Self-ionization of Water and pH
Objectives:
• 1. Identify the ions present in pure water and
give their concentrations.
• 2. Define pH and explain how the pH scale is
used.
• 3. Identify what a buffer is and how the
strength of it is related to the buffer capacity.
Water has many unusual properties. One of these properties is
water's ability to act as either an acid or a base.
(H2ODH3O++ OH-)
• Amphoteric is the name given to a substance that can act as either an acid
or a base.
• Water becomes hydronium and hydroxide ions through self ionization
– The concentrations of each for pure water are equal at a pH of 7
Ion-Product
• The ion-product (Kw) can be used to find the
concentration of acid and base in any solution
based on the following assumptions:
– The formula for Kw for ALL solutions at 25oC is Kw =
[H3O+][OH-] = 1x10-14
• pH of pure water is 7, so the concentration of H3O+ is 1 x10-7
• The concentration of base therefore is
[OH-] = 1x10-14/1x 10-7 = 1 x10-7
– EX: If a solution has 1 x10-2 H3O+ions, what is the
concentration of the base ions?
• [OH-] = 1x10-14/1x 10-2 = 1 x10-12
pH Scale
• The pH scale was developed to help people interpret the amount of
hydronium ions compared to the amount of base in a solution. The
scale was developed by Soren Sorenson, a Danish biochemist, and is
based on the -log of the hydronium ion (H3O+) concentration in
solution.{-log[H3O+]}.
– The pH scale contains values between 0 and14.
– Each whole number change in the pH of a substance represents a 10
fold increase in the number of hydronium ions.
– Scale is from 0 - 14
• 0-7 acidic (solutions with a higher hydronium ion (H3O+)concentration than
1 x10-7 are acidic ; 0 most acidic)
• 7 neutral
• 7-14 basic (solutions with a higher hydroxide ion (OH-)concentration than
1 x10-7 are basic ; 14 most basic)
– can be measured using a chemical indicator
• a weak acid or base that changes color as the pH changes (gain or lose H+ ions)
pH Scale
• pH scale can be used to determine the amount
of hydronium and hydroxide concentrations in
solution with the equation pH + pOH = 14 and
the ion product (Kw). Consider the following
examples.
• example: A solution with 1 x 10-2 H3O+ ions,
what is the pH and pOH of the solution?
– to get the pH simply change the negative
exponent to a positive pH
• pH = 2
• pOH = 14 -2 =12
• A solution with 1 x 10-2OH- ions
• Use the ion product constant 1st to find the
concentration of hydronium
–
Kw=1 x 10-14/1 x 10-2OH-= 1 x 10-12 H3O+ ions
• pH = 12
• pOH = 2