Chapter 10 Acids and Bases 10.6 The pH Scale

pH Scale The pH scale: Is used to indicate the acidity of a solution. Has values that usually range from 0 to 14. Indicates an acidic solution when the values are less than 7. Indicates a neutral solution with a pH of 7. Indicates a basic solution when the values are greater than 7.

pH Range Acidic Neutral Basic 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Neutral [H3O+]>[OH-] [H3O+] = [OH-] [H3O+]<[OH-] Acidic Basic

pH Scale pH is: Defined as the negative log of the hydrogen ion concentration. pH = - log [H3O+] The value of the negative exponent of [H3O+] for concentrations with a coefficient of 1. [H3O+] = 1 x 10-4 pH = 4.0 [H3O+] = 1 x 10-11 pH = 11.0

Calculations of pH A calculator can be used to find the pH of a solution with a [H3O+] of 1 x 10-3 as follows. 1. Enter 1 x 10-3 by pressing 1 (EE) 3 (+/-). The EE key gives an exponent of 10 and the +/- key changes the sign. 2. Press the log key to obtain log 1 x 10-3. log (1 x 10-3) = -3 3. Press the +/- key to multiply by –1. (-3) +/- = 3

Significant Figures in pH When expressing log values, the number of decimal places in the pH is equal to the number of significant figures in the coefficient of [H3O+]. [H3O+] = 1 x 10-4 pH = 4.0 [H3O+] = 1.0 x 10-6 pH = 6.00 [H3O+] = 2.4 x 10-8 pH = 7.62

Calculating [H3O+] from pH The [H3O+] can be expressed with the pH as the negative power of 10. [H3O+] = 1 x 10-pH If the pH is 3.0, the [H3O+] = 1 x 10-3 On a calculator 1. Enter the pH value 3.0 2. Use (+/-) to change sign 3.0 (+/-) = -3.0 3. Use the inverse log key (or 10x) to obtain the [H30+]. = 1 x 10-3 M

pOH The pOH of a solution: Is related to the [OH-]. Is similar to pH. pOH = - log [OH-] Added to the pH value is equal to 14.0. pH + pOH = 14.0

pH and pOH The sum of the pH and pOH values is equal to 14.0 for any aqueous solution.