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Dissociation Constant and Autoionization of Water

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  • 0:07 Auto-Ionization of Water
  • 1:02 Kw Constant
  • 2:06 Determining…
  • 4:03 Example #1
  • 5:06 Example #2
  • 5:58 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn the meaning of auto-ionization of water, sometimes called self-ionization, where water acts as a proton donor and acceptor to form both hydronium and hydroxide ions. Learn what the auto-ionization constant is and how to use it to determine the concentration of either hydroxide or hydronium ions in a solution when given the other value.

Auto-Ionization of Water

You have learned that water is both an acid and a base. As such, it can either give or receive a proton. So, what happens when a water molecule interacts with another water molecule? Hydronium ions and hydroxide ions are formed in equal amounts by the transfer of a proton between the two of them.

Water can act as both an acid and a base to yield hydronium and hydroxide ions.
Auto-Ionization of Water

The reaction above is reversible and goes back and forth all the time. The reaction where a hydroxide ion and a hydronium ion are made is called auto-ionization of water, or sometimes it is called self-ionization. Auto-ionization of water is when two water molecules combine to create one hydroxide ion and one hydronium ion through the transfer of a proton.

Kw Constant

Even pure water contains ions. The water molecules are in equilibrium with the ions. In pure water, the ions are in equal concentrations, which experiments have shown to be 1.0 x 10-7 for each one.

There is a self-ionization constant of water, Kw, which has no units. It is the product of the hydronium ions times the hydroxide ions, or Kw = [H3 O+] [OH-]. This means that anything that increases one of the ions, either the hydronium ions or the hydroxide ions, decreases the other one. The concentration of hydronium ions in a solution is the acidity of the solution. The concentration of hydroxide ions in a solution is the basicity of the solution.

The self-ionization constant of water
Self-Ionization Constant

When I put brackets around the hydronium and hydroxide ions, it means concentration. In other words, [H3 O+] = X reads as 'the concentration of H3 O+ is X.'

Determining Concentration of Ions

You can determine the concentration of either OH- or H3 O+ by using the Kw, or self-ionization constant of water. It is easy to figure out the concentration of either the hydronium or hydroxide ions in a solution when given part of the information.

Problem: What is the [OH-] concentration in a 3 x 10-5 M solution of HCl?

First, look at the information given in the problem.

The basic equation is HCl + H2 O --> H3 O+ + Cl-.

HCl is a strong acid, so it dissociates completely, and you get the same concentration of H3 O+ ions as concentration of the acid as a whole, so [H3 O+] = 3 x 10-5 M.

You learned that [H3 O+] [OH-] = Kw = 1.0 x 10-14.

So, you know [H3 O+] = 3 x 10-5 M.

Kw = 1.0 x 10-14

[OH-] = ? This is what you are solving for.

If we know [H 3 O +] and Kw, we can solve for [OH -].
Determining Concentration

Put in your known values and solve for [OH-].

(3 x 10-5) [OH-] = 1.0 x 10-14

[OH-] = (1.0 x 10-14) / (3 x 10-5) M

[OH-] = 3.33 x 10-10 M

Notice that the concentration of H3 O+ ions is higher than the concentration of OH- ions, so this solution is an acidic solution.

Example

Try another one.

Problem: What is the [OH-] concentration in a 0.450 M solution of HNO3?

Given:

HNO3 + H2 O --> H3 O+ + NO3 -

[H3 O+] = 0.450 M

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