pKa Calculator
Turn acid Ka into pKa with strength band shown.
What it does
Acid strength lives on a log scale. Ka spans orders of magnitude, from strong acids near ten to weak acids near 1e-10. Taking minus log compresses that span into handy pKa numbers where lower means stronger.
The tool above takes Ka and returns pKa, log Ka, and a plain strength band. The default 0.000018 matches acetic acid, so the page opens showing 4.74. Type a new Ka in decimal or e form to update all three results.
The math is pKa = -log10(Ka). For 1.8e-5, log10 is -4.7447, so pKa is 4.74. Hydrochloric near Ka 1e6 gives pKa near -6, water at 1e-14 gives pKa 14 for its conjugate scale context.
Use pKa to pick buffers: best near pKa plus or minus 1. Do not mix Ka with pH. Ka describes the acid, pH describes the solution after concentration and water enter.
This page shows the formula, one full hand checked example at 1.8e-5, a reference table, and practice checks that run through the tool.
How to use
- Type Ka above 0, decimal or e form.
- Read pKa as minus log10.
- Read log Ka as the signed log.
- Read band as strong to very weak.
- Compare acids by lower pKa first.
- For buffers target pH near pKa.
Formula
- Ka
- Acid dissociation constant, above 0.
- pKa
- Minus log10 of Ka.
- band
- Strong below 0, weak above 4 here.
Logs here are base 10. Natural logs need divide by ln 10. Temperature and ionic strength shift measured Ka, so state conditions with lab values.
Worked example
Example: Ka 1.8e-5 (acetic)
Acetic acid Ka 0.000018. Find pKa.
- Write Ka = 0.000018.
- Log10: log10(1.8e-5) = -4.74473.
- Negate: pKa = 4.74473.
- Round: 4.74.
- Log Ka reads -4.74.
- Band weak acid.
Reference table
| Ka | pKa | Band here |
|---|---|---|
| 1 | 0 | Moderately strong |
| 0.001 | 3 | Moderately strong |
| 0.000018 | 4.74 | Weak acid |
| 0.0000001 | 7 | Weak acid |
Limits
- Ka only. pH needs concentration too.
- Water 25 C context assumed. Temp shifts Ka.
- Dilute ideal assumed. Activity corrections ignored.
- Single acid only. Mixtures need full equilibrium.
Common mistakes
- Using natural log without converting to base 10.
- Typing 1.8e-5 as 1.85 or 18e-5.
- Reading higher pKa as stronger. Lower is stronger.
- Mixing Ka with pH.
- Entering 0 or negative Ka, which the tool rejects.
Try it yourself
Quick checks. Answers sit tucked under each question.
Ka 1e-7?
pKa 7, log -7, Weak acid.
Ka 0.001?
pKa 3, log -3, Moderately strong.
Questions
Ka to pKa?
Minus log10. Here 1.8e-5 gives 4.74.
Lower stronger?
Yes. Smaller pKa means larger Ka.
Acetic pKa?
About 4.74.
pKa 7?
Very weak in this banding, Ka 1e-7.
e form?
Yes. Type 1.8e-5 directly.
Buffer?
Best within about 1 of pKa.