http://www.ck12.org Chapter 21. Acids and Bases
KOH(s)→K+(aq) + OH−(aq)Now, the increased concentration of the OH−ions requires a decrease in the concentration of the H+ions in order
to keep the value of the ion-product [H+][OH−] at its equilibrium value. Sample problem 21.1 illustrates how Kw
can be used to find the concentration of either H+or OH−in an aqueous solution when the other concentration is
known.
Sample Problem 21.1: Using Kwin an Aqueous Solution
Hydrochloric acid (HCl) is a strong acid, meaning that it is essentially 100% ionized in solution. What are the values
of [H+] and [OH−] in a 2.0× 10 −^3 M solution of HCl?
Step 1: List the known values and plan the problem.
Known
- [HCl] = 2.0× 10 −^3 M
- Kw= 1.0× 10 −^14
Unknown
- [H+] =? M
- [OH−] =? M
Because HCl is 100% ionized, the concentration of H+ions in solution will be equal to the original concentration of
HCl. Each HCl molecule that was originally present ionizes into one H+ion and one Cl−ion. The concentration of
OH−can then be determined from [H+] and Kw.
Step 2: Solve.
H+ = 2.0× 10 −^3 M
Kw= [H+][OH−] = 1.0× 10 −^14
[OH−] =
Kw
[H+]=
1. 0 × 10 −^14
2. 0 × 10 −^3
= 5. 0 × 10 −^12 M
Step 3: Think about your result.
[H+] is much higher than [OH−] because the solution is acidic. As with other equilibrium constants, the units for
Kware customarily omitted.
Practice Problems- State whether the following solutions are acidic or basic.
a. [H+] = 4.0× 10 −^8 M
b. [OH−] = 3.0× 10 −^5 M
c. [H+] = 7.1× 10 −^6 M - A certain aqueous solution has a hydroxide ion concentration of 5.6× 10 −^5 M. Calculate [H+] in this solution.
The pH Scale
Expressing the acidity of a solution by using the molarity of the hydrogen ion is cumbersome because the quantities
are generally very small. Danish scientist, Søren Sørenson (1868-1939), proposed an easier system for indicating