Test: Acid/base Equilibria - MCAT MCQ

Le Châtelier's principle states that equilibrated solutions tend to resist changes in the relative concentrations of chemical species.

The reaction thus seeks to minimize the effect of the added OCl^- ions, tilting the equilibrium to the left.

The concentration of HOCl increases.

Recall the Henderson-Hasselbach equation, which is valid for the dilute solution described here,

Recall that pKα =−log K_α = 4

Solving the Henderson-Hasselbach equation yields [A^-] = 10^-6 M

Recall that an acid has both K_α and a K_b the relative values of which depend on the nature of its interaction with water.

Recall that K_w = 10^-14

Recall K_αK_b = Kw, and so K_b = 2.5 x 10^-6

Whether a chemical is an acid or base depends on whether K_α or K_b is greater.

Because K_αK_b = K_w, = 10^-14, for this chemical K_α = 3.125 x 10³, suggesting that it is an acid.

Because K_α >> 1, the species is known as a strong acid.

A Bronsted-Lowry acid tends to donate hydrogen ions, which have positive charges in solution.

This leaves the rest of the molecule with a net negative charge.

A Lewis acid tends to accept electrons in solution, leaving the molecule with a net negative charge

Thus both X and Y will tend to acquire a net negative charge in solution

Ammonia is a weak base

Hydrochloric acid is a strong acid

The hydrochloric acid will donate a proton to the ammonia to form ammonium,

Ammonium reacts with water to form hydronium,

A mixture of a strong acid and a weak base acquires an overall acidic pH < 7

The autoionization of water occurs naturally due to attractive forces between constituents of water molecules.

Even if all ions are removed from a sample of water (including hydronium ions), a short time later the water will re-ionize until it approaches an equilibrium concentration of ions.

In equilibrium, K_α  = K_w = 10^-14

Recall the Henderson-Hasselbach equation, which is valid for the buffer solution described here, 

Recall that pKα = - log K_α = 1

Now solve the other term,

The pH is thus pH = 1 + 2 = 3

pH is the negative logarithm of the molar concentration of hydrogen ions

Equivalently, the pH is the logarithm of the molar concentration of hydronium

pH = - log[H₃O⁺] = -4

When a strong acid or base equilibrates in solution, equilibrium is driven to the right, towards the products.

This implies that, if the products were put into a new solution, they would barely react because equilibrium disfavors the spontaneous formation of strong acid or strong base molecules in solution.

Strong bases ionize almost irreversibly, and so their products are weak conjugate acids.