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Which Of The Following Statements Are Correct?

  1. Sep 20, 2004 #1
    Hello People,

    I wasn't quiet sure bout this question so i thought i'd ask it here aswell!!

    The question:

    The ionisation constant of water has the values 1x10^-14 mol2dm-6 at 25 C
    and 1.7x10^-14 mol2dm-6 at 75 C,which of the following statements are correct:

    a)the pH of water at 25 C is equal to that at 75 C
    b)water ionises more at 75 C than at 25 C
    c)at 75 C water contains more H+ ions than OH- ions
    d)the ionisation of water is an endothermic process?
  2. jcsd
  3. Sep 20, 2004 #2
    Hi Jack,


    [tex] K_w=[H^+][OH^-][/tex]

    Let´s calculate the pH of pure water. The charge balance tells us that [H] must be equal to [OH] (theese systems are allways neutral), so:
    [tex] K_w=[H^+][OH^-][/tex]

    We solve:


    We see, If Kw increases, [H] increases, then pH decreases. pH of hot water is lower than pH of cold water. "a" is incorrect.

    "b" is correct, water ionises more at 75 than at 25. "c" is incorrect, the charge balance must be satisfied.

    Normally if a process is enhanced by temperature increase, that process is endotermic. I would say that "d" is correct (I can´t assure that).
  4. Sep 20, 2004 #3


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    Science Advisor
    Gold Member

    Hello, I agree with altered-gravity. He's made a good explanation indeed. Water will always be neutral; since 14 becomes 13,6 and pH will be the middle point of 13,6, for example.

    My physical chemistry is not very good, but his explanation is logical about the last one. Increasing temperature (and thus, more thermal energy) is used by the system to increase the physical constants. If it would be an exothermic system, the process would be slowed down as you increase the temperature.

    Regards, chem_tr
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