pH

Definitions and Formulas

• The pH scale is defined as the negative log of the concentration of H+:

pH = -log[H+] • The pOH scale is defined as the negative log of

the concentration of OH-, [OH-].

pOH = -log[OH-] • With this scale, calculating the pOH can be done

in the same manner as the pH scale.

pH + pOH = 14

Examples

What is the pH of a solution that contains 25 g of hydrochloric acid (HCl) dissolved in 1.5 L of water?

Quiz

1. Determine the pH of a .0034 M HNO3 solution.

2. Determine the pOH of the solution in #1.

3. What is the pH of a 2.4 x 10-5 M HCl solution?

Auto-Ionization of Water

[OH-]= Kw [H30+]= Kw

[H30+] [OH-]

Example: What is the hydroxide ion concentration in an aqueous solution that is 4.0 x 10-5 M H30+?

[OH-] = (1.0 x 10-14)

(4.0 x 10-5) =2.5 x 10-10 M

ion product constant for water Kc (55.5M) = [H+][OH-] = Kw

Kw= [H30+] [OH-]

Kw= 10 -14

In water, Kc= [H+][OH-]

55.5 M

Practice

What is the hydronium ion concentration in an aqueous solution containing 3.5 x 10-7 OH-?

[H30+] = (1.0 x 10-14)

(3.5 x 10-7) =2.9 x 10-8

What is the hydroxide ion concentration in an aqueous

solution containing 8.34 x 10-12 H30+ ?

[OH-] = (1.0 x 10-14)

(8.34 x 10-12) =1.20 x 10 -3

Strong Acids

• dissociate completely in water to form hydronium ion(s)

• Examples– HCl– HNO3

– H2SO4

Strong Bases

• dissociate completely in water to form hydroxide ions

• Examples:– NaOH– KOH– Ca(OH)2