Unit 5- lecture 5 Using the pH scale to characterize acids and bases.

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Presentation transcript:

Unit 5- lecture 5 Using the pH scale to characterize acids and bases

The pH scale is a way of expressing the strength of acids and bases The pH scale is a way of expressing the strength of acids and bases. It is based on the concentration of H+ each pH unit is a factor of 10 for [H+] < 7 = acid 7 = neutral > 7 = base

Equation to relate pH with H+ pH= - log[H+] [H+] =10-pH [H+] = H+ concentration of a substance in Molarity (M)

pH calculations – Solving for H+ Ex. 1: If the pH of Coke is 3, [H+] =? [H+] = 10-pH [H+] = 10-3 = .001 M = 1 x 10-3 M Ex. 2: If the pH of ammonia is 11.5, [H+] =? [H+] = 10-pH [H+] = 10-11.5 = 3.2 x 10-12 M

pH calculations – Solving for pH Ex 3: If [H+] = 2.5 X 10-10 M, what is the pH? pH = - log [H+] pH = -log [2.5 X 10-10 ] pH = 9.6 Example 4: If [H+] = 8.0 X 10-5 M, what is the pH? pH = - log [8.0 X 10-5 ] pH = 4.1

Because pH is on a logarithmic scale… this means that for every 1 point change in pH the [H+] changes by a power of 10 Example: when pH change from 1 to 3… - it is a 100 times change in H+ - (less acidic by 100 times) When pH changes from 12 to 9 - it is a 1000 times change in H+ - more acidic by 1000 times.

1. If a substance has a pH of 2, how many more times acidic is it then a substance with a pH of 6? 2. If a substance has a pH of 4, how many more times Hydrogen concentration does is it have than a substance with a pH of 7? 3. Which substance is more acidic, that with a pH of 2 or a pH of 5. How many more times acidic is it?

pOH Since acids and bases are opposites, pH and pOH are opposites! So if something is acidic pOH is > 7, if its basic pOH is < 7, and if its neutral pOH = 7. pOH looks at the perspective of a base pOH = - log [OH-] [OH-] = 10-pOH Since pH and pOH are on opposite ends, pH + pOH = 14

pH [H+] [OH-] pOH

Ex. 1: What is the pH of a solution with a pOH of 12.5? pH + pOH = 14 pH = 1.5, acid Ex 2: What is the pOH of a solution with an [OH-] of 5 x10-3 ? pOH = -log [OH-] = -log [5x10-3] = 2.3, base

Ex 3: What is the [OH-] of a solution with a pOH of 11.5? [OH-] = 10-pOH = 10-11.5 = 3.16 x 10-12 M , acid Ex 4: What is the [OH-] of a solution with a pOH of 4.5? = 10-4.5 = .000032 M or 3.2 x10-5 M , base

All water solutions contain both H3O+ and OH- In a neutral solution [H+] = [OH-] In an acidic solution [H+] > [OH-] In a basic solution [H+] < [OH-]