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Weak acid - Weak base Buffer

  1. Oct 20, 2009 #1
    Hi !! i am new to these forums.. so i just want to ask whether i am posting my query in the correct forum??

    I have a doubt regarding this weak acid - weak base buffer.
    When the moles of Acid > Base,
    We use the formula pH = pka + log10 ([salt]/[acid]) to calculate the pH of the Acidic Buffer formed.

    Actually, in the derivation of this formula for calculating pH of an acidic buffer.. we assume 100% dissociation of salt..
    But here, the Weak acid- Weak base salt formed does not dissociate to full extent.

    Kindly explain to me why we are using this formula although the salt doesn't dissociate completely.
     
  2. jcsd
  3. Oct 20, 2009 #2

    Borek

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    Salt dissociation is complete. I suppose what you are really asking about is why we ignore hydrolysis of the acid. Wel... in general we shouldn't. But there is a wide area where this approximation works OK.

    --
     
  4. Oct 20, 2009 #3
    But I was taught that the salt of Weak acid- Weak base doesn't dissociate completely.

    And for this formula to be valid, we have to assume 100% dissociation of salt.
     
  5. Oct 20, 2009 #4

    symbolipoint

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    The point is that the anion of this salt came from a weak acid; therefore some hydrolysis(?) will occur and the anion of the weak acid will combine with some hydrogen ion from the water, reforming some of the molecular weak acid.
     
  6. Oct 20, 2009 #5
    i understood your point respected BOREK :)
    yeah i got it now that the salt gets completely dissociated in water..
    but the hydrolysis of acid is ignored.
    Thank u
     
  7. Oct 20, 2009 #6
    And i am sorry for saying that i was taught wrong.. It was mis-interpreted by me.
     
  8. Oct 20, 2009 #7

    symbolipoint

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    blitz.km,
    Did you tell us exactly what course you are studying? If you are in General Chemistry (first year college level rigorous course), then you might check an alternate book or two of General Chemistry. Also, or if you are also in "Quantitative" or Analytical Chemistry, you should check into a (possibly different) analytical or quantitative Chemistry textbook about neutralization titrations or weak acid and base equilibrium. Give yourself a large amount of time to learn this stuff, since it does not become learned quickly for most people.

    Did you yet check Borek's instructional information in his website?
     
  9. Oct 21, 2009 #8
    symbolipoint,

    I am still in school in class 12th. I am from India.
    I have taken up Physics, Chem. and Maths....i am yet to join college.
    We have been taught Ionic Equilibrium in school...but i just love to keep gathering more knowledge about the topics i learn.
    So i read a few books of my cousin who's in college. And had just a small query regarding this concept.

    I am still finding a reason why do salts of Weak acid- Weak base dissociate completely in water??
    (can u help me?)

    And yeah, i looked at some of the material uploaded by Borek, but not much bcoz i joined these forums just yesterday.
     
  10. Oct 21, 2009 #9

    Borek

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    Ionic salts usually dissociate 100% - that's just the way it is, that's how ionic salts dissolve. Water molecules are dipoles, so they are easily attracted by ions. Ions get surrounded by water molecules (it is called hydration) and water molecules separate them, lowering electrostatic force that keeps ions together in a crystal.

    What happens then in the case of weak acids is that they react with water molecules, hydrolysing and increasing solution pH:

    A- + H2O <-> HA + OH-

    --
     
  11. Oct 21, 2009 #10

    symbolipoint

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    Borek, I think your discussion is helpful. I am still confused a little about dissociation of sodium acetate. More to my way of thinking, this sodium salt is soluble and is present in solution as ions. Is this hydration? I guess for all my time studying, I am still lacking some finer, detailed understanding. The acetate hydrolizing with water to become protonated as the weak acid again, I understand... and then comes the somewhat difficult but learnable algebraic relationship simplifiable to a quadratic equation (Ka, Kw,).

    You probably have a more detailed and clearer understanding of this than most people.
     
  12. Oct 22, 2009 #11

    Borek

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    Sodium cation is surrounded by water molecules - they are dipoles, with oxygens being slightly negative, so they are electrostatically attracted and ordered around the Na+. That means two things - first, none of the anions can get close, second, water has a high dielectric constant, so the attraction between sodium cations and anions that even get close is much smaller than it would be if there were no water present. Number of water molecules surrounding the cation is called solvation number and can be determined experimentally.

    In the case of other metals situation can get more complicated, as water molecules are not only dipoles attracted just by electrostatic forces, but they are also quite good ligands, so they are able to complex many of the d-block metals. For example when we speak about complexation of copper by ammonia in water solution we really speak about water ligands being replaced by ammonia ligands. But that's another story (one that doesn't make it all easier to understand).

    Ka & Kw give cubic equation :wink:

    --
     
  13. Oct 22, 2009 #12
    Is solvation same as hydration ??
    If not, please explain Sir.
     
  14. Oct 22, 2009 #13

    Borek

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    Solvation is more general - it may happen in any solvent. Hydration is in water. So technically hydration is kind of a solvation.

    And don't sir me :wink:

    --
     
  15. Oct 22, 2009 #14
    Thank you. :)
     
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