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Creating a Buffer

  1. Apr 28, 2015 #1
    1. The problem statement, all variables and given/known data

    Using sodium citrate and citric acid, create a buffer solution that will withstand an addition of 20ml of 0.1mol/L HCl or NaOH solution without a significant increase in pH (+/-2). Bonus marks for using the smallest amount of buffer solution.

    2. Relevant equations

    Reaction: H3C6H5O7 ↔ 3H+ + C6H5O73-

    ka of Acetic Acid = 7.4x10-4

    pka of Acetic Acid = 3.13

    ph = pka + log[A-/HA]

    3. The attempt at a solution

    I'm not sure where to begin. The Henderson-Hasselbalch equation above might be needed, but our teacher never showed us it, so most likely we have to create this solution without it. My understanding is that a solution needs to be created with the same concentration of hydronium ions from the citric acid as with the concentration of citrate, the conjugate base.

    Also, citric acid apparently is triprotic, losing 3 hydrogens in water. However, it is a weak acid, meaning it does not ionize completely. What is my first step in solving this overall question?

    Update:

    I used the H-H equation after receiving info that the pka and pH in buffer solutions must be equal. Making the two equal, I calculated that the concentration of acid divided by the concentration of its conjugate base must equal 1. How can I do this step in the lab? How do I make the ratio equal to 1?
     
    Last edited: Apr 28, 2015
  2. jcsd
  3. Apr 29, 2015 #2

    epenguin

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    Gold Member

    pH (+/-2). .?? Can you check you have written the question out accurately as asked you because this does not make sense to me. A pH change of 2 (hundredfold change of [H+]) is not insignificant. Where you wrote acetic you presumably meant citric? If possible change your post so others can see the question in the first post.

    This is a problem about buffering capacity. It is true that maximum buffering capacity occurs where pH = pK . For a monobasic acid HA it is simple to make from HA and NaA. You have to think about how to make it from H3Cit and Na3Cit. Think about now much of what species (mainly) there needs to be in solution. Actually there are three different buffers you can make for three different pKs, but since you give only one pK presumably this is the pH required.

    Perhaps the question asked should be quoted verbatim.
     
    Last edited: Apr 30, 2015
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