pH, Indicators, and Titrations

Chemistry Joke

Special thanks to Kate Pancho!

Definition of pH

pH is a number scale from 0 – 14 that represents the acidity or basicity of a solution.

0 – 6 is acidic 7 is neutral 8 – 14 is basic

pH is a measure of the hydrogen ion concentration of a solution

pH = - log [H+] The lowercase p stands for negative

log [H+] represents the concentration of

hydrogen ions in units of molarity (mol/L)

Definition of pH

Definition of pOH

pOH is also a number scale that represents acidity or basicity of a solution.

It is the opposite of the pH scale pOH is a measure of the hydroxide ion

concentration of a solution. pOH = - log [OH-]

pH and pOH

pH + pOH = 14 [H+] [OH-] = 1 x 10-14 M If [H+] increases, [OH-] must

decrease.

Calculating pH and pOH If [H+] is 10-5 M, the pH is 5.

› Is this acidic, basic, or neutral?

If the pH is 5, what is the pOH?

› pH + pOH = 14, so the pOH = 9.

If [H+] is 10-7 M, what is the pH?

› Is this acidic, basic or neutral?

If the pH is 7, what is the pOH?

Calculating pH and pOH If [OH-] = 10-5 M, what is the pOH? If [OH-] = 10-6 M, what is the pH?

› The pOH = 6, so the pH = 8 If the hydrogen ion concentration is 1 x 10-4

M, is the solution acidic, basic, or neutral? › pH = 4, so acidic

If the hydroxide ion concentration is 1 x 10-5 M, is the solution acidic, basic, or neutral?› pOH = 5; pH = 9, so the solution is basic

Calculating [H+] and [OH-]

Remember that 10a x 10b = 10(a+b)

If [OH-] = 10-6 M, what is the [H+]?› [H+] [OH-] = 1 x 10-14 M› [H+] = 10-8 M

If [H+] = 10-9 M, what is the [OH-]? › [OH-] = 10-5 M

Fill in the chart on your notesheet.

Indicators

An indicator is a compound that changes color within a specific pH range.

Some common indicators:› Phenolphthalein—colorless to bright pink

at a pH range of 8 – 10.› Alizarin yellow—yellow to lilac at a pH

range of 10 – 12› Bromphenol blue—yellow to blue at a pH

range of 3 - 4

Indicator Demo

Titrations

Titration is a method used to determine the concentration of [H+] or [OH-].

Procedure:› 1. Measure a volume of acidic solution of

unknown concentration.› 2. Add an indicator to the solution.› 3. Add, drop by drop, a measured volume

of base of known concentration just until the indicator turns color.

Titrations The solution of known concentration is

called the standard solution. The point where the indicator changes

color is called the end point. Once the titration is complete, the

unknown molarity of the acid can be calculated.

MA x VA = MB x VB › Molarity of acid x volume of the acid is

equal to the molarity of the base x the volume of the base.

Titration Demo

Chemistry Joke

Q: What is this?

A: A fish out of water!!