Question

In each of the following cases, is the concentration of acid before and after dissociation nearly the same or very different? Explain your reasoning: (a) a concentrated solution of a strong acid; (b) a concentrated solution of a weak acid; (c) a dilute solution of a weak acid; (d) a dilute solution of a strong acid.

Short answer

(a) Different, (b) Nearly the same, (c) Nearly the same, (d) Different

Step-by-step solution

Understanding Concentration and Dissociation

Before solving the problem, let's understand that 'concentration' refers to the amount of acid in a given volume of solution, while 'dissociation' refers to the extent to which acid molecules break into ions in water.

Evaluating Strong Acid in Concentrated Solution

In a concentrated solution of a strong acid (a), the acid is fully dissociated. Therefore, the concentration of the acid before and after dissociation is very different. Most of the acid exists as ions.

Evaluating Weak Acid in Concentrated Solution

In a concentrated solution of a weak acid (b), the acid is partially dissociated. Therefore, the concentration of the acid before and after dissociation is nearly the same, as significant portions remain non-dissociated.

Evaluating Weak Acid in Dilute Solution

In a dilute solution of a weak acid (c), though the acid is still partially dissociated, the ratio of dissociated to undissociated acid is higher compared to a concentrated solution. However, it will still be nearly the same.

Evaluating Strong Acid in Dilute Solution

In a dilute solution of a strong acid (d), the acid also fully dissociates. Hence, the concentration before and after dissociation is very different, similar to the concentrated solution of a strong acid.

Key concepts

Strong Acids

A strong acid is one that fully dissociates in water. This means that nearly all the acid molecules break apart into ions. Examples of strong acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃).

When you have a concentrated solution of a strong acid, the molecules dissociate completely. Therefore, the concentration of acid before dissociation is significantly higher than after dissociation. This is because the strong acid turns into hydrogen ions (H⁺) and anions in solution.

In summary:

• Concentrated solution of strong acid: full dissociation, large difference in concentration before and after.
• Dilute solution of strong acid: still full dissociation, but lower concentration of ions compared to the concentrated scenario.

Weak Acids

Weak acids do not fully dissociate in water. This means they only partially break into ions. Examples of weak acids include acetic acid (CH₃COOH) and formic acid (HCOOH).

For a concentrated solution of a weak acid, only a small percentage of the molecules dissociate. As a result, the concentration of the acid before and after dissociation remains nearly the same. Most of the acid remains in its molecular form.

When it comes to a dilute solution of a weak acid, the dissociation is still partial. However, due to the lower initial concentration, the extent of dissociation is relatively higher compared to the concentrated solution. Yet, the concentration of the acid before and after dissociation is still almost the same.

In summary:

• Concentrated solution of weak acid: partial dissociation, small difference in concentration before and after.
• Dilute solution of weak acid: partial dissociation still, but dissociation ratio higher than in concentrated solution, concentration nearly same before and after.

Concentration

The term 'concentration' refers to how much acid is present in a given volume of solution. It's usually expressed in molarity (M), which is moles of solute per liter of solution.

High concentration means more acid molecules are present in a smaller volume. Low concentration means fewer acid molecules in the same volume.

In the context of dissociation:

• For strong acids, regardless of whether the solution is concentrated or dilute, the dissociation is always complete, leading to a significant difference in concentration before and after dissociation.
• For weak acids, the initial concentration affects the ratio of dissociated to undissociated molecules, but the overall concentration before and after dissociation remains close to each other, primarily because weak acids dissociate only partially.