Calculate Ka Values for The Following Compounds Phenol

This guide explains how to calculate the dissociation constant (Ka) for phenol and other compounds, including the formula, assumptions, and practical applications.

What is Ka?

The dissociation constant (Ka) is a measure of how strongly an acid dissociates in water. It represents the equilibrium constant for the reaction where an acid donates a proton to water, forming the conjugate base and hydronium ion.

HA ⇌ H⁺ + A⁻ Ka = [H⁺][A⁻]/[HA]

Ka values range from very small (weak acids) to very large (strong acids). Phenol is a weak acid with a Ka value typically around 1.3 × 10⁻¹⁰ at 25°C.

How to Calculate Ka

To calculate Ka, you need to measure the concentration of the proton (H⁺), the conjugate base (A⁻), and the remaining undissociated acid (HA) at equilibrium. The formula for Ka is:

Ka = [H⁺][A⁻]/[HA]

For phenol, the dissociation reaction is:

C₆H₅OH ⇌ C₆H₅O⁻ + H⁺

Ka is calculated by measuring the pH of a solution of known phenol concentration and using the Henderson-Hasselbalch equation to determine the equilibrium concentrations.

Phenol Ka Values

The Ka value for phenol varies slightly with temperature and solvent, but typical values are:

At 25°C in water: 1.3 × 10⁻¹⁰
At 20°C in water: 1.5 × 10⁻¹⁰
At 25°C in ethanol: 1.0 × 10⁻⁹

These values indicate that phenol is a weak acid, with only about 0.0000000001% of phenol molecules dissociating in water at 25°C.

Comparison of Ka Values

Here's a comparison of Ka values for common acids:

Acid	Ka Value	Strength
Hydrochloric Acid (HCl)	1.0 × 10⁷	Strong
Acetic Acid (CH₃COOH)	1.8 × 10⁻⁵	Weak
Phenol (C₆H₅OH)	1.3 × 10⁻¹⁰	Weak
Ammonia (NH₃)	1.8 × 10⁻⁵	Weak base

Frequently Asked Questions

What is the Ka value for phenol?: The Ka value for phenol is typically around 1.3 × 10⁻¹⁰ at 25°C in water.
How is Ka different from pKa?: Ka is the dissociation constant, while pKa is the negative logarithm of Ka. pKa = -log(Ka).
Can Ka values be measured experimentally?: Yes, Ka values can be measured experimentally using techniques like potentiometry or spectroscopy.
What factors affect Ka values?: Ka values are affected by temperature, solvent, and the presence of other substances that can donate or accept protons.
How do I calculate Ka from pH?: You can use the Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]).