pH Scale and Calculations

1
pH Scale
pOH
pH

• Unit 1
• Page 68-70

2
pH Scale

• We use this scale to measure the strength of an
  acid or base.
• pH is defined as the logH
• (negative log of the hydrogen ion concentration,
  in moles per liter)
• pH can use the concentration of hydronium ions or
  hydrogen ions.

3
pH Scale
7
Acid
Base
0
14

Zumdahl, Zumdahl, DeCoste, World of Chemistry
2002, page 515
4
pH Scale
Acidic
Neutral
Basic or Alkaline
5
pH of Common Substances
Timberlake, Chemistry 7th Edition, page 335
6
pH and pOH of Common Substances
pH H1 OH1- pOH
14 1 x 10-14 1 x 10-0 0 13 1 x
10-13 1 x 10-1 1 12 1 x 10-12
1 x 10-2 2 11 1 x 10-11 1 x 10-3
3 10 1 x 10-10 1 x 10-4 4
9 1 x 10-9 1 x 10-5 5 8 1 x
10-8 1 x 10-6 6 6 1 x 10-6
1 x 10-8 8 5 1 x 10-5 1 x 10-9
9 4 1 x 10-4 1 x 10-10 10
3 1 x 10-3 1 x 10-11 11 2 1 x 10-2
1 x 10-12 12 1 1 x 10-1 1 x
10-13 13 0 1 x 100 1 x 10-14 14
NaOH, 0.1 M Household bleach Household
ammonia Lime water Milk of magnesia Borax Bakin
g soda Egg white, seawater Human blood,
tears Milk Saliva Rain Black coffee Banana Tomato
es Wine Cola, vinegar Lemon juice Gastric juice
More basic
7 1 x 10-7 1 x 10-7 7
More acidic
7
Acid Base Concentrations
10-1
pH 3
pH 11
OH-
H3O
pH 7
10-7
concentration (moles/L)
OH-
H3O
OH-
H3O
10-14
H3O lt OH-
H3O OH-
H3O gt OH-
acidic solution
neutral solution
basic solution
Timberlake, Chemistry 7th Edition, page 332
8
pH
pH -log H
pOH -log OH-
pH pOH 14
Kelter, Carr, Scott, Chemistry A World of Choices
1999, page 285
9
Self-Ionization Of Water

• Even the purest of water conducts electricity.
  This is due to the fact that water self-ionizes,
  that is, it creates a small amount of H3O and
  OH-.
• H2O H2O H3O OH-
• In pure water, the concentration of H is always
  equal to the concentration of OH-.
• This equilibrium is very important because it
  creates a concentration of H whose negative
  logarithm is 7, the pH of a neutral solution.

10
pH and pOH

• pH - logH3O H3O 10-pH
• pOH - logOH- OH- 10-pOH
• pH pOH 14.00
• neutral solution H3O OH- 10 7 M pH
  7.0
• acidic solution H3O gt 10-7 M pH lt
  7.0
• basic solution H3O lt 10-7 M pH gt 7.0

11
Calculating pH and pOH

• pH The negative logarithm (base 10) of the
  hydrogen ion concentration, when the
  concentration is in moles per liter, or its
  Molarity.
• pOH The negative logarithm (base 10) of the
  hydroxide ion concentration, when the
  concentration is in moles per liter, or its
  Molarity.

12
Procedure to find pH or pOH
First Find the Molarity of the hydrogen (or
hydroxide) ions, which is the H concentration
(or the OH- concentration). Example A
0.01 M solution of HCl has a
concentration of H ions equal to 0.01 mol/L.
Then Find the pH by taking the negative log
(base 10) of the molar concentration. Example
The log100.01 2
13
Molarity to pH
Two pH calculation methods Longhand or
Calculator 1. Longhand method a. Write
molarity in exponential form. .01
10-2 b. The logarithm of 10-2 is -2. c.
The negative of -2 is 2 d. The pH is 2. (Use
for molar concentrations that are in powers of
ten 10-6, 10-12)
14
Molarity to pH Calculator
2a. Method 1 (Casio Classroom calculator)
a. enter molarity (.01) in your calculator
.01 b. press the key that says log read
-2 c. press the /- key to change the
sign 2 d. read the pH as 2 2b. Method 2
(Casio fx-3000MS calculator) a. Press the log
button b. Enter the number .01 c. Press the
key and read -2 d. Change the sign and
read the pH as 2 (Use for molar concentrations
that are in other than powers of ten.)
15
Molarity to pH Calculator
2c. Method 3 (TI 83 calculator) a. Press
the log button b. Enter the number .01 c.
Press the key and read -2 d. Press the (
- ) key e. Press the enter key and read
2 (Use for molar concentrations that are in
other than powers of ten.)
16
Procedure Casio Classroom
Calculator method (check YOUR calculator
algorithm) Task Find pH and pOH when H
concentration is 0.00004 M a. 0.00004 in
scientific notation 4 x 10 -5
Calculator Action
Readout b. Enter 4 in your calculator c.
Press the EXP key d. Enter the number 5
e. Press the sign change (/-) key f.
Press the log key g. Press the
/- key h. Read the pH i. 14 4.4
9.6
4
4 00
4 05
4 -05
- 4.397940009
4.397940009
4.4
pOH
17
Procedure Casio fx-3000MS
Calculator method (check YOUR calculator
algorithm) Task Find pH and pOH when H
concentration is 0.00004 M a. 0.00004 in
scientific notation 4 x 10 -5
Calculator Action
Readout b. Press the log key c. Enter the
number 4 d. Press the EXP key e. Enter
the number -5 f. Press the key
g. Press the /- key (or x -1) h. Read
the pH i. 14 4.4 9.6
log
log 4
log 4E
log 4E-5
- 4.397940009
4.397940009
4.4
pOH
18
Procedure TI-83 Calculator
Calculator method (check YOUR calculator
algorithm) Task Find pH and pOH when H
concentration is 0.00004 M a. 0.00004 in
scientific notation 4 x 10 -5
Calculator Action
Readout b. Press the log key c. Enter the
number 4 d. Press the EE key e. Enter
the ( - ) 5 f. Press the enter key
g. Press the ( - ) 2nd ANS keys h.
Read the pH i. 14 4.4 9.6

log
log (4
log (4E
log (4E-5
- 4.397940009
4.397940009
4.4
pOH
19
Check Your Calculation!
To check your answer, reverse the procedure.
The result should be your starting molar
concentration. (4 x 10 -05 or 0.00004)
(Calculator)
(Readout) a. Enter pH of 4.4 b. Press
the /- sign (x -1) c. Press the 10x key
(shift 10x ) d. Read molarity
4.4
- 4.4
3.9810717 -05
4 x 10 -05
20
Finding pOH pH pOH 14 If you know the pH,
then you know the pOH. For pH 2 If you
know the pOH, you know the pH. For pOH 11
pOH 12
pH 3
21
Your Turn!
Find the pH and pOH if the molar concentration of
H is 2.1 x 10-06 Since the pH is the negative
logarithm, base 10, of the molarity, then
log10 H log10 2.1 -06 -5.677780705 a.
Enter 2.1 into your calculator (Casio
Class) b. Add exponent notation c.
Press 6 key to change exponent to 6 d.
Change sign of exponent e. Press the log
function key f. Press the sign change
key g. Read the pH as h. Since pOH is
14 pH, then the pOH
2.1
2.1 00
2.1 06
2.1 -06
- 5.677780
5.677780
5.6
8.4
22
Your Turn!
Find the pH if the molar concentration of OH- is
3.5 x 10-09 Since the pOH is the negative
logarithm, base 10, of the molarity, then
log10 OH- log10 3.5 -09 -8.455 a.
Enter 3.5 into your calculator b. Add exponent
notation c. Press 9 key to change exponent
to 9 d. Change sign of exponent e.
Press the log function key f. Press the sign
change key g. Read the pOH as h.
Since pH is 14 pOH, then the pH
3.5
3.5 00
3.5 09
3.5 -09
- 8.45593
8.45593
8.5
5.5
23
Questions ?