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Henry's law

At a constant temperature, the amount of gas that dissolves in a liquid is directly proportional to the partial pressure of that gas in equilibrium with the liquid.

P = KH  C   
    for example:                take P =  1 atmosphere = 1 * 10^5 Pascal.
                                    KH = Henry's constant.    C = concentration of gas in liquid.
Hcl    C =  1 * 10^-5 M
         pH  =  - log C    (logarithm to base 10)
               =  - log  10^-5  = 5