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What is difference between pH and pKa , pOH and pKb?

  1. Oct 25, 2016 #1
    • You have to show your attempts at answering the question, this is a forum policy. This is just a case of checking definitions.
    Do pH and pKa refer to the same concept? The same question for pOH and pKb also.
    What does kWh actually mean. In have read that it is the ionic product and is the product of [H+] and [OH-] but what does it indicate. Does it have any full form?
     
  2. jcsd
  3. Oct 25, 2016 #2
    This is not a complete answer, but pH and pOH are more or less inverse. The lower case p referred to "partial pressure of", and H vs OH was for Hydrogen and Hydroxide (acid vs base). pKa has come along much later, and is generally considered to be a better measure of acidity, but I can't define it better than that. Both pH and pKa were originally intended to measure "acidity", but as the definition of acid morphed, the measurement preference changed from pH to pKa.

    -Jeff
     
  4. Oct 25, 2016 #3
    pH and pKa measure different things.
    pH is a property of a specific solution. pKa is a fundamental property of a substance.
    The definition is that pKa is the pH at which half of the acid is dissociated.
     
  5. Oct 25, 2016 #4

    epenguin

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    Not very surprisingly pKa has the same relation to pH as Ka has to H - or really to [H+]...

    I have only seen the constant that you wrote written as Kw. It is defined as the product of [H+] and [OH-], which is constant for aqueous solutions (whatever the pH).

    But this is not the place to find out about these things - they are in any elementary physical chemistry textbook and loads of online sources with the same coverage. Read a textbook and if you have any difficulties, If something does not make sense to you or you can't do the problems (having tried) then come here. We can only supplement textbooks we cannot write them, we cannot really be as good as they are.
     
  6. Oct 28, 2016 #5

    James Pelezo

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    Since pH is such a dominate reference for acidity and alkalinity in many areas of science it may be instructive to review the pH values and ranges of different chemical systems in nature. Here is a reference sheet I've passed out to my classes over the years and find it is a good basis for many interesting discussions. It is not by any means complete, but represents some common systems the student may have heard about.


    pH values and ranges for natural systems.*

    Classification of soil pH ranges*

    The United States Department of Agriculture Natural Resources Conservation Service, formerly Soil Conservation Service classifies soil pH ranges as follows: [17]
    upload_2016-10-28_14-6-31.png

    Living systems*
    upload_2016-10-28_14-6-31.png

    Seawater
    See also: Ocean acidification

    The pH of seawater is typically limited to a range between 7.5 and 8.4.[18] It plays an important role in the ocean's carbon cycle, and there is evidence of ongoing ocean acidification caused by carbon dioxide emissions.[19] However, pH measurement is complicated by the chemical properties of seawater, and several distinct pH scales exist in chemical oceanography.[20]

    *From => https://en.wikipedia.org/wiki/PH
     
  7. Oct 28, 2016 #6
  8. Oct 28, 2016 #7

    Borek

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    Why would negative pH complicate anything?
     
  9. Oct 28, 2016 #8
    It would if you were taught, and 'believe', that the pH range goes from 0 to 14
     
  10. Oct 28, 2016 #9

    Bystander

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    "pH = 0" corresponds to what concentration?
     
  11. Oct 28, 2016 #10
    Are you asking me because you would like to know?
     
  12. Oct 28, 2016 #11

    Bystander

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    What's the pH of a 10 M HCl solution?
     
  13. Oct 28, 2016 #12
    I think that's a bit condescending and patronizing. I don't do pissing contests. I'm sorry I entered your domain of homework helper and science advisor. I suggest you google the answer, or better yet, try to work it out for yourself.
     
  14. Oct 28, 2016 #13

    jim mcnamara

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    @Bystander
    FWIW - negative pH is a real value and has use in Soil Science for example.

    pubs.acs.org/doi/pdf/10.1021/ed083p1465
     
  15. Oct 29, 2016 #14

    Borek

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    Sure, but that's just a common misconception, that probably comes from the fact people don't care about how the logarithm works. Sadly, in popular thinking math is considered as not needed for chemistry, add to that fact that most people ignore the obvious connections, and you have a disaster waiting to happen. I still don't consider negative pH a complication - it is a way people ignore what they already know that produces problems.

    And Bystander was just trying to force you to find out by yourself that pH values outside of 0-14 are obvious when you combine the scale definition with real life examples, nothing condescending nor patronizing about it. It is called a Socratic method and that's a way we work here.
     
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