Hydrogen Ions and Acidity
Hydrogen Ions from Water
 Water is highly polar – what does that mean?
 Water particles are in continuous motion
 If they possess enough energy, a H+ can be transferred
from one water molecule to another
 The particle that remains once the H+ leaves is OH The water molecule gaining the H+ becomes
hydronium (H3O+)
H2O(l) + H2O(l) → H3O+(aq) + OH-(aq)
Self-ionization of water
 Another way of writing the ionization of water is:
H2O(l) ↔ H+(aq) + OH-(aq)
 Hydronium and hydrogen ions can be written
interchangeably
 Both examples show water forming ions
 The equilibrium concentration for both hydrogen and
hydroxide ions is very small at 25°C
 Each is only 1 x 10-7 M
 When both [H+] and [OH-] are equal in concentration,
the solution is said to be neutral
Ion Product Constant for Water
 In any aqueous solution, as hydrogen ion
concentration increases, hydroxide ion concentration
decreases
 If hydrogen ions were to be added, the equilibrium
would shift and the hydroxide ion concentration
would decrease
 The product of the hydrogen-hydroxide
concentration is 1 x 10-14
 In other words, the Kw = [H+][OH-] = 1 x 10-14
Sample Problem, p. 596
Complete 9, 10, p. 596
Acidic and Basic Solutions
 When acids form aqueous solutions, there is more H+
ions than OH- (coming from the ionization of water)
 The [H+] is greater than 1 x 10-7 M
 Basic solutions are the opposite
 The hydroxide ion concentration is greater than the
hydrogen ion concentration
 The [OH-] is greater than 1 x 10-7 M
 They are otherwise known as alkaline solutions
Calculating pH (power of hydrogen)
 Ranges from 0 – 14
 Neutral solutions have a pH of 7
 The closer the pH is to zero, the more acidic it is; the
closer it is to 14, the more basic it is
 To calculate pH from the hydrogen ion concentration,
pH = -log[H+]
Figure 19.9, p. 597
Calculating pOH
pH + pOH = 14
pOH = -log[OH-]
 Express concentrations in scientific notation
 When writing the pH or pOH, the concentration of
the solution is equivalent to the same number of digits
after the decimal of the value itself
 Ex. 1.0 x 10-5 = 5.00 (two digits in the base of the
scientific notation – two digits after the decimal)
When is pH not a whole number?
 Most of the time…
As a result, it is not easy to make a simple mental calculation.
Instead, use the logarithmic equation to convert from
concentration to pH or pOH.
Ex. [H+]
pH
= 4.2 x 10-10M
= -log[[4.2 x 10-10]
= -9.37675
= 9.38
Questions 11-12, p. 599
Calculating [H+] from pH
 Rearrange the equation pH = -log[H+]
[H+] = - antilog pH
Using Kw, in addition to pH/pOH, [H+]/[OH-], you can
solve for any unknown
Sample Problem 19.3, p. 600
Sample Problem 19.4, p. 601
Questions 15-16, p. 601