Notes:

Conjugate Acid/Base Explained in words:
http://www.science.uwaterloo.ca/~cchieh/cact/c123/baseacid.html

Buffer Problem

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Notes on finding the pH of salts:

Problem #1: The Ka of formic acid (HCOOH) is 1.8 x 10¯4. What is the pH of 0.35 M solution of sodium formate (NaHCOO)?
Solution:
Calculate the Kb of the formate (HCOO¯) ion:
  • 1.00 x 10¯14 = (1.8 x 10¯4) (x)x = 1.00 x 10¯14 / 1.8 x 10¯4 = 5.56 x 10¯11
Calculate the [OH¯]:
  • 5.56 x 10¯11 = [(x) (x)] / (0.35 - x)neglect the minus x
  • x = 4.41 x 10¯6 M
Calculate pOH, then pH:
  • pOH = - log 4.41 x 10¯6 = 5.36pH = 14 - pOH = 14 - 5.36 = 8.64

Problem #2: Calculate the pH of a 0.36 M CH3COONa solution given the pKa of acetic acid is 4.745.
Solution:
Using the pKa, calculate the Ka of acetic acid:
  • Ka = 10¯pKa = 10¯4.745 = 1.80 x 10¯5
Using the Ka of the acid, find the Kb of the base (the CH3COONa):
  • 1.00 x 10¯14 = (1.80 x 10¯5) (x)x = 5.56 x 10¯10
Use the Kb expression to calculate the [OH¯]:
  • 5.56 x 10¯10 = [(x) (x)] / (0.36 - x)neglect the minus x
  • x = 1.415 x 10¯5 M (I threw in a guard digit.)
Calculate pOH, then pH:
  • pOH = - log 1.415 x 10¯5 = 4.85pH = 14 - pOH = 14 - 4.85 = 9.15
Comment on the solution to Example #3:
You can use the pKa to get the pKb
  • 4.745 + x = 14.000x = 9.255
Use the pKb to get the Kb:
  • Kb = 10¯pKb = 10¯9.255 = 5.56 x 10¯10
Go to rest of solution, at "Use the Kb expression," just above.

Problem #3: What is the pH of a 0.0510 molar solution of salt NaCN (the Ka for HCN is 6.166 x 10¯10)?
Solution:
CN¯ is the salt of a weak acid and hydrolyzes thusly:
  • CN¯ + H2O ⇔ HCN + OH¯
Determine the Kb for this reaction (using Kw = KaKb):
  • 1.00 x 10¯14 = (6.166 x 10¯10) (x)x = 1.6218 x 10¯5 (I kept a couple guard digits.)
Write the Kb expression and solve for [OH-]:
  • Kb = ([HCN] [OH¯)] / [CN¯]1.6218 x 10¯5 = [(x) (x)] / 0.0510
  • x = 9.0946 x 10¯4 M
Determine pOH (with negative log) and then pH (using pH + pOH = 14):
  • -log 9.0946 x 10¯4 = 3.0412pH = 14 - 3.0412 = 10.959 (to three sig figs)



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Explanation of Conjugate Acids/Bases



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