Question

The boiling point of 0.2 mol kg⁻¹ solution of X in water is
greater than equimolal solution of Y in water. Which one of
the following statements is true in this case ? [2015]
(a) Molecular mass of X is greater than the molecular mass
of Y.
(b) Molecular mass of X is less than the molecular mass of
Y.
(c) Y is undergoing dissociation in water while X undergoes
no change.
(d) X is undergoing dissociation in water.

Answer 1

Answer:

(a) is the answer

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Answer 2

The boiling point of 0.2 mol kg⁻¹ solution of X in water is  greater than equimolal solution of Y in water. X is undergoing dissociation in water.

• Option (d) is correct.
• we know that,
• molarity = moles of solute / mass of solution
• and
• given, molarity of X = 0.2 mol kg⁻¹  = molarity of Y
• given, ΔTb (X) > ΔTb (Y)
• where ΔTb = elevation in boiling point.
• we have,
• ΔTb =Kb m
• where,
• m = molality of the solution
• Kb = molal boiling point constant
• as ΔTb∝i
• where, i = vant Hoff factor
• we have,
• iΔTb =iKb m
• as ΔTb (X) > ΔTb (Y)
• ⇒ i(X) > i(Y)
• hence, X is undergoing dissociation in water.