Question

Boiling point of a 2% aqueous solution of a non-volatile solute A is equal to the boiling point of 8% aqueous solution of a non-volatile solute B. The relation between molecular weights of A and B is

• A. $M_A = 4M_B$
• B. $M_B = 4M_A$
• C. $M_A = 8M_B$
• D. $M_B = 8M_A$

Answer 1

Answer: (B)

$M_B = 4M_A$

Answer 2

$(ΔT_b)_A = (ΔT_b)_B$
$K_bM_A = K_bM_B$
$⇒ M_A = M_B$

$⇒ \frac{\frac{2}{M_A}}{100}×1000 = \frac{\frac{8}{M_B}}{100} × 1000$

$⇒ M_B = 4M_A$

The correct answer is (B): $M_B = 4M_A$