pKa Calculator
Calculate pKa from the acid dissociation constant Ka using pKa = -log10(Ka). Free online pKa calculator for chemistry.
What is pKa?
pKa is the negative base-10 logarithm of the acid dissociation constant ($K_a$). It measures how strongly an acid donates protons in solution: a lower pKa means a stronger acid, and a higher pKa means a weaker acid. The relationship is defined by:
$$\text{p}K_a = -\log_{10}(K_a)$$
Chemists use pKa values to compare acid strengths, design buffer solutions, and predict reaction behavior. For example, acetic acid has $K_a = 1.8 \times 10^{-5}$, which gives $\text{p}K_a = 4.76$. You can use this calculator to convert any $K_a$ value to pKa instantly.
How to Calculate pKa from Ka
- Identify the acid dissociation constant ($K_a$) for your weak acid.
- Take the base-10 logarithm of $K_a$.
- Multiply by -1 to get pKa.
Once you know pKa, you can use it in the Henderson-Hasselbalch equation with our Buffer pH Calculator to determine the pH of a buffer solution.
Common Acids and Their pKa Values
| Acid | Ka | pKa |
|---|---|---|
| Hydrochloric acid (HCl) | Very large | ≈ -7 |
| Acetic acid (CH₃COOH) | 1.8 × 10⁻⁵ | 4.76 |
| Carbonic acid (H₂CO₃) | 4.3 × 10⁻⁷ | 6.37 |
| Water (H₂O) | 1.0 × 10⁻¹⁴ | 14.00 |
Frequently Asked Questions
What is the difference between Ka and pKa?
Ka is the equilibrium constant for acid dissociation and can span many orders of magnitude. pKa is the negative log of Ka, which compresses that range into a more convenient scale for comparing acid strengths. Lower pKa values indicate stronger acids.
Can pKa be negative?
Yes. Very strong acids such as hydrochloric acid and sulfuric acid have Ka values much greater than 1, which gives negative pKa values. Most weak acids used in buffers have positive pKa values between 3 and 12.
How is pKa used in buffer calculations?
pKa appears in the Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]). Buffers work best when the desired pH is within about one pH unit of the pKa. Use our Buffer pH Calculator to apply this equation.