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Buffer Systems

  1. Apr 12, 2005 #1
    Here's my question:

    State whether each of the following solutions is a buffer system or not.

    a) KH2PO4/H3PO4
    H3PO4 is a weak acid
    H2PO4- is the conjugate base derived from the salt
    thus, this is a buffer system

    b) NaClO4/HClO4
    not a buffer system b/c HClO4 is a strong acid

    c) NH3/NH4N03
    not really sure about this one?
    NH3 is the base and NH4NO3 is the acid, i suppose
    but I'm not sure how to determine the relative strength/weakness of these.

    Could someone tell me whether these are correct or not? And please help me with part c?
     
  2. jcsd
  3. Apr 12, 2005 #2

    Borek

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    Staff: Mentor

    a, b OK.

    c - it is a buffer. pKb for ammonia is 4.75 IIRC so the pKa is 9.25 - for the reaction

    NH4+ <-> NH3 + H+
     
    Last edited by a moderator: Aug 13, 2013
  4. Apr 12, 2005 #3

    Gokul43201

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    Staff Emeritus
    Science Advisor
    Gold Member

    NH4NO3, by itself, is a salt and NH3 (or NH4OH) is the conjugate base derived from it.

    Edit : Oops ! Didn't see Borek's post. Nevermind.
     
  5. Apr 15, 2005 #4
    NH4N03 is weakly acidic and partially dissociates because NH4 itself is a weak electrolyte, remembering that NH3 is a weak base. Therefor [c] is also a buffer system.
     
  6. Apr 15, 2005 #5
    You need to use the Henderson Hasselbalch equation to find the pH of a buffer solution. There are two similar forms of the equation for acidic and basic buffers. Thats how you can determine the strength (if you call it so) of a particular solution.

    If you have an acid and its conjugate base, the buffer solution is called an acidic buffer. If you have a base and its conjugate acid, the buffer solution is called a basic buffer. As a base and its conjugate acid bear conjugate relationship, this is not really a great way to say it. However, buffers like ammonia/ammonium salt (nitrate, chloride etc) are basic buffers as ammonia for all practical purposes is an extremely weak acid and a strong base.

    Cheers
    Vivek
     
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