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pKa Calculator

Calculate pKa from pH, Ka, or lookup acid dissociation constants

Calculate pKa

Calculation Method

pH of Solution

pH scale: 0-14

Conjugate Base Concentration [A⁻]

Molarity of conjugate base

Weak Acid Concentration [HA]

Molarity of weak acid

pKa Results

5.8000

pKa

Acid Dissociation

1.585e-6

Dissociation Constant

Calculation Method

Method: Henderson-Hasselbalch equation

Equation: pKa = pH - log₁₀([A⁻]/[HA])

Calculation Steps

1. Given: pH = 4.8, [A⁻] = 0.01 M, [HA] = 0.1 M

2. Calculate log ratio: log₁₀(0.01/0.1) = -1.0000

3. Apply Henderson-Hasselbalch: pKa = 4.8 - -1.0000 = 5.8000

4. Calculate Ka: Ka = 10^(-pKa) = 10^(-5.8000) = 1.585e-6

Acid Strength Interpretation

Weak acid - partially dissociates in water

Rule: Lower pKa = Stronger acid, Higher pKa = Weaker acid

Buffer Information

pH vs pKa: pH = 4.8, pKa = 5.8000

Buffer Effectiveness: Good buffer range (pH ± 1 from pKa)

Acid-to-Base Ratio: [HA]/[A⁻] = 10.000

Key Equations

Henderson-Hasselbalch: pH = pKa + log₁₀([A⁻]/[HA])

pKa from Ka: pKa = -log₁₀(Ka)

Ka from pKa: Ka = 10^(-pKa)

Example: Acetic Acid pKa Calculation

Given Information

Acid: Acetic acid (CH₃COOH)

pH: 4.8

Acetic acid concentration: 0.1 M

Acetate ion concentration: 0.01 M

Step-by-Step Calculation

1. Apply Henderson-Hasselbalch equation: pH = pKa + log₁₀([A⁻]/[HA])

2. Rearrange: pKa = pH - log₁₀([A⁻]/[HA])

3. Calculate ratio: log₁₀(0.01/0.1) = log₁₀(0.1) = -1

4. Calculate pKa: pKa = 4.8 - (-1) = 5.8

Result: pKa of acetic acid ≈ 4.76 (literature value)

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pKa Scale

Very Strong Acids

pKa < 0

HCl, HBr, HI, H₂SO₄

Strong Acids

pKa = 0-2

Mostly dissociated

Weak Acids

pKa = 2-7

HF, acetic acid

Very Weak Acids

pKa = 7-14

NH₄⁺, water

Extremely Weak

pKa > 14

Alcohols, alkanes

Common pKa Values

HCl-6

H₂SO₄-3

HF3.2

Acetic acid4.76

Carbonic acid6.37

NH₄⁺9.25

HCN9.3

Water14

Quick Tips

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Lower pKa = stronger acid

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pKa = pH when [HA] = [A⁻]

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Best buffer: pH within ±1 of pKa

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pKa = -log₁₀(Ka)

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Temperature affects Ka, not pKa definition

Understanding pKa

What is pKa?

pKa is the negative logarithm of the acid dissociation constant (Ka). It measures how strongly a Brønsted acid holds onto its proton (H⁺). Lower pKa values indicate stronger acids that more readily donate protons.

Key Concepts:

• Acid Strength: Determined by proton-donating ability
• Dissociation: HA ⇌ H⁺ + A⁻
• Equilibrium: Ka = [H⁺][A⁻]/[HA]
• Logarithmic Scale: pKa = -log₁₀(Ka)

Henderson-Hasselbalch Equation

pH = pKa + log₁₀([A⁻]/[HA])

Relates pH, pKa, and concentration ratio of conjugate base to acid.

Buffer Applications

Most effective when pH is within ±1 unit of pKa (10-90% dissociation).

Special Cases

When [HA] = [A⁻], then pH = pKa (50% dissociation point).

Applications in Chemistry

Buffer Design

Select acid-base pairs with pKa near desired pH for maximum buffering capacity.

Drug Development

pKa affects drug absorption, distribution, and bioavailability in biological systems.

Environmental Chemistry

Predict acid behavior in natural waters and soil systems for pollution control.

Relationship Summary

pKa vs pH

pH = pKa: [HA] = [A⁻] (50% dissociation)

pH < pKa: [HA] > [A⁻] (mostly acid form)

pH > pKa: [HA] < [A⁻] (mostly conjugate base)

pKa vs Ka

Low pKa: High Ka (strong acid)

High pKa: Low Ka (weak acid)

Conversion: pKa = -log₁₀(Ka)

Inverse: Ka = 10^(-pKa)

Related Chemistry Calculators

pH Calculator

Calculate pH from concentration or H⁺ activity

Buffer Capacity Calculator

Calculate buffer capacity and effectiveness

Henderson-Hasselbalch Calculator

Calculate pH of buffer solutions

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