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