Answer
$\text { (a) } 2.3 \times 10^{-4}$
Work Step by Step
Recall Henry’s law: $$x_{CO_2}=\frac{p_{CO_2}}{H}.$$ Given $H =12.8\text{ MPa}=12.8\times 10^6\text{ Pa}$, total air pressure $100\text{ kPa}=0.1\text{ MPa}$ and the $CO_2$ mole fraction in air $3\%=0.03$, we have: $$p_{CO_2}=0.03\times 0.1\text{ MPa}=0.003\text{ MPa}.$$ Apply Henry's law: $$x_{CO_2}=\frac{0.003}{12.8}\approx 2.3\times 10^{-4}.$$ The correct answer is (a).
