Name _____________________________________________________ Date _____________ Period ________
Calculating pOH of a Base
We’ve already discussed properties of bases such as tasting bitter, NOT reacting with metals,
and neutralizing acids. Chemicals or substances having these properties are said to be basic.
There are two common definitions for bases:
1. Bases increase the concentration of the hydroxide ion, [OH-] (Arrhenius)
2. Bases are PROTON ACCEPTORS (Brønsted-Lowry)
Most bases encountered in everyday life are dissolved in water, also known as being an
aqueous solution. We’ve also already discussed that bases are also defined as having a pH
higher than 7, because
(1) their hydrogen ion [H+] concentration is less than 10-7 moles per liter (M) when
measuring pH (meaning the H+ ion is less in a base than an acid)
Bases and acids can also be measured using the pOH scale. Bases are defined as having
their hydroxide concentration [OH-] more than 10-7 moles per liter (M) when measuring pOH.
pOH means “potential hydroxide ion” scale. It measures how many hydroxide ions are present
in a solution. pOH is defined as the negative logarithm of the hydroxide ion concentration.
See the following equation:
−
pOH = − log[OH ]
This equation only works if you know the concentration, or Molarity of the basic
solution you are working with. Let’s do some example calculations together.
Calculations
1. What is the pOH of an basic solution that contains 2.3 x 10 -4 OH - ions?
• Are you working with an base? ____________
• Are you given the concentration, or Molarity of that solution? ____________
• Next, plug the concentration or Molarity into the equation.
!
pOH = − log[2.3 × 10 −4 ] = ___________________
2. What is the pOH of a 0.25 M NaOH solution?
• Are you working with an base? ____________
• Are you given the concentration, or Molarity of that solution? ____________
• Next, plug the concentration or Molarity into the equation.
!
pOH = − log[OH − ] = − log[______] = ____________
3. What is the pOH of a 0.085 M Mg(OH)2 solution?
• Are you working with an acid? ____________
• Are you given the concentration, or Molarity of that solution? ____________
• Plug the concentration into the equation:
!
pH = − log[H + ] = − log[______] = ____________
So, what happens if you’re not given the concentration, or Molarity, of an basic solution?? You
will have to calculate it first, before you can use the pOH equation.
Molarity (M)=
moles of solute (moles)
Liters of solution (L)
Practice
1. What is the pOH of a 15L solution containing 0.45 moles of NaOH?
• Calculate the molarity first:
• Plug the Molarity of this basic solution into the pOH equation:
2. What is the pOH of a 755 mL solution containing 1.25 g of KOH?
• Calculate the molarity first:
• Plug the Molarity of this basic solution into the pOH equation:
Working Backwards
If the pOH of a basic solution is known, one can also calculate the concentration of the
hydroxide ion, OH -. Refer to the equation below:
[OH − ] = 10 − pOH
1. What is the hydroxide ion, OH- concentration of a basic solution with a pOH of 1.5?
2. What is the hydroxide ion, OH- concentration of a NaOH solution with a pOH of 1.2?
2