# L2 - Hydrogen Ions and Acidity

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```					   Hydrogen Ions and Acidity                              Self-ionization of Water
• As we have seen, water is amphoteric.
• In pure water, a few molecules act as bases
and a few act as acids.
H2O(l) + H2O(l)        H3O+(aq) + OH−(aq)
• This is referred to as self-ionization.

• Sometimes, the self-ionization of water is               Ion-Product Constant
described as:
• In any aqueous solution, as [H+]
H2O(l) ⇄    H+   (aq)   +   OH-   (aq)      increases, [OH-] decreases. If additional
ions (either hydrogen or hydroxide ions)
• The self-ionization of water occurs to a very
are added to the solution through the
small extent.
addition of an acid or base, the
• In pure water at 25oC, the hydrogen ion             concentrations of H+ or OH- must change
concentration, [H+], and hydroxide ion              to balance the addition.
concentration, [OH-], are each only 1.0 x 10-7M   • For aqueous solutions, the product of the
• Since the [H+] and [OH-] concentrations are         hydrogen ion concentration and the
equal in pure water, water is neutral.              hydroxide concentration is a constant
value equaling 1.0 × 10−14

Ion-Product Constant                                       [H+] vs. [OH-]
• This special constant is referred to as the
ion-product constant for water, Kw.
°
• At 25°C,
Kw = [H+] x [OH-] = 1.0 × 10−14
• Looking at the equation above:
What happens to the hydroxide concentration
if acid is added to water?
What happens to the hydrogen ion
concentration if base is added to water?

1
Because the ion product of H+ and                    Calculations Using Kw
OH- is constant if:
[H+] > 1.0 x 10-7    the solution is acidic          Kw = [H+] x [OH-] = 1.0 × 10−14

[H+] < 1.0 x 10-7    the solution is basic
• Because the product of hydrogen ions
- Or -                                           and hydroxide ions is equal to a constant
value, If we know one value, we can find
[H+] > [OH-]         acidic solution             the other.

[H+] < [OH-]         basic solution

Pg. 596
in your
Text

Work the practice problems above.

Try this:                                                         pH
Determine the hydrogen ion and hydroxide
ion concentrations in a solution that is 3.0   • “Hydrogen Power,” or power of hydrogen.
x 10-2 M NaOH.                                 • Is a way to express concentration of
acid/base solutions on a log scale,
resulting in easier numbers to work with.
• pH is defined as the negative base-10
logarithm of the hydronium ion
concentration.
pH = −log [H3O+]
MOVIE

2
pH                              pH
• An acid has a
• In pure water,                                  higher [H3O+]
than pure
Kw = [H3O+] [OH−] = 1.0 × 10−14            water, so its
pH is <7
• Because in pure water [H3O+] = [OH−],         • A base has a
lower [H3O+]
[H3O+] = (1.0 × 10−14)1/2 = 1.0 × 10−7       than pure
water, so its
• Therefore, in pure water,                       pH is >7.

pH = −log (1.0 × 10−7) = 7.00

Pg. 599
in your
Text

Now try this:
What is the pH of a solution that is 1.6
x 105 M OH- ?

Pg. 600
in your
Text

Now try this:
What is the hydroxide concentration in
a solution that has a pH of 10.5 ?

3
Work practice problems number 15a and                       The Other “p” Scale
16a on pg 601 in your text.
• The “p” in pH tells us to take the
negative log of the quantity (in this case,
hydrogen ions).
• The same thing can be done with
hydroxide ions.
pOH −log [OH−]

Watch This!                               How Do We Measure pH?
Because                                                                      • For less accurate
[H3O+] [OH−] = Kw = 1.0 × 10−14,                                        measurements,
one can use:
we know that                                                                     Litmus paper
• “Red” paper
−log [H3O+] + −log [OH−] = −log 1.0 x 10-14 = 14.00                                 turns blue
above ~pH = 8
• “Blue” paper
or, in other words,                                                                 turns red below
~pH = 5
pH + pOH = 14.00
An indicator

pH Indicators
pH indicators change colors at different acid
or base concentrations

4
How Do We Measure pH?

For more accurate
measurements,
one uses a pH
meter, which
measures the
voltage in the
solution.

5

```
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