pH

1
pH

• pH -logH
• Used because H is usually very small
• As pH decreases, H increases exponentially
• Sig figs only the digits after the decimal place
  of a pH are significant
• H 1.0 x 10-8 pH 8.0 2 sig figs
• pOH -logOH-
• pKa -log K

2
Relationships

• KW HOH-
• -log KW -log(HOH-)
• -log KW -logH -logOH-
• pKW pH pOH
• KW 1.0 x10-14
• 14.00 pH pOH
• H,OH-,pH and pOH
• Given any one of these we can find the other
  three.

3
Basic
Acidic
Neutral
4
Acid Base Strength

• A helpful way of looking at dissociation is to
  look at the general equation
• HA(aq) H2O(l) H3O(aq) A-(aq)
• Acid Base Conjugate acid Conjugate
  base
• This is an equilibrium.
• Competition for H between H2O and A-
• The stronger base controls direction.
• If H2O is a stronger base it takes the H
• Equilibrium moves to right.
• A strong acid is completely dissociated in an
  aqueous solution therefore equilibrium will move
  to the right.
• The chart on pg 616 shows the relative strengths
  of acids.

5
Calculating pH of Solutions

• Always write down the major ions in solution.
• Remember these are equilibria.
• Remember the chemistry.
• Dont try to memorize there is no one way to do
  this.

6
Protons

• The proton is fundemental to both Arrhenius and
  the B-L definitions.
• Dissociation of an Arrhenius acid HA gives an
  aqueous hydrogen ion, or hydrated proton, written
  as H(aq)
• HA(aq) H(aq) A-(aq)

7
Dissociation of water

• One of the most important properties of water is
  its ability to act as a proton donor (acid) or
  proton acceptor (base). This is called
  amphoteric
• H2O H2O H3O OH-
• or 2H2O H3O OH-
• To mathematically figure out the equilibrium
  constant you use the equation
• K H3O OH-
• H2O2

8
Ion-product constant of water

• Kw KH2O2H3OOH-
• At 25ºC KW 1.0 x10-14
• In EVERY aqueous solution.
• Neutral solution H OH- 1.0 x10-7
• Acidic solution H gt OH-
• Basic solution H lt OH-

9
Practice Problem

• Calculate the pH pf an aqueous ammoinum solution
  that has a OH- concentration of 1.9 x 10-3

10
Homework Problems

• 15.5,15.6, 15.8, 15.9, 15.10, 15.11