pH scalepH scale

• Logarithmic scale expressing the H+ concentration, [H+].

• If the pH changes by a factor of 1, the [H+] changes by a factor of 10.

• pH = -log[HpH = -log[H++]]

Water & Self-Ionization

• H2O (l) + H2O (l) H3O+ (aq) + OH-

(aq)

– HH33OO++ is called hydronium ion

– OHOH-- is called hydroxide ion

• In reality, H+ and H3O+ are used interchangeablyinterchangeably.

• HH++ is called a proton or a hydrogen ion.

-

pH of Water

• Water ionizes to produce both the hydronium ion and the hydroxide ion in equal amounts.

• Ionization happens to a very small extent

• [H+] = [OH-] = 1 X 10[H+] = [OH-] = 1 X 10-7-7 M M • At this concentration:At this concentration:

pH = -log[H+] = 7

pH to Molarity

• pH = -log[H+]. Solve for [H+].

• -pH = log[H+]

• Antilog(-pH) = [H+]

• Say pH = 5. Then –pH = -5.

• Antilog(-5) = 10-5.

• The –pH becomes the power of 10!The –pH becomes the power of 10!

•If the pH = 4If the pH = 4

• The [H+] = ?

• The pOH = ?

• The [OH-] = ?

1010

Antilog(-pH) = [HAntilog(-pH) = [H++]]Antilog(-4) = 10Antilog(-4) = 10-4-4

Antilog(-10) = [OHAntilog(-10) = [OH--]]1010-10-10 = [OH = [OH--]]

Difference in pH = 22, which means

the [H+] differ by 1022 or 100100.

The pH of 5 solution has an [H+] that is 100 X greater than that of the pH 7 solution.

• If the pH changes from 5 to 7, how do the [H+] compare?

Significance of pH differences

• Difference in pH =5 and pH =7

– Difference is 2 units of pH

• The [H+] differ by 1022 or 100100

• The pH of 5 solution has an [H+] that is 100 X greater than that of the pH 7 solution.

pH

[H+]1 M

or

1X1000 M

0.1 M

or

1 X 10-1-1

M

0.01 M

or

1 X 10-2-2

M

.001 M

or

1 X 10-3-3

M

pH 00 11 22 33

Ion Product Constant for Water

• Value of the equilibrium constant expression for the self-ionization of water.

H2O (l) + H2O (l) H3O+ (aq) + OH-

(aq)

Keq= [H+] [OH-]

[H2O]

Kw= [H+] [OH-] = 10-14

•If the pH = 2

The [H+] = ? 1 x 10-2M

• If the [H+] = 1 X 10-5 M

• The pH = ? 5

How many time stronger is an acidic solution withA pH of 2 than an acidic solution with a pH of 5?

PROBLEM: Relative acid strength

1000x

pOH

• By analogy, pOH is defined as –log[OH-]

• Express [OH-] in scientific notation

• If [OH-] = 1.0 X 10-3 M

• Then pOH = -log(10-3) = -(-3) = 3.

• If the [OH-] = 1 X 10-3

• The pOH = ?

• The pH = ?

• The [H+] = ?

pOH = -log[OHpOH = -log[OH--]]= -log(10= -log(10-3-3) = --3 = 3) = --3 = 3

14 – 3 = 1114 – 3 = 11

Antilog(-11) = 10Antilog(-11) = 10-11-11 M M

• If the [H+] = 1 X 10-5 M

• The pH = ?

• The pOH = ?

• The [OH-] = ?

55

14 – 5 = 914 – 5 = 9

Antilog(-9) = 10Antilog(-9) = 10-9-9

pH + pOH = 14pH + pOH = 14

• pH = 3, pOH =

• pH = 7, pOH =

• pH = 10, pOH =

• pH = 9, pOH =

• pH = 1, pOH =

1111

77

44

55

1313

Acid, Base, or Neutral?

• All H2O contains some H+ and some OH-

• Pure H2O – concentrations are very low.

• [H+] = [OH-] = 1 X 10-7 Molar.

• When [H+] = [OH-], solution is neutral

• Acidic solution: H+ > OH-

• Basic solution: OH- > H+

[H+] pH [OH-] pOH Acidic or

Basic

3

10-5 M

10-2 M

6

11111010-3-3 1010-11-11 AA

55991010-9-9 BB

22 12121010-12-12 M M AA

881010-8-8 M M 1010-6-6 M M BB

How to safely test pH

• Instrumental– use a pH meter

• Indicators– use a series of indicators

• Reaction– Acid if reacts with metals

(other than Cu, Ag, or Au)

Indicator

• A substance that changes color over a narrow pH range.

• Use several indicators to narrow down the pH range of the substance.