Consider separate solutions of $0.500 \mathrm{M} \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{aq}), 0.100 \mathrm{M} \mathrm{Mg}_{3}(\mathrm{PO}_{4})_{2}(\mathrm{aq}), 0.250 \mathrm{M} \mathrm{KBr}(\mathrm{aq})$ and 0.125 $\mathrm{M} \mathrm{Na}_{3} \mathrm{PO}_{4}(\mathrm{aq})$ at $25^{\circ} \mathrm{C}$. Which statement is true about these solutions, assuming all salts to be strong electrolytes?

(C) They all have the same osmotic pressure.

(A) $0.125 \mathrm{M} \mathrm{Na}_{3} \mathrm{PO}_{4}(\mathrm{aq})$ has the highest osmotic pressure.

(B) $0.500 \mathrm{M} \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{aq})$ has the highest osmotic pressure.

(D) $0.100 \mathrm{M} \mathrm{Mg}_{3}(\mathrm{PO}_{4})_{2}(\mathrm{aq})$ has the highest osmotic pressure.

JEE Main · 2014 · 06 Apr 2014 · Offline · Solutions

Consider separate solutions of

0.500 M C_{2} H_{5} OH (aq), 0.100 M Mg_{3} (PO_{4})_{2} (aq), 0.250 M KBr (aq)

and 0.125

M Na_{3} PO_{4} (aq)

2 5^{\circ} C

. Which statement is true about these solutions, assuming all salts to be strong electrolytes?

Answer: (C)

Osmotic pressure is

π = i C R T

, so compare the product

i C

for each solution. Ethanol does not dissociate,

i = 1

, giving

1 \times 0.5 = 0.5

. Potassium bromide gives two ions,

2 \times 0.25 = 0.5

Mg X_{3} (PO X_{4}) X_{2}

gives five ions,

5 \times 0.1 = 0.5

Na X_{3} PO X_{4}

gives four ions,

4 \times 0.125 = 0.5

. Every one comes to the same

0.5 R T

, so all four solutions share the same osmotic pressure.

JEE Main · 2014 · 06 Apr 2014 · Offline · Single correct option