1.1 What is Chemistry?

http://www.ck12.org Chapter 21. Acids and Bases

present in extremely low concentrations). Mathematically, this means that we can ignore the 1.0× 10 −^7 M H+
already present. For example, if x has a value of 0.0012, the quantity (1.0× 10 −^7 + x) is equal to x after rounding to
the correct number of significant figures. This second assumption further simplifies the equation above, and we can
now solve for x:

1. 8 × 10 −^5 =[[ 0 x.][ 50 x]]

9. 0 × 10 −^6 =x^2

x= 0. 0030

The concentration of H+at equilibrium is equal to 1.0× 10 −^7 + 0.0030 M, which after rounding is simply 0.0030
M. Then, find the pH as usual:

pH=−log[H+]

pH=−log[ 0. 0030 ]

pH= 2. 52

A 0.50 M solution of acetic acid would have a pH of 2.52.

As long as we can make the two simplifying assumptions shown above, determining the pH of a weak acid solution
becomes much simpler. As we saw in the example problem,

Ka=[x]

2

[C]

where x is the equilibrium concentration of H+and C is the original concentration of the weak acid. Overall, this
reduces to a much simpler expression for finding [H+]:

[H+] =

√

C×Ka

Calculations involving weak bases can be carried out in the same fashion.

Lesson Summary

• Strong acids completely transfer their acidic protons to the solvent when dissolved in water. When a weak
  acid is dissolved in water, most of the molecules will retain their acidic protons, and only a small percentage
  will dissociate.


• Strong bases completely dissociate in water, releasing all hydroxide anions into the solution. The most
  common strong bases are soluble metal hydroxides. Weak bases partially deprotonate water to form small
  amounts of hydroxide and the protonated base.


• The pH of an acidic solution can be calculated from the original concentration of the acid and the value of Ka.
  Only the original concentration is required for strong acids.


• The pH of a basic solution can be calculated from the original concentration of the base and the value of Kb.
  Only the original concentration is required for strong bases.

Lesson Review Questions

1. Describe the difference between a strong and a weak acid. How does the Kavalue for a given acid correspond
   to acid strength?