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Buffers

buffered solutions - contains weak conjugate acid-base pair  

  • resists drastic pH changes
  • able to neutralize both H+ and OH- ions
  • conjugate acid-base pair can't consume each other
  • usually created by combining a weak acid/base w/ its salt
  • pH of buffer = log ([base] / [acid]) - log (Ka)
    • pH = pKa + log ([X-] / [HX])
  • buffer capacity - amount of acid/base the buffer can neutralize before pH begins changing
  • add strong acid >> [HX] increases, [X-] decreases
  • add strong base >> [X-] incresaes, [HX] decreases

If the buffer system in blood is made up of carbonic acid and sodium bicarbonate, what is the base to acid ratio if the pH is measured to be 7.41?  

  • Given:
    • pH = 7.41
    • Ka of carbonic acid = 4.3 x 10-7
    • Ka = [H+] [X-] / [HX]
  • [H+] = 10-7.41
  • 4.3 x 10-7 = [H+] [HCO3-] / [H2CO3]
  • [HCO3-] / [H2CO3] = 4.3 x 10-7 / 10-7.41 = 11

How many moles of NaOBr should be added to 1.00 L of 0.050 M HOBr to form a buffer w/ pH 8.80?  

  • Given:
    • pH = 8.80
    • Ka HOBr = 2.5 x 10-9
    • Ka = [H+] [X-] / [HX]
  • assume that the NaOBr has negligible effect on the volume
  • [H+] = 10-8.8
  • 2.5 x 10-9 = [H+] [NaOBr] / [HOBr]
  • [NaOBr] = (2.5 x 10-9) (0.050) / (10-8.8) = 0.079

In a buffer of acetic acid and sodium acetate where the pH is 5, find the molarity of sodium acetate if the molarity of acetic acid is 0.10  

  • Given:
    • pH = 5
    • [acetic acid] = 0.10
    • Ka = 1.8 x 10-5
    • Ka = [H+] [X-] / [HX]
  • [H+] = 10-5
  • 1.8 x 10-5 = [H+] [sodium acetate] / [acetic acid]
  • [sodium acetate] = (1.8 x 10-5) (0.1) / (10-5) = 0.18
Subject: 
Chemistry
Subject X2: 
Chemistry

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