A mixture of gases O₂, H₂ and CO are taken in a closed vessel containing charcoal. The graph that represents the correct behaviour of pressure with time is :

Solution

Chemistry ma’am · 4 steps

Charcoal pulls gas molecules out of the space, so pressure falls fast at first and then levels off.

1. What charcoal does. Activated charcoal has a huge porous surface. It adsorbs gases on that surface (physisorption), so molecules leave the gas phase. Fewer free gas molecules in a closed vessel means lower pressure.
2. Shape of the fall. At the start almost all surface sites are empty, so adsorption is fast and the pressure drops steeply. As the sites fill up, adsorption slows and the curve flattens.
3. Why not zero. Adsorption is reversible. Once the rate of adsorption equals the rate of desorption, the pressure becomes constant. H₂, with a very low critical temperature, is adsorbed only weakly, so some gas always stays free.
4. So. The pressure falls steeply, then levels off at a value above zero. That is graph (c).

The idea to keep. Adsorption in a closed vessel always lowers the gas pressure, quickly at first and then more slowly, until it settles at an equilibrium value. Easily liquefied gases are adsorbed more than gases like H₂.

Answer from the NTA final answer key. Solution by PrepWiser.