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A buffer is prepared by mixing 65.7 mL of 0.450 M HC7H5O2 with 35.0 mL of 0.643 M KC7H5O2.              Ka = 6.3 x 10-5    Determine the pH for this buffer.  What will be the pH after the addition of 0.015 mol HBr? Assume the volume does not change.

A buffer is prepared by mixing 65.7 mL of 0.450 M HC7H5O2 with 35.0 mL of 0.643 M KC7H5O2.              Ka = 6.3 x 10-5    Determine the pH for this buffer.  What will be the pH after the addition of 0.015 mol HBr? Assume the volume does not change.

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ISBN:9781305957404
Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
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A buffer is prepared by mixing 65.7 mL of 0.450 M HC7H5O2 with 35.0 mL of 0.643 M KC7H5O2.              Ka = 6.3 x 10-5   

  1. Determine the pH for this buffer. 
  2. What will be the pH after the addition of 0.015 mol HBr? Assume the volume does not change. 
Expert Solution
Step 1

pH = pKa + log [Salt]/[Acid]

pKa = -log Ka

 

Step 2

Answer 1:

Number of mole of HC7H5O2  = Molarity x volume in L = 0.450  x  0.0657 = 0.029565

Number of mole of KC7H5O2  = Molarity x volume in L = 0.643  x  0.035 = 0.022505

 

Total volume after mixing =  65.7 mL + 35.0 = 100.7 mL = 0.1007 L

[HC7H5O2] = 0.029565/0.1007

[KC7H5O2] = 0.022505/0.1007

pKa = -log Ka = -log 6.3 x 10-5  = 4.20

pH = pKa + log [KC7H5O2]/[HC7H5O2]

      = 4.20 + log 0.022505/0.029565

      = 4.20 + log 22505 - log 29565

      = 4.20 + 4.35 - 4.47

      = 4.08

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