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ICSE Class 10 Chemistry Question 33 of 72

Mole Concept & Stoichiometry Miscellaneous Exercises — Question 33

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Question 32

A compound X consists of 4.8% carbon and 95.2% bromine by mass.

(i) Determine the empirical formula of this compound working correct to one decimal place (C = 12; Br = 80)

(ii) If the vapour density of the compound is 252, what is the molecular formula of the compound?

Answer
Element% compositionAt. wt.Relative no. of atomsSimplest ratio
carbon4.8124.812\dfrac{4.8}{12} = 0.40.40.4\dfrac{0.4}{0.4} = 1
bromine95.28095.280\dfrac{95.2}{80} = 1.191.190.4\dfrac{1.19}{0.4} = 2.9 = 3

Simplest ratio of whole numbers = C : Br = 1 : 3

Hence, empirical formula is CBr3

(ii) Empirical formula weight = 12 + 3(80) = 252 g

Molecular weight = 2 x V.D. = 2 x 252 = 504

n=Molecular weightEmpirical formula weight=504252=2\text{n} = \dfrac{\text{Molecular weight}}{\text{Empirical formula weight}} \\[0.5em] = \dfrac{ 504}{252} = 2

Molecular formula = n[E.F.] = 2[CBr3] = C2Br6

∴ Molecular formula = C2Br6