pH SCALE Chemistry
Dissociation Water will split apart to give a H +1 ion and an OH -1 ion. H 2 O (l) H +1 (aq) + OH -1 (aq) The equilibrium constant for this is K w (like K sp ) K w = [H +1 ] x [OH -1 ] = 1.0 x 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 ]? K w = [H +1 ] x [OH -1 ] = 1.0 x so: 1.0 x = (0.1) x [OH -1 ] [OH -1 ] = 1.0 x / 0.1 = 1.0 x 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 M
pH Scale
pH = -log 10 [H + ] [H + ] = 10 -pH Scale is logarithmic Every step in pH is factor of 10 of [H + ] Example: pH = 3 [H + ] = 1 x M pH = 4 [H + ] = 1 x M Low pH More Acidic! High pH More Basic!
pH Calculations Determine the pH of a solution that has [H + ] = 3 x 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 H 2 SO 4 (pH = -18)?
[OH - ] Water breaks down into H + & OH - H 2 O H + + OH - [H + ] x [OH - ] = 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-] = M, determine: 1. pH 2. [H + ] 3. Which one gives the highest pH? a) 1 M HCl b) 1 M CH 3 COOH c) 1 M NH 3 d) 1 M NaOH