Question

Henry's law constant for oxygen and nitrogen dissolved in water at 298K are 2000000000Pa and 5000000000Pa , respectively. A sample of water at a temperature just above 273K was equilibrated with air (20% oxygen and 80% nitrogen) at 1 atm. The dissolved gas separated from a sample of this water and then dried. Determine the composition of this gas.

Answer 1

Henry's law can be put into mathematical terms (at constant temperature) as

$$p = k_{\mathrm{H}} c$$

where $p$ is the partial pressure of the solute in the gas above the solution, $c$ is the concentration of the solute and $k_{\mathrm{H}}$ is a constant with the dimensions of pressure divided by concentration.

So if partial pressure of oxygen is $(20\% / 100\%) = 0.2$ and for nitrogen it is 0.8, the concentrations of it can be found:

C for $\mathrm{O}_2$ is $\mathrm{p/K = 0.2 / 2000000\,kPa = 0.0000001\,M}$

C for $\mathrm{N}_2$ is $\mathrm{p/K = 0.8 / 5000000\,kPa = 0,00000016\,M}$

But for finding composition of final gas mixture you need to know mass of water sample or volume of starting gas mixture.