KEY2CHEM

Acid-Base Chemistry

An acid can be defined as a substance that donates a proton ($$\require{mhchem}\ce{H+}$$) to generate hydronium ion ($$\require{mhchem}\ce{H3O+}$$) in aqueous solution. The extent of ionization (dissociation) of an acid determines whether it is classified as a strong acid (fully dissociates) or a weak acid (partially dissociates). The extent of dissociation, particularly of a weak acid, is an equilibrium process.

Example 1.

Which diagram best describes the solution that forms when $$\require{mhchem}\ce{HCl}$$ () is dissolved in water ($$\require{mhchem}\ce{H2O}$$, )?

A

B.

C.

Solution

B.

$$\require{mhchem}\ce{HCl}$$ is a strong acid, which dissociates completely into ions ($$\require{mhchem}\ce{H3O+}$$ and $$\require{mhchem}\ce{Cl-}$$) when dissolved in water.

Example 2.

In a $$0.50\text{ M}$$ solution of the weak acid acetic acid ($$\require{mhchem}\ce{CH3COOH}$$), which species will have the highest concentration?

A. $$\require{mhchem}\ce{CH3COOH}$$

B. $$\require{mhchem}\ce{H3O+}$$

C. $$\require{mhchem}\ce{OH-}$$

Solution

A. $$\require{mhchem}\ce{CH3COOH}$$

A weak acid dissociates only slightly into its ions; most of the acid exists as undissociated molecules.

Example 3.

Which statement about a strong base is true?

A. A strong base is more harmful than a weak base.

B. A strong base dissociates to a greater extent than a weak base.

C. A strong base is more concentrated than a weak base.

Solution

B. A strong base dissociates to a greater extent than a weak base.

A strong  base dissociates completely into ions while a weak base will generate a smaller than stoichiometric concentration of ions.