Untitled - Kelly Walsh High School

Acids and Bases 223
The pH of pure water is 7.00 and on the pH scale, this is neutral. A solution that
has a [H 3O ] is greater than pure water will have a pH less than 7.00 and is
acidic. A solution that has a [H 3O ] is less than pure water will have a pH
greater than 7.00 and is basic.
It is also possible to calculate the pOH of a solution. It is defined as pOH =
log [OH ]. The pH and the pOH are related:
pH pOH pK w 14.00 at 25 o C
Calculate the pH and pOH of a 0.025M nitric acid solution.
Write the dissociation reaction for nitric acid, a strong acid:
HNO 3(aq) l H (aq) NO 3 (aq)
Since nitric acid is a strong acid, the contribution to the [H ] from the nitric acid
will be 0.025 M. The contribution of water to the [H ] will be insignificant.
Thus, [H ] 0.025
pH log [H ] log [.025] (1.60) 1.60
pOH 14.00 pH 14.00 1.60 12.40
A solution had a pH 8.75. Calculate the [H ] and [OH ] of the solution.
We may determine the [H ] of the solution from the pH using the inverse log
function on our calculator or by using the relationship:
[H ] 10 pH
[H ] 10 8.75 1.8 10 9 M
To calculate the [OH ] we will use the K w relationship: K w [H 3O ] [OH ]
K w
[OH ] 5.6 106M [H3O 1.00 1014
] 1.8 109 15-5 Acid-Base Equilibrium (K a and K b)
The K a—Acid Dissociation Constant
Strong acids completely dissociate (ionize) in water. Weak acids partially dissociate
and establish an equilibrium system. There is a large range of weak acids
based upon their ability to donate protons. Consider the general weak acid HA
and its reaction when placed in water:
HA(aq) H 2O(l) K H 3O (aq) A (aq)