Answer: It is found that for any aqueous solution (neutral, acidic or basic), the product of H+ ion concentration and OH- ion concentration is always constant and equal to 10-14 at 25°C.This is called ionic product of water(Kw)
Kw= [H+] [OH - ] = 10-14
Here The pH of hydrochloric acid solution is 3. Let OH- ion concentration = p
[H+] [OH - ] = 10-14
1.0 × 10-3 x p = 10-14
p = [10-14]¸[ 1.0 × 10-3] =1.0 × 10-11
Thus for a solution if H+ concentration is known, OH- ion concentration is fixed (because their product is always constant).Hence we express the acidic, basic or neutral character of any solution by expressing its H+ ion concentration in terms of pH where p stands for potenz ie power.