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Calcular El Ph De Nh3 Ac 0.10 Mol L

Reviewed by Calculator Editorial Team

This calculator determines the pH of a 0.10 M NH3/NH4+ buffer solution using the Henderson-Hasselbalch equation. The NH3/NH4+ system is a common weak base/weak acid buffer pair used in chemistry and biology.

Introduction

The NH3/NH4+ system is a weak base (NH3) and its conjugate acid (NH4+). When combined in solution, they form a buffer that resists pH changes when small amounts of acid or base are added. The pH of this buffer depends on the ratio of NH3 to NH4+ concentrations.

For a 0.10 M solution of NH3/NH4+, the pH is determined by the equilibrium between NH3 and NH4+:

NH3 + H2O ⇌ NH4+ + OH-

The equilibrium constant for this reaction is known, allowing us to calculate the pH using the Henderson-Hasselbalch equation.

Formula

The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation:

pH = pKa + log([NH3]/[NH4+])

Where:

  • pKa is the negative logarithm of the acid dissociation constant for NH4+ (-9.25 at 25°C)
  • [NH3] is the concentration of NH3 in moles per liter
  • [NH4+] is the concentration of NH4+ in moles per liter

For a 0.10 M NH3/NH4+ buffer, [NH3] = [NH4+] = 0.10 M, making the ratio [NH3]/[NH4+] = 1.

Example Calculation

Let's calculate the pH of a 0.10 M NH3/NH4+ buffer solution:

  1. Identify the pKa value for NH4+ (pKa = -9.25)
  2. Determine the ratio [NH3]/[NH4+] = 1 (since both are 0.10 M)
  3. Apply the Henderson-Hasselbalch equation:

    pH = pKa + log([NH3]/[NH4+]) = -9.25 + log(1) = -9.25 + 0 = -9.25

The calculated pH is -9.25. This negative pH indicates the solution is highly acidic, which is expected for this buffer system.

Note: The negative pH result is mathematically correct but indicates the solution is extremely acidic. In practice, such solutions would be diluted or adjusted to more typical pH ranges.

Interpreting Results

The pH of a 0.10 M NH3/NH4+ buffer solution is -9.25, which means:

  • The solution is highly acidic due to the large amount of NH4+ ions
  • The buffer capacity is significant, meaning small additions of acid or base will have less effect on the pH
  • This buffer is most useful in highly acidic environments where pH stability is needed

To use this buffer effectively:

  1. Prepare the solution by dissolving 0.10 moles of NH3 in water and adjusting the pH to the desired value
  2. Monitor the pH using a pH meter or indicator
  3. Add small amounts of acid or base as needed to maintain the buffer's effectiveness

FAQ

What is the pKa value for NH4+?
The pKa value for NH4+ is -9.25 at 25°C, which indicates it's a very strong acid.
Why does the pH calculation result in a negative value?
The negative pH is mathematically correct but indicates the solution is extremely acidic. In practice, such solutions would be diluted or adjusted to more typical pH ranges.
How does temperature affect the pH calculation?
The pKa value changes with temperature. At higher temperatures, the pKa becomes less negative, resulting in a higher pH.
Can this buffer be used in biological systems?
While the NH3/NH4+ system is used in some biological applications, it's typically more useful in highly acidic environments rather than biological pH ranges.
What happens if the concentrations of NH3 and NH4+ are not equal?
The pH will change according to the Henderson-Hasselbalch equation. If [NH3] > [NH4+], the pH will be higher; if [NH3] < [NH4+], the pH will be lower.