Today’s lesson is brought to you by
7, 14, and the letters
Kw, pH
Kw: the ion-product constant of water
Q: will pure distilled water conduct electricity
A: As the demo shows, it will (slightly).
• If water conducts electricity, ions must exist
• Water exists as an equilibrium, which is
referred to as the self-ionization of water:
H2O + H2O  H3O+(aq) + OH–(aq)
[H3O+] [OH–] or Kw = [H O+] [OH–]
3
Kc =
[H2O]2
Simplified reaction: H2O  H+(aq) + OH–(aq)
[H+] [OH–]
+] [OH–]
[H
or
Kw
=
Kc =
[H2O]
Note: H+ is just shorthand for
Kw
• What is the value of Kw?
• It has been measured at 1.0 x 10–14 (25 °C)
• Notice (pg. 600) that by definition:
when
the solution is
[H+] > [OH–]
acidic
[H+] < [OH–]
basic
[H+] = [OH–]
neutral
• Pure water is neutral since [H+] and [OH–]
must be identical (both come from one H2O)
• As temperature increases Kw increases
Q: Rewrite the equilibrium of water with heat as
a product or reactant (based on above point)
Q: Do PE 1 (refer to example 15.1)
Answers
As temperature increases Kw increases.
It must be that: H2O + heat  H+ + OH–
(increase temp = shift right =  Kw)
PE 1
Kw = [H+] [OH–]
1.0 x 10–14 = [H+] [7.8 x 10–6]
[H+] =1.28 x 10–9
Thus, the solution is basic: [H+] < [OH–]
pH
• Notice (PE1) that a change in [H+] is matched
by a change in [OH–], since Kw is constant
(see middle columns of table 15.2 – pg. 603)
• [H+] is commonly referred to because it is
critical to chemical and biochemical reactions
• A quick method of denoting [H+] is via pH
• By definition pH = –log [H+]
• The pH scale, similar to the Richter scale,
describes a wide range of values
• An earthquake of ‘6’ is10x as violent as a ‘5’
• Thus, the pH scale condenses possible
values of [H+] to a 14 point scale
• Also, it is easier to say pH=7 vs. [H+]=1x10–7
Calculations with pH
• pH = –log [H+], what is pH if [H+] = 6.3 x 10–5?
Enter 6.3 x 10–5 (‘6.3’, ‘exp’, ‘5’, ‘+/-’)
Then hit ‘log’, followed by ‘+/-’ Ans: 4.2
• What is [H+] if pH = 7.4?
• To solve this we must rearrange our equation
[H+] = 10–pH mol/L
Enter ’10’, ‘xy’, ‘7.4’, ‘+/-’, ‘=‘ 3.98x10–8 M
• Finally, notice on pg. 603 that pH + pOH = 14
• This is related to Kw = 1.0 x 10–14
• You do not need to know how equations are
derived; you need to know how to use them
Equations and practice
• You will need to memorize the following:
[H+] = 10–pH
[OH–] = 10–pOH
pH = – log [H+]
pOH = – log [OH–]
pH + pOH = 14
• Read footnote on 601 about significant digits
• Do PE 2 – 4 on pg. 605
• Use examples 15.2 – 15.4 as reference
PE 2 answers
c) pH = – log [H+]
= – log [7.2x10-8]
= 7.14
pOH = 14 – pH
pOH = 14 – 7.14 = 6.86
(slightly basic)
d)
Ba(OH)2  Ba2+ + 2OH–
b)pOH = – log [OH–]
pOH = – log [OH–]
= – log [0.0050]
= – log [0.00070]
= 2.3
= 3.15
pH = 14 – pOH
pH = 14 - pOH
= 14 – 2.3 =11.7
= 14 – 3.15 = 10.85
= – log [H+]
= – log [0.020]
= 1.6989 = 1.70
pH + pOH = 14
pOH = 14 – pH
pOH = 14 – 1.70
=12.3
a)pH
PE 3 answers
a) [H+] = 10–pH = 10–2.90 = 1.29 x 10–3 mol/L
pOH = 14 – pH = 14.00 – 2.90 = 11.1
[OH–] = 10–pOH = 10–11.1= 7.94 x 10–12 mol/L
b) [H+] = 10–pH = 10–3.85 = 1.41 x 10–4 mol/L
pOH = 14 – pH = 14.00 – 3.85 = 10.15
[OH–] = 10–pOH = 10–10.15 = 7.08 x 10–12 mol/L
c) [H+] = 10–pH = 10–10.81 = 1.55 x 10–11 mol/L
pOH = 14 – pH = 14.00 – 10.81 = 3.19
[OH–] = 10–pOH = 10–3.19 = 6.46 x 10–4 mol/L
PE 3, 4 answers
d) [H+] = 10–pH = 10–4.11 = 7.76 x 10–5 mol/L
pOH = 14 – pH = 14.00 – 4.11 = 9.89
[OH–] = 10–pOH = 10–9.89= 1.29 x 10–10 mol/L
e) [H+] = 10–pH = 10–11.61 = 2.45 x 10–12 mol/L
pOH = 14 – pH = 14.00 – 11.61 = 2.39
[OH–] = 10–pOH = 10–2.39 = 4.07 x 10–3 mol/L
PE 4
a) acidic, b) acidic, c) basic, d) acidic, e) basic
Measuring pH
• pH can be measured in several ways
• Usually it is measured with a coloured acidbase indicator (see pg. 606) or a pH meter
• Coloured indicators are a crude measure of
pH, but are useful in certain applications
• pH meters are more accurate, but they must
be calibrated prior to use
• Calibration means setting to a standard
• A pH meter is calibrated with a solution of
known pH often called a buffer
• “Buffer” indicates that the pH is stable
Using pH meters
1. Always rinse pH meter in distilled water prior
to placing it in a solution (buffer or otherwise)
2. Place the pH meter in a buffer with about the
same pH as that of your solution (4, 7, or 10)
3. Turn on the pH meter only when in solution
• Start with with buffer 7. Hit “cal” once.
• Wait upto a minute until it automatically sets
• Rinse, dry, and place in second buffer (4/10)
• Hit “cal” once. Wait until it automatically sets
• There is no need to use “read”.
4. Measure the pH of your solution
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