Fish generally need an O2 concentration in water of at least 4 mg/L for survival. What partial pressure?

Question

2.Fish generally need an O2 concentration in water of at least 4 mg/L for survival. What partial pressure of oxygen above the water (in atmospheres at 0°C) is needed to obtain this concentration? The solubility of O2 in water at 0 °C and 1 atm partial pressure is 2.21 x 10-3 mol/L.

Answer 1

First change 4 mg/L to mol/L, by dividing 0.004 g by the weight of 1 mol O2. Call this FishMol.
Henry's Law says solubility of a gas in a liquid is directly proportional to partial pressure of the gas over the liquid, so divide FishMol by the solubility (2.2*10^-3 mol/L-Atm) and you get the required O2 partial pressure in Atm.
The answer could be 0.05656 but I could be lying or wrong.

Answer 2

2e-3 mol dissolved O2/ L = 1 atm O2 gas (conversion factor in a moment) (4 mg / L) / (16 g / mol) = 0.25 mmol O2 / L (0.25 mmol O2 / L) (1 atm O2 gas / 2e-3 mol O2/ L) = 0.125 atm O2 gas