Acids and Bases: Ionization Constants
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What is the pH of bleach?
Acid and base ionization constants also allow you to calculate the expected pH of a solution.
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What is a spectator ion? |
We can solve this problem by creating the same type of table (an ICE table) and following the same steps as before.
Step 1: Determine what is present in the solution initially (before any ionization occurs).
In
the table below, fill in the concentrations of OCl-, HOCl,
and OH- present initially (To enter an answer using scientific
notation, replace the "x 10" with "e". For example, if the answer is 1
x 10-5, type "1e-5".).
The
concentration of hypochlorite ion present before ionization is given
in the problem.
This
is a solution of hypochlorite ion; before any ionization occurs, there
is no HOCl present.
While
there is no hydroxide present from the ionization of the hypochlorite
ion, there is some present since this is an aqueous solution. What
is the hydroxide concentration in pure water?
Good!
Step 2: Determine how initial concentrations will change when ionization occurs.
Ionization
will decrease the concentration of the reactants and increase
the concentration of the products.
The
stoichiometry of the reaction shows the when one mole of hypochlorite
(OCl-) ionizes, one mole of hydogen hypochlorite (HOCl)
and one mole of hydroxide ions (OH-) form.
Good!
Step 3: Determine what is present in the solution after equilibrium has been reached.
This can be done since the equilibrium concentrations will equal the initial concentration plus the change that occurs with ionization:
| Initial + Change = Equilibrium |
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| Initial | 0.71 M | 0 M | 1 x 10-7 M | ||||
| Change | - x | + x | + x | ||||
| Equilibrium | 0.71 M - x | x | 1 x 10-7 M + x | ||||
These expressions for the equilibruim concentrations can be inserted into the base ionization constant expression:
This expression could be solved directly using complicated mathematics. However, some terms may be able to be ignored.
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How do I know which terms can be ignored? |
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This
x represents the hydroxide ions produced from the ionization of hypochlorite,
which will likely be much more than the amount due to the autoionzation
of water.
This
term can not be ignored.
Good!
Are there any others?
Good!
With these terms ignored, the expression is easy to solve:
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Before proceeding with the calculation, we need to make sure the terms we ignored are negligible. We ignored the contribution from the autoionization of water to the hydroxide concentration; 1 x 10-7 + 5.01 x 10-4 is still equal to 5.01 x 10-4 so the 1 x 10-7 can be ignored. We also ignored the ionization change from the concentration of the hypochlorite ion; 0.71 - 5.01 x 10-4 is still equal to 0.71 so the ionization change can be ignored. |
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Click to see another check of the approximation |
Looking back, the original problem was to calculate the pH of the bleach solution. This is easily calculated since in any aqueous solution, the product of the concentrations of hydronium ions and hydroxide ions is equal to a constant:
Kw = [ H3O+ ] [ OH-]
What
is the pH of household bleach? (Hint: look back at the table to see
what
the equilibrium concentration of hydroxide ions is.)
