pH Scale & Molarity
Unit 1:
Biochemistry
You must know!
How to interpret the pH scale.
 The importance of buffers in biological
systems.

pH Scale

In an aqueous solution some water and
other molecules tend to ionize
– Ex: H2O  H+ (hydrogen ion) and OH(hydroxide ion)
The concentration of H+ is used to
calculate pH
 The pH scale ranges from 1 - 14

pH Scale
Between each number of the pH scale the H+
concentration changes 10 fold.
 It is a negative, logarithmic scale

pH = -log [H+]
Ex: a solution has a concentration of 10-7 Molar
H+ therefore the solution has the pH = ?
 Water is neither acidic nor basic, but neutral

Acids

Acid
– Solution that has a high concentration of H+ ions
Chemical formula usually begins with H
 Has a pH of 6.9 or less
 Examples: HCl, H2SO4, HNO3, HCH3O3

Bases

Base
– Solution that has a low concentration of H+ (high
concentration of OH-)
Chemical formula usually ends with OH
 Has a pH of 7.1 or higher
 Examples: NaOH, KOH, Mg(OH)2

If an acid is added to water, the pH drops
 If a base is added to water, the pH rises

Buffers

Buffers
– Substance that keeps the pH relatively stable when
an acid or base is added.
Most biological solutions have a natural ability to
buffer: blood, milk, saliva, egg white
 Chemical reactions in living things (respiration,
photosynthesis) depend on a stable pH
 Buffers add H+ ions when pH rises or remove H+
ions when pH drops

Molarity
Used to determine a concentration in
chemical solutions
 1M (one molar) NaCl solution means that
6 x 1023 molecules of NaCl are dissolved in
1 liter H2O.
 Mass of a substance (indicated on Periodic
Table of Elements): Na = __, Cl = ___
so NaCl = Na + Cl = 58 grams/mole
 To make a 1M NaCl solution 58g NaCl will
be dissolved in 1L water
