pH
chapter 6
lesson 7
Objective
You will be able to:
calculate pH from concentration and
concentration from pH
What does pH stand for?
pH stands for the “power of hydronium”.
It is the concentration of the hydronium ion.
The symbol [H3O+] means concentration of
hydronium.
Water ionizes as follows:
2H2O (l) ➔ H3O+ (aq) + OH- (aq)
In distilled water, the concentration of
hydronium is 1 x 10-7 mol/L.
What is the pH of water?
Solutions with higher concentrations of hydronium
are acidic.
Typical acids and their pH are:
hydrochloric acid = pH 1
acetic acid = pH 3
hydrocyanic acid = pH 5
What are the concentrations of the hydronium ions
in these acids?
Notice how the concentration changes for each
change in the pH number.
Each change in the pH number changes the hydronium
ion concentration by a factor of 10!
How many more times concentrated is the
hydrochloric acid (pH=1) than the acetic acid (pH=3)?
It is easy to read the pH of a solution that has values like
those in the previous example.
What if the concentration is more complicated?
Then we must use the formula:
Example:
pH = - log[H3O+]
What is the pH of a solution where the [H3O+] is
7.20 x 10-9 mol/L?
Significant Digits with pH measurments.
Notice that the pH is reported to 3 decimal places
even though the concentration has just 3 significant
digits. This is because only the decimal places of a pH
are considered significant.
Ex. What is the pH of the acid when 0.022 g of
hydrogen iodide is dissolved in 700 mL of water?
Assignment
Read text p. 227-230
Do p. 230 all