Ncert Exercises & Solutions

2.41 Determine the osmotic pressure of a solution prepared by dissolving 25 mg of K₂SO₄ in 2 litre of water at 25° C, assuming that it is completely dissociated.

When K₂SO₄ is dissolved in water, $K^{+} and {SO}_{4}^{2 -}$ ions are produced.

$K_{2} {SO}_{4} \to 2 K^{+} + {SO}_{4}^{2 -}$

Total number of ions produced = 3

$∴$ i =3

Given, w = 25 mg = 0.025 g

V = 2 L

T = $25 ° C$ = (25 + 273) K = 298 K Also,

we know that:

R = 0.0821 L atm K^-1mol^-1

M = (2 × 39) + (1 × 32) + (4 × 16) = 174 g mol^-1

Appling the following relation,

$π = i \frac{n}{v} RT$
$= i \frac{w}{M} \frac{1}{v} RT$
$= 3 \times \frac{0.025}{174} \times \frac{1}{2} \times 0.0821 \times 298$
$= 5.27 \times 10^{- 3} atm$

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