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Ph of buffer solution

  1. Apr 14, 2007 #1
    Question

    A buffer solution is prepared by mixing 380.0 ml of 0.250 M propionic acid, C2H5COOH, and 120.0 ml of 0.350 M sodium propionate, C2H5COONa. Calculate the pH of the prepared buffer solution. For propionic acid, Ka = 1.34 X 10^-5.

    Work thus far

    Code (Text):

      We have this

      C2H5COOH <--> C2H5COO- + H+

      Since the C2H5COONa goes to completion and is left
      with C2H5COO-, we can determine the initial amount
      of C2H5COO-, and then we use the x-method to determine
      the pH.

      Initial Molarities:

                     .250 M .380 L     .250 * .380
       [C2H5COOH] = --------------- = ------------ = .19 M
                     (.380 + .120)L        .5      


                     .350 M * .12 L     .350 * .12
       [C2H5COO-] = ---------------- = ------------ = .084 M
                     (.380 + .120) L      .5

      C2H5COOH   <-->     C2H5COO-    +        H+

        .19                 .084               0       initial
         -x                  x                 x       change
       .19-x               .084+x              x       equilibrium

      so,

                             (.084 + x) x
         Ka = 1.34 X 10^-5 = --------------
                              (.19-x)

         ==> x = 3.029 X 10^-5, pH = -log(x) = 3.002

     
    does this look okay to you?
     
  2. jcsd
  3. Apr 14, 2007 #2

    symbolipoint

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    Yes. ..... ..
     
  4. Apr 14, 2007 #3
    thank you! :-)
     
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