1. Kw, pH, and pOH calculations
Chem-To-Go Lesson 39
Unit 10

2. H2O(l) + H2O(l)  H3O+(aq) + OH-(aq)
Kw = [H3O+] [OH-] = 1.0x10-14
H2O(l) + H2O(l)  H3O+(aq) + OH-(aq)
If a reaction reaches equilibrium, then a constant ratio of products to reactants exists.
The constant ratio can be calculated in the lab by measuring the concentration of each substance in the reaction and setting up an expression. The expression are called “k” expressions.
Since only solutions (NOT pure liquids, solids, or gases) have concentrations, the expression should only consider aqueous solutions.
The expression for this equation is: k = [H3O+] [OH-]
After many lab tests to confirm, the Kw value = 1.0x This means that water reacting with water will always produce a ratio of ions equal to 1.0x10-14.

3. Applications of Kw Kw = [H3O+] [OH-] = 1.0x10-14
ACIDS produce H3O+ (H+) when dissolved in water.
BASES produce OH- when dissolved in water.
Look closely at the K expression.
Make a prediction.
Which ion will increase if an ACID is dissolved in the water?
How will the concentration of the other ion respond?
Which ion will increase if a BASE is dissolved?

4. Hydronium & Hydroxide Rule
ACIDS = [H3O+] > [OH-]
When a substance dissolves in water and causes the H3O+ concentration to increase, then the OH- concentration will decrease. This signals that the substance is an ACID. When a substance dissolves in water and causes the OH-concentration to increase, then the H3O+ concentration will decrease. This signals that the substance is a BASE.
BASES = [OH-] > [H3O+]

5. Using Kw to Calculate Ion Concentrations
What is the concentration of OH- ions in saturated limewater if [H3O+] = 3.98x10-13M? Is the limewater acidic, basic, or neutral?
HINT: If the problem involves OH- and H3O+ ions, then you’ll use the kw equation to solve.

6. Using Kw to Calculate Ion Concentrations
What is the concentration of H3O+ ions in a wheat flour and water solution is [OH-] = 1.0x10-8M? Is wheat flour and water acidic, basic, or neutral?
If the problem involves OH- and H3O+ ions, then you’ll use the kw equation to solve.

7. pH Concept The pH scale measures how acidic or basic a substance is.
The pH scale ranges from 0 to 14.
A pH of 7 is neutral.
A pH less than 7 is acidic.
A pH greater than 7 is basic.

8. pH Scale The pH scale is logarithmic.
pH = O 10,000,000x
pH = ,000,000x
pH = ,000x
pH = ,000x
pH = x
pH = x
pH = x
pH = NEUTRAL
pH = x
pH = x
pH = x
pH = ,000x
pH = ,000x
pH = ,000,000x
pH = ,000,000x
The pH scale is logarithmic.
As a result, each whole pH value below 7 is ten times more acidic than the next higher value.
The same holds true for pH values above 7, each of which is ten times more alkaline (another way to say basic) than the next lower whole value.

9. Calculating pH pH = -log[H+] OR pH = -log[H3O+]
pH is defined as the negative logarithm of the hydrogen ion (a synonym for hydronium ion) concentration.
We learned to calculate these concentrations using Kw. Therefore, a pH problem might give me the [H+] concentration directly OR indirectly via [OH-].

10. Using the pH equation
Analysis of a sample of maple syrup reveals the concentration of OH- ions is 5.0x10-8 M. What is the pH of this syrup? Is it acidic, basic, or neutral?

11. Using the pH equation
In a sample of bananas and water, it is found that [H3O+] = 2.51x10-5 M. What is the corresponding pH value, and are the bananas and water acidic, basic, or neutral?

12. pH’s sister - pOH pOH = -log[OH-] pH + pOH = 14
Calculate the pH corresponding to each of the pOH values listed.
a. pOH = 4.32
b. pOH = 8.90

13. pH’s sister - pOH
For the ion concentration given, calculate the concentration of the other ion, the pH, and the pOH of the solution.
[H+] = 4.29x10-11 M (Example)
[OH-] = 7.36x10-2 M (On your own)