Chemistry

I have the answer to this question but the book does not explain WHY this is a good buffer solution? It uses pH=pKa - log(base/acid) and shows 7.2=7.2 - log(1).
I don't understand why they can only use pKa7.2 and ignore the pKa 12.7.

You are instructed to prepare 100ml of a 0.02 M sodium phosphate buffer, pH 7.2, by mixing 50ml of 0.02 M Na2HPO4 and 50ml of 0.02 M NaH2PO4. Explain why this procedure provides an effective buffer at the desired pH and concentration.

Provided Table info:
H2PO4(-): Ka(M) 6.23x10^-8 pKa 7.2
HPO4(2-): Ka(M) 2.20x10^-13 pKa 12.7

  1. 👍 0
  2. 👎 0
  3. 👁 228
asked by Kelsey
  1. First, a correction. The Henderson-Hasselbalch equation is pH = pKa + log (base)/(acid). That negative sign is either a typo in the book or by you in the post. What you are asking isn't exactly clear. I think you want to know why
    a. is the solution a buffer
    b. why it is effective
    c. why use pk2 and not pK3?

    a. The solution is a buffer BECAUSE you have an acid and its conjugate base.
    b. Since 7.2 = 7.2 + log 1 (log 1 is zero) is 7.2, the most effective buffer is when pKa = pH. To get that you make (base) = (acid) and that ALWAYS makes (base)/acid) = 1 and that ALWAYS makes log 1 = 0 and that ALWAYS makes pH = pKa. Since base = acid, adding acid OR base affects the pH the same way so it is effective against BOTH acids and bases equally.
    c. Why use pk2 and not pK3?
    Look at k2 expression here.
    H2PO4^- ==> H^+ + HPO4^2-
    then
    k2 = (H^+)(HPO4^2-)/(H2PO4^-)

    Here is the buffer equation if a base is added.
    H2PO4^- + OH^- ==> HPO4^- + H2O and here is the secret.
    Note that k2 is the ONLY k expression (write out k1 and k3 expressions to see this if you will) that includes H2PO4^- and HPO4^2 and nothing else-. k1 for H3PO4 = (H^+)(H2PO4^-)/(H3PO4) and you don't have H3PO4 anywhere in that buffer. Also k1 = (H^+)(PO4^3-)/(HPO4^-) and you don't have PO4^3- anywhere in that buffer. You have only the salts NaH2PO4 and Na2HPO4 so k1 and k3 are not applicable. Get it?

    1. 👍 0
    2. 👎 0

Respond to this Question

First Name

Your Response

Similar Questions

  1. Basic Differential Equation

    Hi all. I need help understanding something my book dosn't go over well. For example dy/dx=-4y is a given solution and I need to find the differential equation. I simply isolate y then integrate. Getting y=e^(-4x) which is

    asked by Joe on June 5, 2015
  2. Math

    I just have a quick question regarding this question: Simplify. State any restrictions on the variables. log(x^2+7x+12)-log(x^2-9) my answer is log[(x+4)/(x-3)] which is correct. However, for the restrictions, I believe that they

    asked by Christine on June 6, 2009
  3. Language Arts

    The following question ask about one or more selections from your literature text book you may use your text book to answer this question what to answer this question both lobs girl and Jeremih song a time flashbacks in a

    asked by Tamari on October 11, 2018
  4. Physics

    Hi!! For this question, I feel like the answer is C but I'm not sure. If the answer is C, could you explain to me why/ if it isn't then why not? A book leans against a crate on a table. Neither is moving. Which of the following

    asked by Kaylen on June 7, 2016
  5. Language arts .

    "This question asks about your independent reading selection. You may use your reading selection to help you answer the question" "Has your independent reading selection grabber your attention? Tell the title and author of your

    asked by Anonymous (: on February 9, 2016
  6. Math

    Actually, i made a mistake. All though we still came up with the right answer, I made a mistake and copied another book answer to another question into this question. The real book answer to this word problem is: ( Let's translate

    asked by Mack on August 9, 2007
  7. Reading

    I am reading this book called "The Last Book in the Universe" and I have to answer some questions and one of the questions asked : (chapter 2) What happened in the big times that changed the way people live? What do we call this

    asked by Sara on August 6, 2012
  8. maths reiny help!!

    You had helped me with a similar question. This one says : A whole seller supplies 90 books while he was ordered to send 80 books by a retailer. If 15%trade discount was given, what was the net discount to the retailer? I solved

    asked by A GOODSTUDENT on August 26, 2016
  9. MATHS REINY

    You had helped me with a similar question. This one says : A whole seller supplies 90 books while he was ordered to send 80 books by a retailer. If 15%trade discount was given, what was the net discount to the retailer? I solved

    asked by A GOODSTUDENT on August 26, 2016
  10. Anatomy and Physiology

    Explain how a drug that sits on acetylcholine receptors will affect muscle function? Is there a good source online to find the above question? My book doesn't go into great detail for me to answer the question.

    asked by Student on February 7, 2014
  11. artist

    I am suposed to incorporate the answer to this question into a short repot on raphael. What makes a Raphael a Raphael? I can't find the answer anywhere on the interent. It is a title of a book so all I get is places to get the

    asked by Riley on April 15, 2009

More Similar Questions