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Chapter 6 (CIC) and Chapter 16 (CTCS)
• Read in CTCS Chapter 16.4-7
• Problems in CTCS: 16.23, 25, 27, 31, 33, 35, 37, 41, 45, 57, 49, 51, 59, 61, 63, 65
pH – Power of Hydrogen
• As [H+] increases, the pH decreases
• Based on logarithmic scale
• Compared to a pH = 1 solution, a pH = 2 solution is 10x less acidic
• An acid has a pH < 7
• A base has a pH > 7
pH of Some Common Solutions
Brown, T.L.; LeMay, H.E.Jr.; and Bursten, B.E. Chemistry the Central Science, 8th Edition, Prentice Hall, Upper Saddle River, NJ, 2000, p 602.
Why Do We Bother With pH?
• [H+] = 3.4 x 10-6 is too small
• p stands for –log and H stands for [H+]
• pH = -log [H+]
• Neutral solution has [H+] = 1 x 10-7 or pH = 7.0
Q: If [H+] = 3.4 x 10-6, what is the pH? How many sig figs should there be?
Q: What is the [OH-] of this solution? What is the pOH?
• Notice that pH + pOH = 14.00 (pKw)
• Derived from [H+] [OH-] = 1 x 10-14 (Kw)
Strong Acids/Bases• Table 4.2 lists these as:
• Note that the strong bases are all of the soluble hydroxides
Strong Acids
HCl HBr HI HClO4
HNO3 H2SO4
Strong Bases
LiOH NaOH KOH RbOH
CsOH Sr(OH)2 Ba(OH)2
• What’s the definition of a strong acid or base?
Q: What is the pH of 0.008 M HCl?
Q: The pH of the HCl soln in lab was about 1.85. What was it’s concentration?
Q: What is the [OH-] in 0.010 M Sr(OH)2?
Q: What is the pH of this solution?
Q: What is the pH of a saturated Ca(OH)2 solution if the solubility is 0.97 g/100. mL?
Weak Acids• Not to be confused with dilute acids!
• 0.0001 M HCl is NOT a weak acid!
HClO + H2O H3O+ + ClO-
• Remember that K = [products]/[reactants]
+ -3
2
H O ClOK =
HClO H O
+ -3 -8
a
H O ClOK = = 3.0 x 10
HClO
If [HClO] = 1.0 M, then
[H3O+][ClO-] = 3.0 x 10-8
So, since there should be equal amounts of [H3O+] and [ClO-], then
[H3O+]2 = 3.0 x 10-8
Or [H3O+] = 1.7 x 10-4
And pH = 3.76
Things to Remember• A Ka must have H+ in the products• Ka’s of strong acids have large values
(greater than 1)• A Kb must have OH- in the products• Kb’s of strong bases have large values
(greater than 1)• You may not be able to simplify the problem
so much and you will have to make approximations
Q: Calculate the pH of a 0.025 M lactic acid solution if its’ Ka = 1.4 x 10-4.
A: 2.75
Q:Calculate the Ka of a 0.085 M phenylacetic acid solution if its’ pH is 2.68.
A: 5.3 x 10-5
Q: Calculate the pH of a 1.0 M methylamine solution if its’ Kb = 4.38 x 10-4.
A: 12.32