Question

At 65°c, the ion-product constant of water, kw, is 1.20 ´ 10-13. the ph of pure water at 65°c is:

Answer 1

The ph of pure water at 65 c is 6.460  
According to Le Châtelier's Principle, if you make a change to the conditions of a reaction in dynamic equilibrium, the position of equilibrium moves to counter the change you have made. Hence, if you increase the temperature of the water, the equilibrium will move to lower the temperature again. It will do that by absorbing the extra heat. That means that the forward reaction will be favored, and more hydrogen ions and hydroxide ions will be formed. The effect of that is to increase the value of Kw as temperature increases.

Answer 2

When Kw = [OH-] [H+] & when we know that the pure water should has the concentration of H [ H+] = the concentration of OH [OH-]So
[H+]=[OH-]
∴ we can assume that Kw = [H+] [H+] or Kw = [OH-] [ OH-] or we can assume [H+] or [ OH-] as z
So, then Kw = Z^2

when we have the value of Kw so, by substitution:
Z^2 = 1.2 x 10 ^-13
Z = √ 1.2x10^-13
   =  3.46x 10^-7 
So we now get [H+] = 3.46X10^-7 
when PH = -㏒[H+]
∴ PH = -㏒(3.46x10^-7) = 6.46