The pH Scale The Power or Potential of Hydrogen Topic 8.3

The Nature of Science Occam’s razor a scientific and philosophic rule requiring that the simplest of competing theories be preferred to the more complex the pH scale was proposed to simplify the comparison of concentrations of H+ ions in solution based on the log10 scale the base 10 logarithm scale is simply an exponential scale (scientific notation) written in a different form

pH scale the pH scale is a way of expressing the strength of acids and bases instead of using very small numbers, we just use the NEGATIVE power of 10 on the Molarity of the H+ ion under 7 = acid 7 = neutral over 7 = base

pH of 1 is 100x more acidic than pH of 3 [H+] ions in scientific notation [H+] ions traditionally 1x100 1.0 1 1x10-1 0.1 2 1x10-2 0.01 3 1x10-3 0.001 4 1x10-4 0.0001 5 1x10-5 0.00001 6 1x10-6 0.000001 7 1x10-7 0.0000001 8 1x10-8 0.00000001 9 1x10-9 0.000000001 10 1x10-10 0.0000000001 11 1x10-11 0.00000000001 12 1x10-12 0.000000000001 13 1x10-13 0.0000000000001 14 1x10-14 0.00000000000001 pH of 1 is 100x more acidic than pH of 3 pH of 4 is 1,000x more basic than pH of 1 pH of 6 is 10,000 less concentrated [H+] than pH of 2

Properties of Water

water will self-ionize to a very small extent in pure water at 25°C, the [concentration] of H+ (or H3O+) and OH- ions are equal [H3O+] = [OH-] both have a concentration of 1.0 x 10-7 M the ionic product constant (Kw) for water at 298 K is: Kw = [H+] [OH-] = 1.0 x 10-7 x 1.0 x 10-7 = 1.0 x 10-14

acid solutions have a greater concentration of H+ ions (or H3O+) than OH- ions basic solutions are opposite and have more OH-, referred to as alkaline solutions when in water

pH calculations – Solving for pH pH = - log [H+] [H+] = 10-pH Example: If [H+] = 1 X 10-10, what is the pH? pH = - log 1 X 10-10 pH = - (- 10) pH = 10 Example: If [H+] = 1.8 X 10-5 pH = - log 1.8 X 10-5 pH = - (- 4.74) pH = 4.74

Try These! Find the pH of these: A 0.15 M solution of hydrochloric (HCl) acid pH = .82 2) A 3.00 X 10-7 M solution of nitric acid (H2NO3) pH = 6.5

pH calculations – Solving for H+ If the pH of Coke is 3.12, what is the [H+]? 10-pH = [H+] [H+] = 10-3.12 = 7.6 x 10-4 M

A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the solution? pH = - log [H+] 10-8.5 = [H+] 3.16 X 10-9 = [H+]

solving for pOH since acids and bases are opposites, pH and pOH are opposites pOH does not really exist, but it is useful for changing bases to pH pOH looks at the perspective of a base pOH = - log [OH-] Since pH and pOH are on opposite ends, pH + pOH = 14

What is the pH of the 0.0010 M NaOH solution? [OH-] = 0.0010 M pOH = - log 0.0010 pOH = 3 pH = 14 – 3 = 11

pH [H+] [OH-] pOH

pH testing There are several ways to test pH Blue litmus paper (turns red = acid) Red litmus paper (turns blue = basic) pH paper (multi-colored) pH meter (7 is neutral, <7 acid, >7 base) Universal indicator (multi-colored) Indicators like phenolphthalein Natural indicators like red cabbage, radishes

pH meter tests the voltage of the electrolyte converts the voltage to pH must be calibrated with a buffer solution and stored in a solution

pH indicators indicators are dyes that can be added that will change color in the presence of an acid or base. some indicators only work in a specific range of pH once the drops are added, the sample is ruined some dyes are natural, like radish skin or red cabbage