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!!
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