Tooth enamel is composed of the mineral hydroxyapatite. It is essentially insoluble in water with K_sp = 2.3 $\times$ 10^-59, but it reacts with weak acids in the mouth as described by one of the reaction equations below. You can determine the equilibrium constant, K, for this reaction from K_sp and K_w (the equilibrium constant for water autoionization). How do you do this? A)K = K_wK_sp B)K = K_w/K_sp. C)K = K_sp/K_w D)K = K_w = K_sp E)K = K_w = K_sp .

Question

Quizplus · Accepted Answer

The equilibrium constant for the reaction between tooth enamel and weak acids can be expressed as:

Tooth enamel (solid) + H+ (aq) + A- (aq) ⇌ HA (aq) + H2O (l)

where H+ represents the hydrogen ion released by the acid and A- represents the anion from the acid.

The equilibrium constant expression for this reaction is:

K = [HA]/[H+] [A-]

The concentration of water is not included in this expression as it is a pure liquid and its concentration remains constant.

The equilibrium constant for water autoionization is:

Kw = [H+][OH-]

where Kw is a known constant.

We can use these two equilibrium constants to relate the concentration of H+ in the two reactions:

K = Kw/[OH-] [HA]/[A-]

However, since enamel is essentially insoluble in water, the concentration of OH- is negligible and can be ignored.

Therefore, the equilibrium constant for the reaction between enamel and weak acids can be expressed as:

K = Kw [HA]/[H+] [A-]

or

K = Kw [H+]/[HA] [A-]

Thus, the correct choice is C: K = Kw [H+]/[HA] [A-].

Tooth Enamel Is Composed of the Mineral Hydroxyapatite ×\times× 10-59, but It Reacts with Weak Acids in the Mouth

Tooth Enamel Is Composed of the Mineral Hydroxyapatite $\times$ 10^-59, but It Reacts with Weak Acids in the Mouth