical formula is CH2Cl

Empirical formula weight = 12 + (2 x 1) + 35.5 = 49.5

molar mass = 98.96

n=Molecular weightEmpirical formula weight=98.9649.5=1.99=2\text{n} = \dfrac{\text{Molecular weight}}{\text{Empirical formula weight}} \\[0.5em] = \dfrac{98.96}{49.5} = 1.99 = 2

So, molecular formula = 2(CH2Cl) = C2H4Cl2