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Calculate Ph of A Mixture Containing 0.130m Honh2 0.130m Honh3cl

Reviewed by Calculator Editorial Team

This guide explains how to calculate the pH of a mixture containing 0.130M HONH2 and 0.130M HONH3Cl. We'll cover the chemical principles, step-by-step calculation methods, and practical applications of this important acid-base mixture.

Introduction

When two acids with different dissociation constants are mixed, their pH depends on the relative concentrations and the strength of each acid. In this case, we're examining a mixture of 0.130M HONH2 (a weak acid) and 0.130M HONH3Cl (a strong acid).

The pH of such mixtures can be calculated using the Henderson-Hasselbalch equation, which relates the pH of a buffer solution to the ratio of conjugate base to conjugate acid concentrations.

How to Calculate the pH

Step 1: Understand the Components

HONH2 is a weak acid with a dissociation constant (Ka) of approximately 1.8 × 10⁻⁴. HONH3Cl is a strong acid that completely dissociates in water.

Step 2: Determine the Initial Concentrations

Both acids are present at 0.130M concentrations in the mixture.

Step 3: Apply the Henderson-Hasselbalch Equation

Henderson-Hasselbalch Equation:

pH = pKa + log10([A⁻]/[HA])

Where:

  • A⁻ = concentration of conjugate base (HONH2⁻)
  • HA = concentration of weak acid (HONH2)
  • pKa = -log10(Ka)

Step 4: Calculate the pH

Since HONH3Cl is a strong acid, it completely dissociates, providing chloride ions and H⁺ ions. The H⁺ ions react with the weak base HONH2 to form HONH2⁺, which then dissociates to form H⁺ and HONH2⁻.

The equilibrium can be represented as:

HONH2 + H2O ⇌ HONH2⁺ + OH⁻

HONH2⁺ ⇌ H⁺ + HONH2⁻

Using the given concentrations and the Ka value, we can calculate the pH of the resulting solution.

Example Calculation

Let's calculate the pH of a mixture containing 0.130M HONH2 and 0.130M HONH3Cl:

  1. Calculate the pKa of HONH2: pKa = -log(1.8 × 10⁻⁴) ≈ 3.74
  2. Since HONH3Cl is a strong acid, it completely dissociates, providing 0.130M H⁺ ions.
  3. The H⁺ ions react with HONH2 to form HONH2⁺, which then dissociates to form H⁺ and HONH2⁻.
  4. At equilibrium, the concentration of H⁺ ions is determined by the Ka of HONH2 and the initial concentrations.
  5. Using the Henderson-Hasselbalch equation: pH = pKa + log([HONH2⁻]/[HONH2])
  6. Assuming the system reaches equilibrium, we find the pH ≈ 3.74

Note: The actual pH may vary slightly depending on the exact equilibrium conditions, but this calculation provides a good approximation.

Interpreting the Results

A pH of approximately 3.74 indicates that the solution is acidic. This is expected since we're mixing a weak acid with a strong acid. The strong acid dominates the pH, but the weak acid still contributes to the buffer capacity of the solution.

This type of mixture is commonly used in chemical buffers and pharmaceutical formulations where a stable pH is required.

Frequently Asked Questions

Q: Why does the strong acid dominate the pH in this mixture?
A: Strong acids completely dissociate in water, providing a high concentration of H⁺ ions that significantly affect the pH. The weak acid contributes to the buffer capacity but doesn't change the pH as dramatically.
Q: How does the pH change if I change the concentrations of the acids?
A: Increasing the concentration of the strong acid will lower the pH further, while increasing the concentration of the weak acid will have a smaller effect on the pH but will increase the buffer capacity.
Q: Can I use this calculation for other acid mixtures?
A: Yes, the principles outlined here can be applied to other acid mixtures, but you'll need to know the dissociation constants (Ka) of the specific acids involved.