pH - christophersonbiology

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Wake-up
Write down each equation below. Identify the
base (B), acid (A), conjugate acid (CA), and
conjugate base (CB).
1. NH3 + HCN  NH4 + CN
1. HSO4 + PO4  HPO4 + SO4
1. HCO2H + OH  H2O + CO2H
pH and pOH
Ways to measure Acidity/Basicity
What is pH?
What is pOH?
What does pH represent?
Parts Hydrogen
pH
Measurement that reveals if a solution is acidic
or basic
What is the pH Scale?
Runs from 0.0 to 14.0 with 7.0 being neutral;
Logarithmic
pH scale
Neutral
Acid
1
Base (Alkaline)
6 7 8
14
Logarithmic
Scale
A ten-fold difference
between each
successive full
number on the scale
Logarithmic Scale – ACID
Each whole pH value BELOW 7 is ten times more
acidic than the next higher value. For example, pH
4 is ten times more acidic than pH 5 and 100 times
(10 times 10) more acidic than pH 6.
Logarithmic Scale: BASE
Each whole pH values ABOVE 7 is ten times more
basic than the next lower whole value. For example,
pH 10 is ten times more alkaline than pH 9 and 100
times (10 times 10) more alkaline than pH 8.
Basic Equations (Reference Table)
pH + pOH = 14
pH = -log[H+]
pOH = -log[OH-]
[H+] = 10-pH
[OH-] = 10-pOH
Example #1
What is the pH of a 0.031 M HCl solution?
Known = 0.031 M HCl
Unknown = pH
pH = -log[H+]
pH = -log [0.031M]
pH = 1.5
Example #2
What is the pH of a 2.50 x 10-6 M HNO3
solution?
Known = 2.50 x 10-6 M HNO3
Unknown = pH
pH = -log[H+]
pH = -log [2.50 x 10-6 M]
pH = 5.60
Example #3
What is the pH of a 0.025 M NaOH solution?
Known = 0.025 M NaOH
Unknown = pH
pOH = -log[OH+]
pOH = -log [0.025 M] = 1.6 pOH
pH + pOH = 14
pH + 1.6 = 14
pH = 12.4
Example #4
What is the pH of a 0.000051 M NaOH
solution?
Known = 0.000051 M NaOH
Unknown = pH
pOH = -log[OH+]
pOH = -log [0.000051 M] = 4.3 pOH
pH + pOH = 14
pH + 4.3 = 14
pH = 9.7
Calculating pOH
• pOH = 14 – pH
• pOH = -log[OH-]
EXAMPLE: What is the pOH of a solution that has a
hydroxide ion concentration of 4.82 x 10-5 M ?
pOH = - log [OH-]
pOH = - log [4.82 x 10-5]
pOH = 4.32
YOU TRY!
• Find the pH of the solution if [H+] = 1 x 10-4 M
pH = 4
• What is the hydrogen ion concentration if pH =
7.1?
7.1 = -log [H+]
10 – 7.1 = [H+] = 7.9 x 10-8 M
• What is the pH if [OH-] = 4 x 10 -11 M?
pOH = -log [OH-]
pOH = -log [4 x 10 -11 ] =
pH scale
Neutral
Acid
1
Strong
Acid
Base
6 7 8
Weak
Acid
Weak
Base
14
Strong
Base
Breakdown of water

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