Acids & Bases Equilibria

pH & pOH

What is the [H3O+] of a solution which has a [OH-] of 1.0 x 10-9 M at 25oC? Is this solution acidic or basic?

Kw = [H3O+][OH-]

1 x 10-14 = [H3O+][1 x 10-9]

[H3O+] = 1 x 10-5 M

Expressing acidity or basidity by moles per liter takes a long time!!

So we use the pH & pOH scale.

Recall pH of:

0 – 7 is acidic 7 – 14 is basic

Opposite for pOH: 0 – 7 is basic 7 – 14 is acidic

pH = - log [H+]

pOH = - log [OH-]

pH + pOH = 14

What is the pH of a solution with a [H3O+] of 1 x 10-5 M?

pH = - log (1 x 10-5)

pH = - log [H+]

pH = 5

What is the pH of a solution that contains 1.0 x10–2 M of H+?

pH = - log (1 x 10-2)

pH = - log [H+]

pH = 2

What is the pH of a solution that contains 5.0 x10–12 M of H+?

pH = - log (5 x 10-12)

pH = - log [H+]

pH = 11.30

What is the pH of a solution that contains 1.0 x10–7 M of H+? pOH?

pH = - log (1 x 10-7)

pH = - log [H+]

pH = 7

pH + pOH = 14

pOH = 7

What is the pOH of a solution that contains 6.4 x10–10 M of OH-? pH?

pOH = - log (6.4 x 10-10)

pOH = - log [OH-]

pOH = 9.19

pH = - log [H+]

pH = - log (1.56 x 10-5)

pH = 4.81

A solution of 0.25M HCN is prepared in water at 25oC. What is the pH of the solution? pOH?

pH = - log (0.25)

pH = - log [H+]

pH = 0.60

pH + pOH = 14

pOH = 13.40

HCN + H2O H3O+ + CN-

ACID BASE

What is the pH of black coffee at 25oC if [OH-] is 2.0 x 10-10 M?

pOH = - log (2.0 x 10-10)

pOH = - log [OH-]

pOH = 9.70

pH = - log [H+]

pH = - log (5 x 10-5)

pH = 4.30

pH + pOH = 14

A solution of 0.09M NH3 is prepared in water at 25oC. What is the pH?

pOH = - log (0.09)

pOH = - log [OH-]

pOH = 1.05

pH + pOH = 14

pH = 12.95

NH3 + H2O NH4+ + OH-

BASE

What is the [H+] of a solution if the pH is 3.7?

[H+] = 1.995 x 10-4 M

[H+] = 10- pH

[OH-] = 10 - pOH

[H+] = 10- 3.7

What is the [H+] solution if the pH is 9.62?

[H+] = 2.40 x 10-10 M

[H+] = 10- pH

[H+] = 10- 9.62

This makes sense, because the [H3O+] < [OH-] so basic &

pH is basic

What is the [OH-] of a solution if the pOH is 5.4? pH? [H+]?

[OH-] = 3.98 x 10-6 M

[OH-] = 10- pOH

[OH-] = 10-5.4

pH + pOH = 14

pH = 8.6

[H+] = 10- pH

[H+] = 10-8.6

[H+] = 2.51 x 10-9 M

Does [H+] x [OH-] = Kw?