## 10.3 Strong and Weak Acids and Bases and Their Salts

### Learning Objectives

By the end of this module, you will be able to:

• Assess the strength of an acid or base based on dissociation in water.

Except for their names and formulas, so far we have treated all acids as equals. However, acids can be very different in a very important way. Consider HCl(aq). When HCl is dissolved in H2O, it completely dissociates into H+(aq) and Cl(aq) ions; all the HCl molecules become ions:

$\large\text{HCl}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)\rightarrow {\text{H}}_{3}{\text{O}}^{\text{+}}\left(aq\right)+{\text{Cl}}^{-}\left(aq\right)$ (100% dissociation)

Any acid that dissociates 100% into ions is called a strong acid. If it does not dissociate 100%, it is a weak acid. HC2H3O2 is an example of a weak acid:

$\large \text{H}\text{C}_{2}\text{H}_{3}\text{O}_{2}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)\rightarrow {\text{H}}_{3}{\text{O}}^{\text{+}}\left(aq\right)+\text{C}_{2}\text{H}_{3}{\text{O}_{2}}^{-}\left(aq\right)$ (~5% dissociation)

Because this reaction does not go 100% to completion, it is more appropriate to write it as an equilibrium:

$\large \text{H}\text{C}_{2}\text{H}_{3}\text{O}_{2}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)\rightleftharpoons {\text{H}}_{3}{\text{O}}^{\text{+}}\left(aq\right)+\text{C}_{2}\text{H}_{3}{\text{O}_{2}}^{-}\left(aq\right)$

As it turns out, there are very few strong acids, which are given in Table 1. If an acid is not listed here, it is a weak acid. It may be 1% ionized or 99% ionized, but it is still classified as a weak acid.

The issue is similar with bases: a strong base is a base that is 100% ionized in solution. If it is less than 100% ionized in solution, it is a weak base. There are very few strong bases (see Table 1); any base not listed is a weak base. All strong bases are OH compounds. So a base based on some other mechanism, such as NH3 (which does not contain OH ions as part of its formula), will be a weak base.

 Table 1. Some Common Strong acids and Strong Bases Strong Acids Strong Bases HClO4 perchloric acid LiOH lithium hydroxide HCl hydrochloric acid NaOH sodium hydroxide HBr hydrobromic acid KOH potassium hydroxide HI hydroiodic acid Ca(OH)2 calcium hydroxide HNO3 nitric acid Sr(OH)2 strontium hydroxide H2SO4 sulfuric acid Ba(OH)2 barium hydroxide

### Key Concepts and Summary

The strengths of Brønsted-Lowry acids and bases in aqueous solutions can be determined by how they dissociate in water.

### Exercises

1. Differentiate between a strong acid and a weak acid.

2. Differentiate between a strong base and a weak base.

3. Identify each as a strong acid or a weak acid. Assume aqueous solutions.

a)  HF

b)  HCl

c)  HC2O4

4.  Identify each as a strong base or a weak base. Assume aqueous solutions.

a)  NaOH

b)  Al(OH)3

c)  C4H9NH2

5.  Write a chemical equation for the ionization of each acid and indicate whether it proceeds 100% to products or not.

a)  HNO3

b)  HNO2

c)  HI3

6.  Write a chemical equation for the ionization of each base and indicate whether it proceeds 100% to products or not.

a)  NH3

b)  (CH3)3N

c)  Mg(OH)2

7.  A lab technician mixes a solution of 0.015 M Mg(OH)2. Is the resulting OH concentration greater than, equal to, or less than 0.015 M? Explain your answer.

8.  A lab technician mixes a solution of 0.55 M HNO3. Is the resulting H+ concentration greater than, equal to, or less than 0.55 M? Explain your answer.

## Glossary

Strong acid: Undergoes 100 % dissociation in water.

Strong base: Undergoes 100 % dissociation in water.

Weak acid: Undergoes <100 % dissociation in water.

Weak base: Undergoes <100 % dissociation in water.