Step 1: Understanding the Question:
We need to find the solubility ($S$) of a gas using Henry's Law.
Step 2: Key Formula or Approach:
Henry's Law states:
\[ S = K_H \times P \]
where $S$ is solubility, $K_H$ is Henry's constant, and $P$ is the partial pressure of the gas.
Step 3: Detailed Explanation:
Given:
$P = 0.8$ atm
$K_H = 6.8 \times 10^{-4}$ mol $dm^{-3}$ $atm^{-1}$
\[ S = (6.8 \times 10^{-4})\text{ mol }dm^{-3}\text{ atm}^{-1} \times 0.8\text{ atm} \]
\[ S = 5.44 \times 10^{-4}\text{ mol }dm^{-3} \]
Since 1 mol $dm^{-3} = 1$ M,
\[ S = 5.44 \times 10^{-4}\text{ M} \]
Step 4: Final Answer:
The solubility of the gas is $5.44 \times 10^{-4}$ M.