pH SCALE

Chemistry

Dissociation

Water will split apart to give a H+1 ion and an OH-1 ion.

H2O(l) H+1(aq)+ OH-1

(aq)

The equilibrium constant for this is Kw (like Ksp )

Kw = [H+1] x [OH-1] = 1.0 x 10-14

No matter how much of H+1 or OH-1 present, this will always be true. Why?

If the [H+1] is 0.1 M what is the [OH-1]? Kw = [H+1] x [OH-1] = 1.0 x 10-14

so: 1.0 x 10-14 = (0.1) x [OH-1] [OH-1] = 1.0 x 10-14/ 0.1 = 1.0 x10-13

What happens if I mix 0.1 M [H+1] and 0.1 M [OH-1]?

Problem: Calculate the [OH-1] if the [H+1] is 0.005 M

pH Scale

pH Scale

pH = -log10[H+]

[H+] = 10-pH

Scale is logarithmic Every step in pH is factor of

10 of [H+] Example:

pH = 3 [H+] = 1 x 10-3 M pH = 4 [H+] = 1 x 10-4 M

Low pH More Acidic! High pH More Basic!

pH Calculations

Determine the pH of a solution that has [H+] = 3 x 10-5 M.

What is the pH of concentrated HCl? ([H+] = 1.0 M)

Determine the [H+] of a solution with pH = 8.5.

What is the [H+] of concentrated H2SO4 (pH = -18)?

[OH-]

Water breaks down into H+ & OH-

H2O H+ + OH-

[H+] x [OH-] = 10-14

Low pOH More Basic

High pOH More Acidic

More Math Questions

What is the pH of a 1 M NaOH solution? ([OH-] = 1.0 M)

Determine the [H+] of a 0.05 M KOH solution. ([OH-] = 0.05 M)

Measuring pH

pH often measured with a pH probe

Can be measured with an acid/base indicator such as universal indicator

pH Indicators

pH Indicators Change

color at a certain pH

HIn H+ + In-

Natural pH Indicators

Natural pH indicators occur in nature: Blueberries, Red Cabbage, Tumeric, etc.

In acidic soils, hydrangeas produce blue flowers.

In basic soils, hydrangeas produce pink flowers.

pH Meters

Place a probe in solution

Usually accurate to about 0.01 pH unit

Detects H+ ions electrically

Questions…

1. Solution X has a pH of 10. Solution Y has a pH of 7. How many times greater is the [H+]X than [H+]Y?

2. [OH-] = 0.001 M, determine:1. pH2. [H+]

3. Which one gives the highest pH?a) 1 M HCl b) 1 M CH3COOH c) 1 M NH3

d) 1 M NaOH