(i) HCHO < CH4< CO2 < Ar
(ii) HCHO < CO2< CH4< Ar
(iii) Ar < CO2< CH4< HCHO
(iv) Ar < CH4 < CO2 < HCHO
Correct Answer: (iii) Ar < CO2< CH4< HCHO
Explanation: When the value of KH increases, solubility of the gas at a given pressure decreases.