Chemistry I Honors
Acids and Bases
pH and Associated Calculations
Molarity
• This is actually “old
business” as we covered
Molarity in our lessons
involving solutions.
• The “Molarity Pyramid”
still works for these
systems.
• We can calculate the
molarity of an acid
solution the same way we
would calculate the
molarity of any other
solution.
Moles of
Solute
Molarity
Liters of
Solution
The Concentration of Hydronium
• This is an easy one.
• If you look back at the first presentation, you
see that acid molecules react with water to
form hydronium ( H3O+ ).
• In every single example presented, one acid
molecule reacted with one water molecule to
produce one hydronium ion.
example:
HCl + H2O  H3O+ + Cl-
Meaning ????
• If you look back at the
reaction on the previous
slide, you will see that the
ratio of H3O+ to HCl is 1 : 1
• Meaning that the
concentration of
hydronium will be exactly
what the concentration of
the acid solution was.
• So, if the HCl solution is
prepared to be 0.50 molar,
the concentration of H3O+
will be 0.50 - molar
No Big Deal There…
What’s Next ?
• What’s next is
something called pH .
• pH is a measurement
that communicates the
relative acidity of an
acid (or base – but that
discussion occurs later)
solution.
• It is simply a number
value – and it does not
have units
• The best place to begin a
discussion of pH is to
understand the pH scale.
• As you will see on the next
slide, the common scale
runs from 0 to 14 with 7 in
the middle.
• Acid solutions have pH
values less than 7 and base
solutions have pH values
greater than 7
• An actual value of 7 is
considered “neutral” –
neither acidic nor basic.
The pH Scale
Note that the word “alkaline” is a synonym for “basic”.
While this visual does not show it, the scale does
extend to 0 on the left and 14 on the right.
More on the pH Scale
• Need to realize that the
“ends” of the scale are
“more”.
– As you move to the left,
the solutions become
more acidic.
– As you move to the
right, the solutions
become more basic.
• That means that a pH =
2 solution is more acidic
than a pH = 3 solution.
(and a pH 1 solution
would be more acidic
than a pH 2 solution)
• Or, a pH 10 solution is
more basic than a pH 9
solution.
What about the Calculations?
• The pH scale is actually a logarithmic scale, so
we will be taking the logs of numbers.
• The equation to calculate pH is:
pH = - log
+
[H3O ]
pH is the opposite of the log of the
concentration of the hydronium in a solution.
The square brackets around the hydronium
mean “concentration of” (in terms of molarity)
Keystrokes
• So, once you have the
molarity of the acid
solution…
• On a TI-30 series
calculator
– Start with the numerical
value of the molarity
– Push the “log” button
– Change the sign
• You have the pH
• On the TI – 80 series
calculator
– Push the sign change
button (-)
– Push the “log” button
– Enter the numerical
value of the molarity
– Hit “enter”
• Now you have the pH
What if You Have a Base?
• Remember from the slide
on the pH scale that
bases have pH values that
are greater than 7.
• We also have to keep in
mind that bases do not
really have
concentrations of H3O+
because that ion is
produced by acids.
• What we learned in Acids
Lesson #1 is that bases
are metal hydroxides that
dissociate in water to
yield OH- ions.
• For monobasic
compounds (have 1 OHin the formula), the [OH-]
will be the same as the
molarity of the base
solution (just like the
acids).
Showing the Base Dissociation
• Again, consider the dissolving of sodium hydroxide
(monobasic) in water.
Na(OH)  Na+1 (aq) + (OH)-1 (aq)
• There are no coefficients written – they are all 1’s.
So… the ratio of (OH)-1 to Na(OH) is 1 : 1.
• Therefore, the [OH-1] will be equal to the molarity
of the Na(OH) solution.
• If the Na(OH) solution has a concentration of 0.30
molar, the [(OH)-1] will be 0.30 molar.
pOH The Calculation for Bases
• Chemistry has a “parallel” calculation (to the
determination of pH) just for Bases.
• It is called pOH and it is calculated virtually the
same way as we calculate pH.
pOH = - log [OH-1]
• The keystrokes on the calculator will be exactly
the same as described for pH. But the results will
be “curious” – look at the example on the next
slide.
Sample Problem
“A solution of Na(OH) is prepared to have a
concentration of 0.0025 molar. What is the pOH
of this solution?”
First - Since the Na(OH) is 0.0025 Molar, the
[OH-1] is also 0.0025 Molar.
Then: pOH = - log [OH-1]
= - log (0.0025)
= 2.60 (no units)
The “Curious” Part
• You have already seen that the pH scale is
structured so that bases have pH values
greater than 7.
• The answer you calculated in the previous
slide was 2.60 (obviously less than 7).
• How is this possible?
Explanation
• The calculation that we did was for pOH , not
pH.
• The pOH scale actually runs backwards
compared to the pH scale
– The zero value will be on the right
– The 14 value will be on the left
– This is because bases are the “chemical opposites”
of acids.
Comparing the Scales
• Notice how the scales run in opposite directions to each other.
• Notice too how low values on each scale correspond to “more”
• Low pH value is more acidic
• Low pOH value is more basic
Here is the Cool Part
• If we draw a vertical line anywhere on the diagram above and
add the values of pH and pOH that the line would intersect on
the two scales, the result of that adding will be “14”. (Try it!)
• That allows us to create an equation that connects pH and pOH
pH + pOH = 14
Back to the Original Problem
“A solution of Na(OH) is prepared to have a
concentration of 0.0025 molar. What is the pOH
of this solution?”
• Looking back at the calculation (5 slides
earlier), the pOH = 2.60
• But, what if the problem had actually asked
you to calculate the pH ? (instead of the
pOH).
Now You Can Do This
• If the pOH is 2.60 , we can convert this
measurement to a pH using the relationship
presented two slides earlier.
pH + pOH = 14
So:
pH + 2.60 = 14
Subtracting: pH = 14 - 2.60
pH = 11.4
Which is in the Base range on the pH scale. (as it
should be – NaOH is a base.)
Summarizing
Acids
Bases
• Have the formula “HX”
• Ionize to form H3O+1
• Will have [H3O+1] equal to
the acid solution molarity
• You will calculate a pH using
the [H3O+1]
• You can convert this to a
pOH using pOH = 14 - pH
• Are metal hydroxides
• Dissociate in water to form
the OH-1 ion
• Will have [OH-1] equal to
the base solution molarity
• You will calculate a pOH
using the [OH-1]
• You can convert this to a pH
using pH = 14 - pOH
Your Task
• There are two different worksheets that will
provide some quick practice in calculations
involving molarity and pH and pOH.