Liquid Solution

Q: The ratio of the value of any colligative property for K₄ [Fe(CN)₆] solution to that of Fe₄[Fe(CN)₆]₃ (prussian blue), solution is nearly

(A) 1

(B) 0.71

(C)1.4

(D) Less than 1

Sol:

Vont’s Hoff factor (i) for K₄[Fe(CN)₆] is 5 (assuming complete ionization)

K₄[Fe(CN₆]    →  4K⁺   +  [Fe(CN)₆]^-4

↓                          ↓

1 molecule       4 cations      1 anion

Similarly, i for Fe₄[Fe(CN)₆]₃ is 7

As we know, colligative properties ∝ i

$\frac{i_{K_4[Fe(CN)_6}]}{i_{Fe_4[Fe(CN)_6}]_3} = \frac{Colligative \; Properties \; of \; K_4[Fe(CN)_6]}{Colligative \; Properties \; of \; Fe_4[Fe(CN)_6]_3} = \frac{5}{7} =0.71$

Ans: (B)

Q: A solution containing 28 g phosphorous in 315 g CS₂ (b.p. = 46.3°C) boils at 47.9°C (K_b for CS₂ is 2.34). What will be molecular formula of phosphorus? (assuming complete association)

(A) P₄

(B) P₈

(C) P₂

(D) None

Sol: Intermolecular forces among molecules of n-heptane and ethyl alcohols are weaker than among hexane and among ethyl alcohol molecules  ∴ V.P. of solution is more than V.P. of pure solvents.

Ans: (B)

Q: At 334 K the vapor pressure of benzene (C₆H₆) is 0.526 atm and that of toluene (C₇H₈) is 0.188 atm. In a solution containing 0.500 mole of benzene and 0.500 mole of toluene, what is the vapor pressure of toluene above the solution \at 334 K ?

(A) 0.188 atm

(B) 0.10 atm

(C) 0.357 atm

(D) 0.094

Sol: P_T = P⁰_T × X_T

= 0.188 × 0.5

= 0.094

 Ans: (D)

Q: At 25°C the vapor pressure of benzene, C₆H₆ (78g/mole), is 93.2 Torr and the of toluene, C₇H₈ (92 g/mol), is 28.2 Torr. A solution of 1.0 mole of C₆H₆ and 1.0 mol of C₇H₈ is prepared. Calculate the mole fraction of C₆H₆ in the vapor above this solution (assume the solution is ideal).

(A) 0.607

(B) 0.768

(C) 0.232

(D) 0.393

Sol: $Y_B = \frac{P_B^o \times X_B}{P} $

$Y_B = \frac{93.2 \times 0.5}{P} $

$Y_T = \frac{28.2 \times 0.5}{P} $

$\frac{Y_B}{Y_T} = 3.3 $

And, Y_B + Y_T = 1

On solving y_B = 0.768

Ans: (A)

Q: Benzene (C₆H₆, 78 g/mol) and tolune (C₇H₈, 92 g/mol) form an ideal solution. At 60°C the vapor pressure of pure benzene and pure toluene are 0.507 atm and 0.184, respectively. The mole fraction of benzene in a solution of these two chemicals that has a vapor pressure of 0.350 atm at 60°C

(A) 0.514

(B) 0.690

(C) 0.486

(D) 0.190

Sol: 0.35 = 0.507 × X_B + 0.184 × X_t    ….(i)

X_B + X_t = 1                   ….(ii)

On solving equation (1) and (2)

X_B = 0.514

Ans:  (A)