Calculate Ph of 0.2500 M Solution of Ch3coona
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Reviewed by Calculator Editorial Team
This calculator determines the pH of a 0.2500 M solution of sodium acetate (CH3COONa). The calculation follows standard acid-base chemistry principles for weak base solutions.
Introduction
Sodium acetate (CH3COONa) is a weak base that dissociates in water to form acetate ions (CH3COO⁻) and sodium ions (Na⁺). The pH of a solution depends on the concentration of the weak base and its dissociation constant.
For a 0.2500 M solution of CH3COONa, we can calculate the pH using the Henderson-Hasselbalch equation, which relates the pH of a buffer solution to the ratio of the concentrations of the conjugate acid and base.
Formula
The pH of a solution of a weak base can be calculated using the Henderson-Hasselbalch equation:
pH = pKa + log10([CH3COO⁻]/[CH3COOH])
Where:
- pKa is the negative logarithm of the acid dissociation constant of acetic acid (CH3COOH)
- [CH3COO⁻] is the concentration of acetate ions
- [CH3COOH] is the concentration of acetic acid
The pKa of acetic acid is approximately 4.76 at 25°C.
Calculation
For a 0.2500 M solution of CH3COONa, we assume complete dissociation to form acetate ions. The concentration of acetate ions is equal to the initial concentration of CH3COONa.
Since CH3COONa is a weak base, it reacts with water to form CH3COOH and OH⁻. The concentration of CH3COOH is equal to the concentration of OH⁻, which is equal to the concentration of CH3COONa.
Therefore, the ratio [CH3COO⁻]/[CH3COOH] is equal to 1, and the pH calculation simplifies to:
pH = pKa + log10(1) = pKa + 0 = pKa
Thus, the pH of a 0.2500 M solution of CH3COONa is equal to the pKa of acetic acid, which is approximately 4.76.
Interpretation
A pH of 4.76 indicates that the solution is slightly acidic. This is expected because CH3COONa is a weak base that partially dissociates to form CH3COOH, which is a weak acid.
The solution is a buffer because it contains both a weak acid (CH3COOH) and its conjugate base (CH3COO⁻). Buffers resist changes in pH when small amounts of acid or base are added.
Note: The actual pH may vary slightly from 4.76 due to temperature effects and the exact concentration of CH3COONa in the solution.
FAQ
What is the pH of a 0.2500 M solution of CH3COONa?
The pH is approximately 4.76, which is equal to the pKa of acetic acid.
Why is the pH of a CH3COONa solution equal to its pKa?
This occurs because the concentration of acetate ions (CH3COO⁻) is equal to the concentration of acetic acid (CH3COOH) formed by the dissociation of CH3COONa.
Is a CH3COONa solution a buffer?
Yes, because it contains both a weak acid (CH3COOH) and its conjugate base (CH3COO⁻), which helps maintain a relatively constant pH.
How does temperature affect the pH of a CH3COONa solution?
Temperature can slightly change the pKa of acetic acid, which would affect the calculated pH. The pKa of acetic acid decreases slightly with increasing temperature.