Self Ionization of Water and the pH Scale

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 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 0 ACIDIC OH- H3O (We are counting how many hydronium ions are in a solution!) OH- H3O 14 BASIC 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? 1. pH 4 2. pH 10 3. 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 The number of zeros the difference in pH. pH 8 pH 11 Ex. 6-4 2 so, 100 times. pH 7 pH 4

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