JEE Main 2021 · 25 February · morning · Single correct · +4 / −1
In Freundlich adsorption isotherm at moderate pressure, the extent of adsorption ( x/m ) is directly proportional to Pˣ. The value of x is :
- (a) ∞
- (b) zero
- (c) 1
- (d) 1/n
Answer and solution
Answer (d) 1/n
Solution
Chemistry ma’am · 3 steps
Freundlich sits between the two pressure extremes, so its power of P is a fraction between 0 and 1.
- Freundlich equation. Freundlich described adsorption with an empirical power law, where k and n are constants for a given adsorbent and gas at a fixed temperature, and n > 1.
x/m = k P^(1/n)
Pressure range How x/m varies Power of P Low x/m ∝ P¹ 1 Moderate x/m ∝ P^(1/n) 1/n, between 0 and 1 High x/m ∝ P⁰, constant 0 - Why a fraction. At low pressure the rise is linear. At high pressure the surface is saturated and adsorption stops changing. At moderate pressure the curve bends between these two, so the power lies between 1 and 0.
- So. At moderate pressure x/m ∝ P^(1/n), so x = 1/n, which is option (d).
The idea to keep. Remember the three powers of P: 1 at low pressure, 0 at high pressure, and 1/n (between 0 and 1) in between. The Freundlich curve lives in that middle range.
Answer from the NTA final answer key. Solution by PrepWiser.