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Can hydrochloric acid be basic?

  1. Feb 27, 2012 #1
    Suppose you have a HCl solution of 2.00 M
    Now suppose you keep on adding water, with no limit on how much water you are adding.

    We will reach a point where the HCl concentration is so low that if you take the negative log of the [H+] , you get a number greater than 7.

    Doesn't this mean that we essentially have a "basic" HCl solution? But I thought that was impossible.

    What's the catch here?


  2. jcsd
  3. Feb 27, 2012 #2


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    Staff: Mentor

    Water autodissociation.
  4. Feb 27, 2012 #3
    1) I'm not really a chemist but I am confident no. If you have such little HCl in water that the concentration of HCl is effectively zero, the H+ concentration is not zero. Water has its own self ionization that would yield ~7.

    Basically this question is, is HCl basic if there is no HCl in solution? This is wrong because ANY amount of acid added on top of water is slightly more acidic than water.

    2) HCl might be basic if you use an even stronger acid as a solvent. For example, sulfuric acid and nitric acid to add nitro groups.
  5. Feb 27, 2012 #4
    Oh I forgot that if you add water you are adding both H+ and OH- due to the water's ionization.

    So if you had a limited amount of HCl and added infinite water, the pH would approach 7 from the left side of the number line?

    Is it possible to model the pH as a function of the volume of water added, assuming you know the concentration of the HCl initially and assuming that initially all the HCl ionizes to completion?

    Thanks for your replies! This site is the best!

  6. Feb 28, 2012 #5


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    Staff: Mentor


    Yes. See pH of a strong acid, equation 7.4 - you will just need to calculate analytical acid concentration (taking dilution into account).
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