Calculate Ph of A Solution Containing 0.1m Ha

Introduction

When calculating the pH of a solution containing a weak acid (HA), we use the Henderson-Hasselbalch equation. This equation relates the pH of a buffer solution to the acid dissociation constant (Ka) and the ratio of the conjugate base (A-) to the weak acid (HA).

For a solution containing 0.1M HA, we need to know the concentration of the conjugate base (A-) to calculate the pH. If the solution is at equilibrium, the concentration of A- will be equal to the concentration of HA.

pH Calculation Formula

The pH of a solution containing HA is calculated using the Henderson-Hasselbalch equation:

Where:

• pKa is the negative logarithm of the acid dissociation constant (Ka)
• [A⁻] is the concentration of the conjugate base
• [HA] is the concentration of the weak acid

For a solution at equilibrium, [A⁻] = [HA], so the equation simplifies to:

Worked Example

Let's calculate the pH of a 0.1M HA solution where the pKa of HA is 4.75.

1. Identify the given values:
   • [HA] = 0.1M
   • pKa = 4.75
2. Since the solution is at equilibrium, [A⁻] = [HA] = 0.1M
3. Apply the Henderson-Hasselbalch equation:

   pH = pKa + log₁₀([A⁻]/[HA]) = 4.75 + log₁₀(0.1/0.1) = 4.75 + log₁₀(1) = 4.75 + 0 = 4.75

4. The calculated pH is 4.75

Interpreting Results

The pH of a 0.1M HA solution is equal to its pKa when the solution is at equilibrium. This means the solution is neither acidic nor basic; it is at the midpoint of its buffering range.

If the concentration of A- is greater than HA, the pH will be higher than pKa. Conversely, if the concentration of HA is greater than A-, the pH will be lower than pKa.