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Chemistry

Calculate:

(a) A compound contains 69.5% oxygen, 30.5% nitrogen and its molecular weight is 92. Then what will be the molecular formula of the compound?

(b) If the empirical formula of a compound is CHO and its vapour density is 29, find the molecular formula of the compound.

Stoichiometry

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Answer

Element% compositionAt. wt.Relative no. of atomsSimplest ratio
O69.51669.516\dfrac{69.5}{16} = 4.344.342.17\dfrac{4.34}{2.17} = 2
N30.51430.514\dfrac{30.5}{14} = 2.172.172.17\dfrac{2.17}{2.17} = 1

Simplest ratio of whole numbers = O : N = 2 : 1

Hence, empirical formula is NO2

Molecular weight = 92

Empirical formula weight = 14 + 2(16) = 14 + 32 = 46

n=Molecular weightEmpirical formula weight=9246=2\text{n} = \dfrac{\text{Molecular weight}}{\text{Empirical formula weight}} \\[0.5em] = \dfrac{92}{46} = 2

Molecular formula = n[E.F.] = 2[NO2] = N2O4

Hence, molecular formula is N2O4.

(b) Empirical formula of the compound = CHO

Empirical formula mass = 12 + 1 + 16 = 29

Molecular formula mass = 2 x vapour density
= 2 x 29 = 58 g

Molecular formula = (Empirical formula)n

∴ Molecular formula = (CHO)2 = C2H2O2

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