pH Scale and all it’s glory!
pH Scale
• pH is a measure of how acidic
or basic a solution is.
• The pH scale ranges from
0 to 14.
• Acidic solutions have pH values
below 7, Basic solutions have
pH values above 7.
• A solution with a pH of 7 is
neutral, such as pure water.
What is it really measuring…
• pH is a measurement of the H+ ion concentration.
The greater the H+ ions in a solution, the more
acidic, and lower the number on the pH scale.
• The actual concentrations are very small so a
logarithmic scale is used, based on a power of ten!
• If a solution has a pH of 1 and a second solution
has a pH of 2, the first solution is not twice as
acidic as the second—it is ten times more acidic.
Calculating the pH of Strong Acids
• To determine the pH of a strong acid, you first
need the Hydrogen ion concentration. Remember
that in a strong acid, all the molecules dissociate
into the hydrogen ions.
• This is expressed in Molarity and abbreviated in
brackets. Ex [H+] = 2.05 M
• To determine the pH:
pH = - log
+
[H ]
Significant Digits
• Significant digits in pH calculations are tricky
because of the logarithmic scale.
• The number of decimal places in the pH is
determined by the significant figures in the
coefficient.
• Example: For a solution with [H+] = 1 x 10−4
pH = −log [1 x 10−4 ]
* One Sig Fig
pH = - [-4.0]
pH = 4.0
so One Decimal
Significant Digits Examples:
[H3O+] = 1 x 10-4
pH = 4.0
[H3O+] = 8.0 x 10-6
pH = 5.10
[H3O+] = 2.45 x 10-8
pH = 7.610
Practice: Calculate the pH
Find the pH of these strong acids:
pH = - log [H+]
1) A 0.15 M solution of Hydrochloric acid
Ans: pH = 0.82 – 2 sig figs = 2 decimal places.
2) A 3.00 X 10-7 M solution of Nitric Acid
Ans: pH = 6.523 – 3 sig figs = 3 decimal places
Calculating the [H+]
• Suppose you know the pH, but what to solve
for the [H+] concentration?
• A strong acid has a pH of 3.12. Determine
the [H+] concentration.
• pH = - log [H+]
• How do you get [H+] by itself?
• First need to divide by -1….
Calculating the [H+]
• -pH = log [H+]
• Take antilog (10x) of both sides and get
rid of the log.
10-pH = [H+] So….
[H+] = 10-3.12 = 7.6 x 10-4 M
* Remember: pH has 2 decimal places, so
coefficient can only have 2 significant
digits:
You Try! Practice Problem
• A solution has a pH of 1.25. What is the
concentration of hydrogen ions in the
solution?
pH = - log [H+]
1.25 = - log [H+]
-1.25 = log [H+] * Take the antilog
10-1.25 = [H+]
5.6 x 10-2 = [H+]
11
Practice Problems:
• 1) Determine the pH of a solution that has a
hydrogen ion concentration of 5.0 x 10-2.
• pH = - log [5.0 x 10-2] = pH = 1.30
• * 2 SF in base = 2 decimal places
• 2) Human blood must maintain a pH range of 7.35
to 7.45. Determine the range of [H+] for blood.
• 7.35 = - log [H+] = 4.5 x 10-8 M
• 7.45 = - log [H+] = 3.5 x 10-8 M
Determining the pOH of Strong Bases:
• Determining the pH is a useful calculation, but is
limited to acids because acids give of the H+ ion.
• If we want to consider bases, we need to consider
the concentration of [OH-] ions instead of [H+]
ions.
• This can be done by solving for the pOH:
•pOH = - log
[OH ]
• It’s the same exact thing, except this time it’s the
hydroxide ion concentration!
13
pH + pOH = ???
• There is a relationship between the pH and
pOH that is important to know, and water
can help us.
• What do we know about the pH of water?
• Water is right in the middle of the pH scale
because it has the exact same [H+] and [OH-]
concentrations. They are…
• [H+] = 1.0 x 107
• [OH-] = 1.0 x 107
14
pH + pOH = ???
• If we convert these to both pH and pOH by
taking their negative log then:
• pH = 7
• pOH = 7
• Again, right in the middle of the scale. So
what is the sum of pH and pOH going to be?
• 14!!! – This will always be true of ANY
substance and can be helpful in determining
calculations.
Using pOH to get [H+]
• What is the hydrogen ion concentration of a
0.0010 M NaOH solution?
• * Note we cannot simply take the –log because
it’s not an acid. We must use the pOH.
• pOH = -log[OH-]  pOH = -log[0.0010] = 3
• But we want the [H+] so pH + pOH = ????
Using pOH to get [H+]
• pH + pOH = 14
• pH = 14 – pOH  pH = 14 – 3 = 11
• Note: This makes sense because 11 is a base on
the pH scale and NaOH is a base b/c of the
hydroxide ion.
• Almost there: pH = -log[H+]
• 11 = -log[H+]
• [H+] = 1.00 X 10-11
Problem
• The pH of a sample of human blood
was measured to be 7.41 at 25 °C.
Calculate pOH, [H+], and [OH-] for
the sample.
Practice Problems:
A chemist dilutes concentrated hydrochloric acid to
make two solutions: (a) 3.0 M and (b) 0.0024 M.
Calculate the [H3O+], pH, [OH-], and pOH of the two
solutions at 25°C.
Practice Problems:
Problem 2: What is the [H3O+], [OH-], and pOH of a
solution with pH = 3.67? Is this an acid, base, or
neutral?