15-2: Acidity, Basicity, and pH

Self-ionization of Water

Water is amphoteric (acts as acid and base)

Naturally forms H3O+ and OH-

[H3O+] and [OH-]

[H3O+] = [OH-] = 1.00 x 10-7 mol/L

Self-Ionization Constant of Water, Kw

Equilibrium between water and its ions

2H2O(l) H3O+(aq) + OH-(aq)

Equilibrium expression:

Kw = [H3O+][OH-] = (1.00 x 10-7) (1.00 x 10-7)

= 1.00 x 10-14

Constant!!!

Self-Ionization Constant of Water, Kw

H3O+

OH-

[H3O+] > 10-7 > [OH-]

Acid

OH-

H3O+

[H3O+] < 10-7 < [OH-]

Base

H3O+ OH-

[H3O+] = 10-7 = [OH-]

Neutral

Concentrations and KW

See Table 4, Pg 540

[H3O+][OH-] = 1.00 x 10-14

Sample Problem – Determining [OH-] or [H3O+] using Kw

What is [OH-] in a 3.00 x 10-5 M solution of HCl?

Solve:

HCl (strong acid) completely dissociates into H3O+ and Cl-

[H3O+] = 3.00 x 10-5 M

KW = [H3O+][OH-] = 1.00 x 10-14

Rearrange to solve for [OH-]:

[OH-] = 3.33 x 10-10 M

Sample Problem – Determining [OH-] or [H3O+] using Kw

What is [H3O+] in a 3.00 x 10-5 M solution of calcium

hydroxide, Ca(OH)2?

Solve:

Ca(OH)2 (strong base) completely dissociates

2 OH- per unit!

[OH-] = 2 x 3.00 x 10-5 M = 6.00 x 10-5

KW = [H3O+][OH-] = 1.00 x 10-14

Rearrange to solve for [H3O+]: 1.67 x 10-10 M

Practice

Calculate the hydronium ion concentration in an aqueous

solution that has a hydroxide ion concentration of

7.24 x 10-4 M.

1.38 x 10-11

What is [OH-] in a 0.450 M solution of HNO3?

2.22 x 10-14

If 1.2 x 10-4 mol of magnesium hydroxide are dissolved in

1.0 L of aqueous solution, what are [OH-] and [H3O+]?

[OH-] = 2.4 x 10-4 [H3O+] = 4.2 x 10-11

pH Values (power of Hydrogen)

Danish chemist Søren Sørenson (1909)

Describes solution based on [H3O+]

pH = -log[H3O+]

↕

[H3O+] = 10-pH

The Meaning of pH Values

Neutral [H3O+] =[OH-] (pH = 7)

Acidic [H3O+] > [OH-] (0 < pH < 7)

Basic [H3O+] < [OH-] (7 < pH < 14)

pH = 0 pH = 7 pH = 14

Very acidic neutral Very

basic/alkaline

Sample Problem – Calculating pH for solutions

What is the pH of a 0.00010 M solution of HNO3, a strong

acid?

Solution:

HNO3 + H2O → H3O+ + NO3

-

HNO3 completely dissociates

[HNO3] = [H3O+] = 1.0 x 10-4

Calculate pH: pH = -log[H3O+] = -log(1.0 x 10-4) = 4.0

Sample Problem – Calculating pH for solutions

What is the pH of 0.0136 M solution of KOH?

Solution:

KOH + H2O → K+ + OH- + H2O

KOH is a strong base, completely dissociates

[KOH] = [OH-] = 0.0136

Calculate [H3O+] using KW:

[H3O+] = 7.35 x 10-13

Calculate pH:

12.1

Practice

Calculate pH if [H3O+] = 5.0 x 10-3 M.

2.3 acidic

What is the pH of a 0.2 M solution of a strong acid?

0.7

Calculate pH if [OH-] = 2.0 x 10-3 M.

11 strong base

What is the pH of a solution that contains 0.35 mol/L of the strong base NaOH?

14

What are the [H3O+] and [OH-] in a sample of rain that has

a pH of 5.05?

Solution:

Calculate [H3O+]: [H3O

+] = 10-pH = 10-5.05 = 8.91 x 10-6 M

Calculate [OH-]: KW

[H3O+]

= 1.00 x 10−148.91 x 10−6

= 1.12 x 10-9

Sample Problem – Calculating [H3O+] and [OH-] from pH

Practice

What is the hydronium ion concentration in a fruit juice

that has a pH of 3.3?

5.0 x 10-4

A commercial window-cleaning liquid has a pH of 11.7.

What are [H3O+] and [OH-]?

[H3O+] = 2.00 x 10-12; [OH-] = 5.00 x 10-3

If the pH of a solution is 8.1, what are [H3O+] and [OH-]?

[H3O+] = 8.0 x 10-9; [OH-] = 1.3 x 10-6

How Do We Measure pH?

For approximate measurements:

Litmus paper

“Red” paper turns blue above ~pH = 8

“Blue” paper turns red below ~pH = 5

pHydrion paper Indicates approximate pH 1-12

Indicator solutions Change color depending on pH

See Table 6, pg 554

aci

d

How Do We Measure pH?

pH meter

One electrode sensitive to [H3O+]

Voltage between electrodes

related to pH of solution

Accurate to 0.1 or better

Make Your Own pH Indicator!

Red cabbage juice changes color

Finely cut/chop red cabbage

Pour hot water to cover cabbage, let sit until cool

Strain out and discard cabbage, keep liquid

To use: Add a few drops of indicator to solution

Dip filter / blotter paper in liquid and dry; put drop of solution on test strip

Homework – Test your indicator

Test 10 things at home

Take / record data in Lab Book

For each sample: List what you sampled

Notes about how you prepared the sample

Attach test strip for sample to page

State pH of sample

Things you might try: Milk

Vinegar

Baking soda

Liquid dish soap

Ammonia cleaner (e.g., Mr. Clean)

Windex

409 Cleaner

Different fruits / vegetables

Water (tap, bottled water, carbonated)

Soda

Coffee, tea

Tooth paste

Laundry detergent

Shampoo / body wash

Cosmetics

Other stuff? Be Creative!!