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pH of water - ionisation product of water question Watch

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    Hi guys,

    Just wondering whether anyone could explain this to me... my textbook says:

    In pure water [H+(aq)] = [OH-(aq)] which I get, it's neither acidic or alkaline

    Kw = 1x10^-14 mol^2dm^-6 which I get because it stated that at 298K Kw = [H+][OH-] so Kw is usually about 1x10^-14 mol^2dm^-6 at 298K

    I then says (and this is the bit I don't understand) Kw = [H+(aq)]^2
    Why is this so and where did it come from?

    Thanks in advance for any help
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    (Original post by Blake Jones)
    Hi guys,

    Just wondering whether anyone could explain this to me... my textbook says:

    In pure water [H+(aq)] = [OH-(aq)] which I get, it's neither acidic or alkaline

    Kw = 1x10^-14 mol^2dm^-6 which I get because it stated that at 298K Kw = [H+][OH-] so Kw is usually about 1x10^-14 mol^2dm^-6 at 298K

    I then says (and this is the bit I don't understand) Kw = [H+(aq)]^2
    Why is this so and where did it come from?

    Thanks in advance for any help
    In pure water there is one H+ for every OH- ion, so [H+] = [OH-]
    thus: Kw = [H+]^2
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    (Original post by Blake Jones)
    Hi guys,

    Just wondering whether anyone could explain this to me... my textbook says:

    In pure water [H+(aq)] = [OH-(aq)] which I get, it's neither acidic or alkaline

    Kw = 1x10^-14 mol^2dm^-6 which I get because it stated that at 298K Kw = [H+][OH-] so Kw is usually about 1x10^-14 mol^2dm^-6 at 298K

    I then says (and this is the bit I don't understand) Kw = [H+(aq)]^2
    Why is this so and where did it come from?

    Thanks in advance for any help
    As if [H+] = [OH-]
    And Kw = [H+][OH-]
    Then multiplying by [OH-] would be the same as multiplying by [H+]

    Thus Kw = [H+]2

    Does that make sense now?
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    (Original post by Qinge)
    In pure water there is one H+ for every OH- ion, so [H+] = [OH-]
    thus: Kw = [H+]^2
    Oh! Because there are 2H atoms effectively so the concentration is double so then it goes to squared as it's a concentration, got it! Thanks mate!
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    (Original post by KaylaB)
    As if [H+] = [OH-]
    And Kw = [H+][OH-]
    Then multiplying by [OH-] would be the same as multiplying by [H+]

    Thus Kw = [H+]2

    Does that make sense now?
    Yeah I get that now, thank you so much you guys! Don't know why I didn't see that before! XD
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    (Original post by Blake Jones)
    Yeah I get that now, thank you so much you guys! Don't know why I didn't see that before! XD
    No problem! :hat2:
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    (Original post by Blake Jones)
    Oh! Because there are 2H atoms effectively so the concentration is double so then it goes to squared as it's a concentration, got it! Thanks mate!
    That makes it look like you didn't get it.

    If you had some water at 298 K, the pH would be 7.00

    [H+] = 10-pH = 10-7.00 = 0.0000007 mol dm-3

    The only way you can make H+ ions from water also makes OH- ions. So the concentration of OH- ions must also = 10-7.00. Both of them have that concentration, neither have double that value.

    Kw = [H+] x [OH-] = 10-7.00 x 10-7.00 = [H+]2
 
 
 
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