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Identify if the given salt is acidic or not

  1. Dec 10, 2011 #1
    I have been coming across some questions which asks to identify whether the given salt is acidic, basic or neutral.
    I solve them like this:-
    For example the salt is NaCl, the answer is neutral as NaOH and HCl are strong base and strong acid respectively.

    But for problems like KF:-
    I was confused on this one. I thought that KOH is a strong base and about HF i didn't know anything about it. I was not able to think whether HF is strong or weak acid.

    Is there any way to find whether the given acid or base is strong or weak?

    Thanks!
     
  2. jcsd
  3. Dec 10, 2011 #2
    HF is a moderate acid (which some authors will consider it weak), so KF is basic.

    There are no definite rules for determining whether an acid is weak or strong. For mineral acids containing oxygen and with a single central atom, you may use Pauling's rule:

    number of oxygens - number of hydrogens

    HClO4 → 4 - 1 = 3 (very strong acid)
    H2SO4 → 4 - 2 = 2 (strong acid)
    H2SO3 → 3 - 2 = 1 (moderate acid)
    H3BO3 → 3 - 3 = 0 (weak acid)

    Notice, however, that this is a very limited rule, and is mostly a rule of thumb than a precise statement of an acid's strength.

    I usually rely on previous knowledge for all the other acids. Carboxylic acids are weak, except when the carbon near the carboxyl is bonded to an electronegative group. I know the strength of other acids by practice.
     
  4. Dec 10, 2011 #3

    Borek

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    Staff: Mentor

    Simplest approach is to remember strong acids - everything you don't remember is weak.
     
  5. Dec 10, 2011 #4
    :rofl: True. That works too.
     
  6. Dec 10, 2011 #5
    I will go with this one. :rofl:

    Thanks about the info! :smile:

    Can you give me some examples of strong carboxylic acids?
     
  7. Dec 11, 2011 #6
    What about the H+ ion concentration theory?
     
  8. Dec 11, 2011 #7

    Borek

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    Staff: Mentor

    Please elaborate, no idea what theory you mean. H+ concentration is a property of a solution, not of an acid. It is useless in the context of predicting acid strength.
     
  9. Dec 11, 2011 #8
    Arrhenius Theory, that a strong acid produces more H+ ions on dissociation(yes, in form of solution in water) than a weak acid. In case of bases it is the OH concentration which is taken into account.
     
  10. Dec 11, 2011 #9
    But here that's not going to be useful for me as i don't know which solution releases more H+ ions.
    All the acids release H+ ions but my job is to find out which one is stronger.
    There's a small article too "Determining acid strength" on wikipedia, that may be useful.
     
  11. Dec 12, 2011 #10

    Borek

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    Staff: Mentor

    And as you were told, it is useless when it comes to predicting acid strength.

    Besides, I have two solutions here - solution of acetic acid with pH 2.4 and solution of hydrochloric acid with pH 4. Following your suggestions as concentration of H+ in acetic acid is higher, it must be a stronger acid than hydrochloric.

    The only problem is - it isn't.

    You are again commenting on things you understand only partially, and you are again confusing others. Please stop.
     
  12. Dec 12, 2011 #11
    I apologize. English is not my first language. I meant [H+] ,the brackets shows mol/litre concentration, or in other words: A strong acid completely ionize in aqueous solution while a weak acid doesn't.
    And I said it in general, to the statement that there is no definite rule. My perception was different. The dissociation constant certainly tells about the acid/base being strong or weak in comparison with any other acid/base.
     
    Last edited: Dec 12, 2011
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