At any one time, there are incredibly small numbers of hydronium ions and hydroxide ions present.

H2O (l) H+(aq) OH-

(aq)

Kw (ion-product constant for water)

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

1.0 x 10 -7 mol/L 1.0 x 10 -7 mol/L

Acids / bases dissolve in water - increase [H+] / [OH-] and cause an equilibrium shift.

Acidic - [H+] is greater than the [OH-]Basic - [OH-] is greater than [H+]

KW = 1.0 x 10-14

KW = [H+][OH¯]

H2O(l) H+(aq) + OH¯(aq)

· Definition pH and pOH.

· Given pH, pOH, [H3O+] or [OH¯], calculate the remaining values.

· Calculate Ka/Kb, given the pH or pOH and the concentration of a weak acid solution.

· Describe how an acid-base indicator works in terms of the colour shifts and Le Chatelier's Principle.

Acidic - [H3O+] > [OH¯]

Alkaline (basic) - [OH¯] > [H3O+]

Neutral - [OH¯] = [H3O+]

1909 - Soren Sorensen developed a simplified system for the degree of acidity of a solution.

pH - the potenz (power) of hydrogen - Germanpotentia hydrogenii - Latin

Convenient way to express [H+]

pH = -log [H+]

Similarly, the concentration of hydroxide can be expressed as pOH:

pOH = -log [OH-]

pH and pOH have no units.

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

pH + pOH = 14

Special case - pH can be read straight from the value of the [H+].

[H+] = 1.0 x 10-x then pH = x [OH-] = 1.0 x 10-x then pOH = x

Remember, in a neutral solution pH = pOH = 7

Acidic - pH < 7Basic - pH > 7

Calculate the pH of an HCl solution whose concentration is 5.0 x 10-6 mol/L.

pH = -log[H+]= -log(5.0 x 10-6 M) = -(-5.30) = 5.30

The pH of a solution is 3.25. Calculate the hydrogen ion concentration in the solution.

[H3O+] = 10-pH

= 10-3.25

= 5.6 x 10-4 M

** also means: [OH-] = 10-pOH

The pH of a solution is 10.30, what is the hydroxide ion concentration?

pOH = 14.00 - pHpOH = 14.00 - 10.30

= 3.70

[OH-] = 10-pOH

= 10-3.70

= 2.0 x 10-4 mol/L

[H+] = 10-pH

KW = [H+][OH¯]

OR

What is the pH of 5.0 x 10-5 M Mg(OH)2 solution?

Mg(OH)2 (s) Mg2+(s) + 2 OH-

(s)

x x 2x

[OH-] = 2x = 2(5.0 x 10-5 M) = 1.0 x 10-4 M

pOH = -log[OH-] = 4.00

pH = 14.00 - 4.00 = 10.00

Measuring pHThere are two ways to measure pH:pH Meters Indicators

The [H+] inside the probe (reference electrode) is compared to [H+] outside the probe.

Probe must be calibrated first.(inserted into known pH solution)

An indicator is a weak acid or base that undergoes a colour change when they gain or lose hydrogen ions.

Natural pH indicators

Beets, Blackcurrant juice, Blueberries , Carrots , Cherries , Curry Powder , Delphinium Petals , Geranium Petals , Grapes , Horse Chestnut Leaves , Hydrangea , Morning Glories , Onion , Pansy Petals , Petunia Petals , Poison Primrose , Poppy Petals , Purple Peonies , Rayhan Leaves , Red cabbage , Red Radish , Rhubarb , Rose Petals , Strawberries ,Tea , Thyme , Turmeric , Tulip Petals , Violet Petals

Red Litmus stays red in the presense of an acid.

It turns blue in presense of a base.

Weak acid is colourless and its conjugate base ion is bright pink.

Phenolphthalein is a commonly used indicator.

· The pH scale goes from 0 - 14. · pH = -log[H3O+] and [H3O+] = 10-pH · pOH = -log[OH¯] and [OH¯] = 10-pOH

· pOH + pH =14

· Indicators are weak acids or bases that change colour in response to changing hydronium ion concentrations.