Answer
The partial pressure of $O_2$ is equal to 0.716 atm.
Work Step by Step
1. Determine the molarity of the solution:
$ O_2 $ : ( 16.00 $\times$ 2 )= 32.00 g/mol
$$\frac{5.00 \space mg \space O_2}{100.0 \space mL \space soln} \times \frac{1000 \space mL}{1 \space L} \times \frac{1 \space g}{1000 \space mg} \times \frac{1 \space mol \space O_2}{32.00 \space g \space O_2} = 1.56 \times 10^{-3} \space M$$
2. Apply Henry's Law:
$$P_{gas} = \frac Ck = \frac{1.56 \times 10^{-3} \space M}{2.18 \times 10^{-3} M/atm} = 0.716 \space atm$$
