Self Ionization of Water and the pH Scale

Ionization of Water Water will naturally break down into equal ions in water: 2H2O  H3O+ (aq) + OH- (aq) This is called the ionization of water.

pH Scale 7 14 NEUTRAL ACIDIC BASIC • scale used by chemists to measure the acidity of a substance; a measure of the amount of H+ in the solution. • scale goes from 0 – 14 NEUTRAL 7 OH- = H3O+ 14 ACIDIC BASIC OH- < H3O+ (We are counting how many hydronium ions are in a solution!) OH- > H3O+ (We are counting how many hydroxide ions are in a solution!)

pH Scale pH describes a balance between hydroxide ions (OH-) and hydronium ions (H3O+) Note: [H+] and [H3O+] are equivalent! We will use them interchangeably. pH stands for “parts hydrogen ion” pOH stands for “parts hydroxide ion”

Is It An Acid or a Base? pH = 4 pH = 10 pH = 7

The pH scale is not linear! The lower the pH, the higher the concentration of hydronium ions. The pH scale changes 10 times between each step.

Example Problem: How many times more acidic is solution A than solution B? A B pH = 4 pH = 6 pH = 3 pH = 5 pH = 8 pH = 11 pH = 7 pH = 4 The number of zeros = the difference in pH. Ex. 6-4 =2 so, 100 times.

Estimating pH from [H3O+] The pH of a solution can be estimated by looking at the concentration of hydronium (H3O+). The exponent is the estimated pH of the substance! Example: [H3O+] = 1.0 x 10-3 M pH = 3

Estimating pH from [H3O+] The pH of a solution can be estimated by looking at the concentration of hydronium (H3O+). Example: [H3O+] = 1.0 x 10-12 M pH = 12

Estimating pH from [H3O+] The pH of a solution can be estimated by looking at the concentration of hydronium (H3O+). Example: [H3O+] = 3.1 x 10-12 M

Estimating pH from [H3O+] The pH of a solution can be estimated by looking at the concentration of hydronium (H3O+). Example: [H3O+] = 3.1 x 10-12 M pH = a little less than 12

Estimating [H3O+] from pH Example: pH = 9.0 So, [H3O+] = 1.0 x 10-9 M

Estimating [H3O+] from pH You try: pH = 7.0 pH = 2.0 pH = 8.0