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### Course: AP®︎/College Chemistry>Unit 8

Lesson 1: Introduction to acids and bases

# Autoionization of water

In the autoionization of water, a proton is transferred from one water molecule to another to produce a hydronium ion (H₃O⁺) and a hydroxide ion (OH⁻). The equilibrium expression for this reaction is Kw = [H₃O⁺][OH⁻], where Kw is the autoionization constant for water. At 25°C, the value of Kw is 1.0 x 10⁻¹⁴. In pure water, the concentrations of H₃O⁺ and OH⁻ are equal, and the water is considered to be neutral. Created by Jay.

## Want to join the conversation?

• How did you get 1.0*10^-14 for the equilibrium constant equation for [H3O+][OH-]?
• It's just Kw at 25°C. It's a constant which is found in any chemistry textbook's data tables.
• im confused how did you get x=2.5 x 10^-9M
• You know [H3O+] = 4.0 x 10^-6 M and that the solution is at equilibrium at 25°C. Therefore, you also know Kw = 1.0 x 10^-14

So,
Kw = [H3O+][OH-]
[OH-] = Kw/ [H3O+]
[OH-] = (1.0 x 10^-14)/(4.0 x 10^-6)
[OH-] = 2.5 x 10^-9 M

Because [H3O+] > [OH-], the solution is acidic.

(At least it‘s what I understood)
(1 vote)
• why was the concentration of water (left reactants) left out of the equilibrium expression for the autoionization of water?
(1 vote)
• Chemicals species which are pure liquids or solids are excluded from equilibrium expressions. Jay mentions this at .

For solids this is physically understood that the concentration of a solid is its density. And since density depends on the identity of a chemical, it does not change. So for simplicity sake they are given a value of 1 in the equilibrium expression and effectively ignored.

For pure liquids the reasoning is similar. The concentration does not change since the solution is primarily composed of that liquid (the solvent). So liquids also have values of 1 and are also ignored.

Hope that helps.
• Why do we need to know this? If I'm going into biology, I don't need to know what a strong acid versus a weak one is, right?
• the equation x squared equals Kw where x is the concentration of H3O and OH can only be used for a neutral solution right? Because otherwise the H3O and OH would have different concentrations?
• Yep, that's correct.
• 1. . How can the solution be acidic / basic if the hydronium and hydroxide are only made with the 1:1 ratio? In the autoionization equation , if 2 moles of water molecule react , there can only be 1mole of hydronium and hydroxide , which means the solution can never be acidic or basic.

2. at how can the one proton be equivalent to one hydronium? It makes no sense. Because , hydronium is consisted of 3 H atom and 1 oxygen atom , but proton is just the one proton , one ion of hydrogen.
(1 vote)
• Jay is just describing how we define an acid, basic, and neutral solutions. Whether the solutions have more hydronium or hydroxide. Not that these solutions are created by autoionization of water.

If you take the net ionic equation of an acid-base reaction, acids can be simplified to just H^(+). It’s not what is happening of course, but it’s a simplification to make equations more condensed.

Hope that helps.
• 1*10^-14 divided by 4*10^-6 is 2. * 10^-21st. how was thgis answer obtained at
• Looks like you multiplied instead of divided.
• how does the electron thing work ? like why does the hydrogen leave its electron behind ?/
(1 vote)
• Oxygen is more electronegative compared to hydrogen so it has an easier time of holding onto electrons. So it's not so much the hydrogen leaving its electrons, it's that the oxygen is taking the electrons.

Hope that helps.
(1 vote)
• What will be the concentration of other temperature apart from 25 degree and 50 degree
(1 vote)
• The trend is that when temperature increases the ionisation constant of pure water increases. You can search up the actual values
(1 vote)
• why would the reaction shift to the right if an increased temperature leads to an increased Kw? Increasing Kw would increase concentrations of OH- and H3O+... so wouldn't that cause the reaction to shift left in favor of H2O + H2O?
(1 vote)