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Understanding pH

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  1. Jan 2, 2016 #1
    Dear PF Forum,
    Actually I want to study how antioxidant can benefit for our health. But, my understanding in chemistry is very limited. Perhaps I have to go back and study pH, first.
    I have read many sources in the internet.
    And this is one of them:
    http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/what-is-pH.shtml
    So, let me understand this first.
    1. In 1 liter of water, there are approximately 1000/18.016 (weight of H2O, including isotope) = 55.51 moles of H2O. Is it true?
    2. For 1 liter water which have pH = 0 (if this is chemically possible) there are 1 moles H+ about 1 .008 gram?
    2. In 1 liter water pH = 1, there are 1/10 moles or 0.1 gram H+?
    3. Neutral water, pH = 7, there are 1 x 10-7 moles H+ or about 0.1 microgram H+?
    4. If number 3 is true. Why is it neutral? Because there are 1.7 microgram Hydroxide as well?

    Thanks for helping me. I might have some questions more to ask.
     
  2. jcsd
  3. Jan 2, 2016 #2

    BvU

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    Gold Member

    You've got it !
    1.yes
    2.yes. we wouldn't call it water any more because of it extreme acidity ... and because usually we also require a charge balance there have to be a lot of negative ions present too.
    2b.yes, same.
    3.yes
    4.yes

    It all boils down to an equilibrium between ##H_2O \leftrightarrows H^+ + OH^-## in the liquid. At room temperature the equilibrium constant ## k = {[H^+] \times [OH^-] \over [H_2O]} \approx 10^{-7}##.
    [edit] big mistake, see Borek #5 below (thanks !). Should be ##\approx 10^{-14}##
    There are some minor details, but you've got the idea.
     
    Last edited: Jan 2, 2016
  4. Jan 2, 2016 #3
    Thanks BvU for the answer.

    Okay, I'll requote the web again.
    http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/what-is-pH.shtml
    If pH is the concentration of hydrogen ions per liter, why pH 7 is neutral?
    There are 0.1 microgram H+ in it, right?
    Answer: Because there are 1.7 microgram OH as well.

    Okay..., perhaps I should ask again.
    What if in 1 liter water, there are free H+ about 0.1 microgram but the amount of OH- is not 1.7 microgram, but 0.85 microgram.
    What is its pH?
    Thanks for any answer.
     
  5. Jan 2, 2016 #4
    You need to remember that water need to be electrically neutral, so in *pure* water such situation in normal circumstances is *not* possible.
    Should other anions like Cl(-) be also present, to keep stuff neutral, such water would become slightly acidic.
    Equlibrium would be achieved and most of OH(-) would recombine with part of H(+) to reconstitute water.
    Some excess of H(+) would be left.
     
  6. Jan 2, 2016 #5

    Borek

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    Staff: Mentor

    Ain't gonna happen. BvU already mentioned why - there is an equilibrium between these things, given by

    H+ + OH- ↔ H2O

    And (approximately)

    [tex][H^+][OH^-] = 10^{-14}[/tex]

    (we ignore concentration of water as it is constant in most cases, and the value of 10-7 posted by BvU seems to be a typo). If you try to introduce amounts of H+ and OH- that don't fit this equation they will react till the equilibrium is reached.

    Neutral solution doesn't mean pH = 7, actually it means [H+] = [OH-]. This is typically close to 7, but the exact pH of the neutral solution depends on the temperature - see the table here: http://www.chembuddy.com/?left=pH-calculation&right=water-ion-product
     
  7. Jan 2, 2016 #6
    Ahh, so there you are. Borek. I would like to thank Ruud Gullit, in "Give Credits to Mentor", but I forgot your user name.
    Okay, let me summarize your reply.
    A. There is an equilibrium between these things, given by H+ + OH- ↔ H2O
    B. And (approximately) [H+][OH-] = 10-14
    So, let me ask again.
    This Equilibrium is not [H+] = [OH-], but [H+][OH-] = 10-14
    Supposed in 1 litre water there's
    1 * 10-4 moles H+
    1 * 10-6 moles OH-
    So the equilibrium is not H+ - OH-
    That H+ decreases by 0.000001 becomes 0.000099 and OH- decreases by 0.000001 all depleted, but.
    ([H+] - n) ([OH-] - n) = 10-14
    Using ABC equation formula, I find that n = 0.00000099989899.
    So H+ becomes: 0.00009900010101
    OH- becomes: 0.00000000010100999795
    Multiplying those numbers: 0.00009900010101 * 0.00000000010100999795 = 10-14

    Is equilibrium like this?

    C. Concentration of water it is constant in most cases
    D If you try to introduce amounts of H+ and OH- that don't fit this equation they will react till the equilibrium is reached.
    E. Neutral solution doesn't mean pH = 7, actually it means [H+] = [OH-].

    Please see B section: Is that how the equilibrium reached?
    Thanks for any help.
     
  8. Jan 2, 2016 #7

    Borek

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    Staff: Mentor

    Yes, you got it right.
     
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