Acids And Bases

To determine the dissociation constant (K_a) of the weak acid HA, given the pH of the solution, follow these steps:

Step 1: Calculate the hydrogen ion concentration [H⁺].

pH is given as 4, so:

pH = -log[H⁺]

[H⁺] = 10^-pH = 10^-4 M

Step 2: Write the equilibrium expression for the dissociation of HA.

HA ⇌ H⁺ + A^-

Step 3: Express the K_a in terms of concentration.

K_a =

Assuming x M of HA dissociates, an equal amount of H⁺ and A^- is formed at equilibrium:

[H⁺] = x = 10^-4 M

[HA] = Initial concentration of HA - x

Given [HA] = 0.01 M, so remaining [HA] ≈ 0.01 M (since x is small).

Step 4: Substitute into the K_a expression.

K_a ≈

K_a = = 10^-6

Therefore, K_a is .

The correct choice is .

To determine the pH of the buffer solution, we use the Henderson-Hasselbalch equation:

Where:
for acetic acid,
= concentration of acetate ion ,
= concentration of acetic acid .

Substitute these values into the equation:

Simplify the expression:

Since :

Therefore, the pH of the buffer solution is 5.04.

In order to determine which option is a strong acid, we must understand the definition and characteristics of strong acids. Strong acids are those that completely dissociate into their ions in water. This means that in solution, they release a large number of hydrogen ions, resulting in a high level of acidity.

Let's evaluate each of the options provided:

• CH₃COOH (Acetic acid): This is a weak acid. It does not fully dissociate in water, resulting in some of the acid remaining in its molecular form.
• HCl (Hydrochloric acid): This is a strong acid. It completely dissociates in water into H⁺ and Cl^- ions, making it highly acidic.
• NH₄⁺ (Ammonium ion): This is not an acid in its typical standalone form. It doesn't dissociate in water to produce hydrogen ions like strong acids do.
• H₂CO₃ (Carbonic acid): This is a weak acid. Like acetic acid, it does not completely dissociate in water, and its degree of ionization is relatively low.

Therefore, the correct answer is HCl, as it is the only option that is a strong acid, capable of complete dissociation in aqueous solution.