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Adding a weak acid to a strong acid

  1. Feb 20, 2004 #1


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    If a weak acid is added to a strong acid, does it make the solution more acidic or less acidic? Assuming that the weak acid is not one of those ions that can turn into a stronger acid or a stronger base (like H2CO3 <--> HCO3 <--> CO3). I was thinking something like adding ethanoic acid to hydrochloric acid.

    I'm not sure if it would make the solution stronger or weaker. I was thinking that maybe the strong acid would make the weak acid behave a little more like a strong base since weak acids are always weak bases. When weak bases are added to strong acids, they behave like strong bases (and vice versa).

    I just need the answer; no expanation is needed.
  2. jcsd
  3. Feb 20, 2004 #2


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    In a aqueous solution the solution would become more acidic since both acids have free energies of dissociation (forget the latter part if you have not reached the topic of free energy). Both ethanoic acid and HCl will contribute to decrease pH.

    weak acids have strong conjugate bases. In this case ethanoic acid does not really fit the description since its base is stabilized by resonance. Nevertheless it is still classified as a weak acid due to its relative pH.
  4. Mar 24, 2009 #3
    The weak acid would act sort of as a weak base in presence of a strong acid. When such two acids are mixed the pH of the mixture will be lower than the strong acid and higher than the weak acid.
    Answer provided by:Gopi Patel, Jen Serafin, Patric Kozy
  5. Mar 24, 2009 #4


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    Um, wannabpharm, you do know you just replied to a post that's five years old?
  6. Mar 24, 2009 #5
    You mean the opposite, that is lower than that in the weak acid and higher than that in the strong acid?
  7. Mar 24, 2009 #6


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    After five years Science needs to resolve this question!

    I think if you mix the two it will give you a pH almost the same as if you put the same volume of water as of weak acid, but will be very slightly more acid than that. In other words the effect is essentially no different from dilution.

    Instead of complicating by dilution let us just compare a given concentration of strong acid with that same concentration of the strong acid but the solution also containing weak acid. (I am assuming the weak acid not extremely more concentrated than the weak.) Now it is true that, compared with when the weak acid was on its own, dissociated ions e.g. Ac- will take on protons to form HAc. But it cannot take on more protons than it releases into the water in the solution of weak acid of that concentration! Not more, so a small bit less, that little bit is the extra protons added to the solution of strong acid when the weak one is added ideally without dilution.

    It is instructive to make a full calculation. I found, subject to correction:uhh:, that when the strong and the weak acids are equimolar the [H+] is higher than that of the strong acid alone by 3Ka .
  8. Mar 25, 2009 #7


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    Must be something wrong, or it works only for some range of concentrations. See attached image. Both acids 10-3M, [H+]=1.017x10-3, that's 1.7x10-5 more than concentration from HCl alone - so Ka, not 3Ka. But it seems like this is accidental, different values for different concentrations.

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