How is pH defined? The pH of a solution is the negative logarithm of the hydrogen-ion concentration. The pH may be represented mathematically, using the following equation: pH = –log[H+] In pure water or a neutral solution, [H+] = 1 × 10–7M, and the pH is 7. pH = –log(1 × 10–7) = 7.0 If the [H+] of a solution is greater than 1 × 10–7M, the pH is less than 7.0. If the [H+] of the solution is less than 1 × 10–7M, the pH is greater than 7.0.

ChemSaver p 40 Calculate the pH of the following acids: pH= -log [H+] [H+] = 3.4 x 10-5 M   The molarity of the acid is 6.5 x 10-4 M

Sample problem: Calculating pH What is the pH of a solution with a hydrogen-ion concentration of 4.2 × 10–10M? (contd.)

pH and pOH Scale

ChemSaver p 39 basic acidic basic acidic neutral neutral

ChemSaver p 40 Calculate the pOH of the following bases: pOH= -log [OH-]   [OH-] = 1 x 10-9 M The molarity of the base is 3.8 x 10-10 M

How are [H+] and [OH–] related in an aqueous solution? The reaction in which water molecules produce ions is called the self-ionization of water. This reaction can be written as a simple dissociation. H2O(l) ⇄ H+(aq) + OH–(aq) The self-ionization of water occurs to a very small extent. In pure water at 25°C, the concentration of hydrogen ions is only 1 × 10–7M. The concentration of OH– is also 1 × 10–7M because the numbers of H+ and OH– ions are equal in pure water. Any aqueous solution in which [H+] and [OH–] are equal is a neutral solution.

For aqueous solutions, the product of the hydrogen-ion concentration and the hydroxide-ion concentration equals 1.0  10–14. [H+]  [OH–] = 1.0  10–14 This equation is true for all dilute aqueous solutions at 25°C. The product of the concentrations of the hydrogen ions and the hydroxide ions in water is called the ion-product constant for water (Kw). Kw = [H+] × [OH–] = 1.0 × 10–14 A solution in which [H+] is greater than [OH–] is an acidic solution. In acidic solutions, the [H+] is greater than 1 × 10–7M. A basic solution is one in which [H+] is less than [OH–]. The [H+] of a basic solution is less than 1 × 10–7M. Basic solutions are also known as alkaline solutions.

You can use this equation to convert between pH and pOH Kw = pH + pOH Kw always equals 14 So.. pH + pOH = 14 You can use this equation to convert between pH and pOH 10

ChemSaver p 40 Calculate the pH or the pOH from the given, then tell if the substance is acidic, basic, or neutral.   pH + pOH = 14 pH = 8 pOH = _______ (acidic/ basic/ neutral) pH = ______ pOH = 2 (acidic/ basic/ neutral) 11

Sample problem: Calculating pH Calculate the pH of a solution with a pOH of 12. 12

ChemSaver p 39-40 Calculate the molarity of… an acid whose pH is 5.3. (H+)= 10-pH   a base whose pOH is 4.5 (OH-)= 10-pOH

Sample problem: Calculating pH Calculate the hydrogen ion concentration of a solution with a pH of 8.5. 14

Sample problem: Calculating pH Calculate the hydrogen ion concentration of a solution with a pOH of 13.5. 15