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

Definition of pH 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 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!!