Chapter 16.2 and 16.3

pH and pOH

What is pH and pOH?pH is the concentration of hydronium

ions in a solutionpOH is the concentration of hydroxide

ions in a solutionpH tells you the strength of an acid or

a base (more hydronium = more acidic)

Strongest acid = 0.0Neutral = 7.0Strongest base = 14.0

Calculating pH and pOHWhen you are given either a

concentration of hydronium ions or hydroxide ions, you can calculate the pH or the pOH

pH = -log [H3O+]pOH = -log [OH-]Each 1 unit change in pH is a 10 fold

change in the hydronium ion concentration

ExampleWhat is the pH of a solution with a

[H3O+] of 1.0 x 10-12? pH = -log [1.0 x 10-12]pH = 12What do you notice about the

answer and the exponent?They’re the same! This only works

when your hydronium ion concentration is in 1.0 x 10-x form

More Examples[H3O+] = 3.4 x 10-5

pH = 4.5[H3O+] = 6.89 x 10-3

pH = 2.16

Using the hydroxide ionSometimes you will be given the

hydroxide ion concentration and asked to find the pH.

Calculate the pOHSubtract the pOH from 14 to get

your pHThe reverse of this works as well. If

you are given a pH and asked to find a pOH, follow the same steps.

ExampleWhat is the pH of a solution that

has the hydroxide ion concentration of 0.025 M?

pOH = -log [OH-]pOH = -log [0.025]= 1.6pH = 14 – 1.6= 12.4

Calculating [H+] from pHIf you are given the pH and asked to calculate the

[H+], you just need to use you calculator.1.Take the inverse log (2nd + log key) 2.Enter a negative sign (-)3.Enter the pH and close parenthesesExample: What is the [H+] for a solution with a

pH of 7.41?[H+] = inverse log (-pH)[H+] = inverse log (-7.41) = 3.9 x 10-8

You try!: What is the [H+] for a solution with a pH of 3.50?

[H+] = inverse log (-3.50)= 3.16 x 10-4

TitrationTitration is a method used for

determining the concentration of a solution by reacting it with a known volume of another solution of a known concentration

Let’s look at the procedure for titration

Titration Procedure1. Measure the amount of the solution

of an unknown concentration in a beaker. Add an indicator.

2. Fill a buret with the titrating solution of a known concentration called the standard solution.

3. Measured volumes of the standard solution are added slowly and mixed into the solution in the beaker. Continue until the indicator changes

Titration TerminologyEquivalence point – when the

moles of hydrogen equal the moles of hydroxide. This point is not always 7. It may be different when using strong acids vs. weak bases and vise versa.

End point- the point where the indicator changes. This point is different for every indicator.