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.