Choose the correct statement about Henry's constant $(K_H)$.
$I$. $K_H$ increases with increase in temperature generally.
$II$. $K_H$ decreases with increase in temperature generally.
$III$. $K_H$ is the characteristic constant of the gas-solvent system.

$K_{H}$ values for $H_2, CO_2, CH_4$ and $O_2$ at $298 \ K$ are $71.18, 1.67, 41.85$ and $34.86 \ kbar$ respectively. Arrange these gases in the increasing order of their solubility.

The oxygen dissolved in water exerts a partial pressure of $20 \ kPa$ in the vapour above water. The molar solubility of oxygen in water is ............ $\times 10^{-5} \ mol \ dm^{-3}$ (Round off to the Nearest Integer). [Given: Henry's law constant $K_H = 8.0 \times 10^{4} \ kPa$ for $O_2$,Density of water $= 1.0 \ kg \ dm^{-3}$]

Observe the following data given in the table $(K_H = \text{Henry's law constant})$. The correct order of solubility of these gases is:

Gas	$K_H$ (kbar at $298 \ K$)
$CO_2$	$1.67$
$Ar$	$40.3$
$HCHO$	$1.83 \times 10^{-5}$
$CH_4$	$0.413$

$A$ gas $X$ is dissolved in water at $2 \ bar$ pressure. Its mole fraction in the solution is $0.02$. Find the mole fraction of water in the solution when the pressure of the gas is doubled at the same temperature.

Calculate the solubility of a gas in water at $0.8 \ atm$ and $25^{\circ} C$. [Henry's law constant is $6.85 \times 10^{-4} \ mol \ dm^{-3} \ atm^{-1}$]