Due to the large range of proton concentration values ([H^{+}]) in
aqueous solution (typically from
10^{-15}
to 10 M), a logarithmic scale of acidity makes the most sense to put the
values into manageable numbers. The pH
scale of acidity
defines pH as the negative common logarithm of the concentration of
H^{+}: pH = - log
[H^{+}].

Before proceeding with our discussion on pH, let's review some properties of
Logarithms (affectionately called "logs"). A common
logarithm is a function that computes what exponent would be on 10 to obtain
the input number.
For example, the logarithm of 100 (written "log 100") is 2 because
10^{2} = 100. Likewise, log 1,000,000 = 6. To add logs, multiply their
arguments: log 100 + log
1000 =
log (100 x 1000)
= log (100,000) = 5. To subtract logs, divide their arguments: log 100 -
log 1000 = log
(100/1000) = log (0.1) = - 1. Given those simple rules, you can now
manipulate
logs well enough
to do any problem involving logs in acid-base chemistry. For this
section of chemistry,
you will get to know and love the log key on your calculator almost as much as
you did in kinetics.

The pH scale in water is centered around 7, which is called
neutral
pH. This value is not
randomly selected, but rather it comes from the fact that the [H^{+}] in
pure
water
is 10^{-7}
(recall that K_{w} = 10^{-14}). An acidic solution
has a pH value less than 7
because [H^{+}] is greater than that of pure water. A basic
solution has a pH greater than 7 because there is a lower [H^{+}] than
that of pure water
(and consequently a larger [OH^{-}]).

Chemists use a pOH scale analogous to the pH acidity scale to gage hydroxide
ion concentrations of aqueous solutions. pOH is defined
as the negative
common logarithm of the concentration of OH^{-}: pOH = - log [OH^{-
}]. In
the pOH scale, 7 is neutral, less than 7 is basic, and greater than 7 is
acidic.
A useful relationship (which you should be able to derive using the definition
of K_{w}) is
that pH + pOH = 14.
This formula will allow you to readily convert between values of pH and
pOH. A
comparison of the
pH and pOH scales is provided in . Note that because
K_{w} is
constant, the product of [H^{+}] and [OH^{-}] is always
equal to
10^{-14}.

As you have probably guessed by now, the prefix "p" in front of a symbol means
"take the negative
log". We have defined pH and pOH as values to describe the acidic or basic
strength of solutions. Now we can talk about pK_{a} (- log
K_{a}) and
pK_{b}
(- log K_{b}) as measures of acidity and basicity, respectively,
for standard acids and bases. An
acid with a
pK_{a} less than zero is called a strong acid because it
almost
completely dissociates
in water, giving an aqueous solution a relatively low pH. Acids with
pK_{a}'s greater than
zero are called weak acids because they only partially dissociate in water,
to make solutions with
larger pH's than those strong acids produce at the same concentration.
Similarly, bases
with
pK_{b}'s
less than zero are strong bases and bases with pK_{b}'s
greater
than
zero are called
weak bases.

Factors Affecting Acidity

Now that we have the tools to be able to describe the acidity and
basicity
of compounds with
known pK_{a}'s, let's analyze trends in acidity and basicity to
reach an intuitive
understanding of those trends.