1

### JEE Main 2019 (Online) 11th January Evening Slot

K2Hgl4 is 40% ionised in aqueous solution. The value of its van't Hoff factor (i) is:
A
1.6
B
2.2
C
2.0
D
1.8

## Explanation

K2Hgl4 is 40% ionised.

$\therefore$ $\alpha$ = ${{40} \over {100}}$ = 0.4

K2[Hgl4] $\to$ 2K+ + [Hgl4]2+

N = ${{2 + 1} \over 1}$ = 3

i = 1 + (N - 1)$\alpha$

= 1 + (3 - 1)0.4

= 1 + 2$\times$0.4

= 1.8
2

### JEE Main 2019 (Online) 12th January Morning Slot

Freezing point of a 4% aqueous solution of X is equal to freezing point of 12% aqueous solution of Y. If molecular weight of X is A, then molecular weight of Y is -
A
4A
B
2A
C
3A
D
A

## Explanation

For same freezing point,

($\Delta$Tf)X = ($\Delta$Tf)Y

$\Rightarrow$ kf mx = kf my

$\Rightarrow$ ${{4 \times 1000} \over {A \times 96}} = {{12 \times 1000} \over {M \times 88}}$

$\Rightarrow$ M = 3.27A $\simeq$ 3A
3

### JEE Main 2019 (Online) 12th January Evening Slot

Molecules of benzoic acid (C6H5COOH) dimerise in benzene. 'w' g of the acid dissolved in 30 g of benzene shows a depression in freezing point equal to 2K. If the percentage association of the acid to form dimmer in the solution is 80, then w is – (Its given that Kf = 5 K kg mol–1, Molar mass of benzoic acid = 122 g mol–1)
A
1.5 g
B
1.8 g
C
1.0 g
D
2.4 g

## Explanation We know,

$\Delta$Tf = i Kf $\times$ ${w \over M} \times {{1000} \over {{w_s}}}$

i = 1 + $\alpha$$\left( {{1 \over n} - 1} \right)$

Here Benzoic acid dimerise, so value of n = 2

$\therefore$  i = 1 + 0.8 $\left( {{1 \over 2} - 1} \right)$

= 1 $-$ 0.4

= 0.6

$\therefore$  2 = 0.6 $\times$ 5 $\times$ ${w \over {122}} \times {{1000} \over {30}}$

$\Rightarrow$  w = 2.44 g
4

### JEE Main 2019 (Online) 12th January Evening Slot

If Ksp of Ag2CO3 is 8 $\times$ 10–12, the molar solubility of Ag2CO3 in 0.1 M AgNO3 is -
A
8 $\times$ 10–12 M
B
8 $\times$ 10–10 M
C
8 $\times$ 10–13 M
D
8 $\times$ 10–11 M

## Explanation $\therefore$   8 $\times$ 10$-$12 = (2s + 0.1)2 s

$\Rightarrow$   s $\times$ 10$-$2 = 8 $\times$ 10$-$12

$\Rightarrow$   s = 8 $\times$ 10$-$10