Can I solve for different variables?

Yes! This calculator supports reverse solving. You can solve for: Conjugate base concentration, K==a==, Weak acid concentration, pH, pK==a==, pK==a==.

pKa

Solve pka relationships quickly using core chemistry formulas.

Formula shown5 inputs definedUpdated Nov 2025By Automated Chemistry GeneratorFree, no sign-up

Quick Summary

Use this when you need:

Convert literature Ka values into pKa for buffer design
Compare acid strengths on a logarithmic scale
Check whether a reported Ka matches its stated pKa

You’ll need:Conjugate base concentration, K==a==, Weak acid concentration, pH, and 1 more

You’ll get:pK==a==

How this is calculated

Formula:

pKa = -log10(Ka)

In plain language:

Taking the negative base-10 logarithm of the acid dissociation constant Ka linearizes the wide dynamic range, so entering Ka lets the calculator output the corresponding pKa.

The formula is always visible so you can verify the math and explain your numbers to anyone who asks.

What these terms mean

Conjugate base concentration

Conjugate base concentration used in the calculation.

K==a==

K==a== used in the calculation.

Weak acid concentration

Weak acid concentration used in the calculation.

pH

pH used in the calculation.

pK==a==

pK==a== used in the calculation.

pK==a==

pK==a== used in the calculation.

When to use this calculator

Convert literature Ka values into pKa for buffer design
Compare acid strengths on a logarithmic scale
Check whether a reported Ka matches its stated pKa

Last updated: November 19, 2025

pKa calculator explained

pKa expresses acid strength on a logarithmic scale: $pK_a = -\\log_{10} K_a$ . This calculator lets you enter an acid dissociation constant {ka}, a buffer pH {ph}, and conjugate concentrations ({conjugate_base_concentration}, {weak_acid_concentration}) to solve for pKa values derived either from $K_a$ or from the Henderson-Hasselbalch relationship.

Use it to convert literature $K_a$ tables into pKa values for buffer design, or to back-calculate $K_a$ from titration data.

How the conversion works

Two direct relationships:

pK_a = -\\log_{10} K_a

and

pK_a = pH - \\log_{10}\\left(\\frac{[A^-]}{[HA]}\\right)

The calculator keeps both forms synchronized so you can switch between them instantly.

Units and conversions

Quantity	Units	Notes
$K_a$	dimensionless (activity-based)	Enter the dissociation constant.
$[A^-]$ , $[HA]$	mol/L	Any consistent concentration works because only their ratio appears.
pH, pKa	dimensionless	Outputs of the calculation.

Worked examples

Convert $K_a$ to pKa
Acetic acid has $K_a = 1.8 \\times 10^{-5}$ .

pK_a = -\\log_{10}(1.8 \\times 10^{-5}) = 4.74

Entering $K_a$ into the calculator returns the same result.

Determine pKa from buffer measurements
A buffer has $pH = 3.80$ with $[A^-]/[HA] = 0.25$ .

pK_a = 3.80 - \\log_{10}(0.25) = 3.80 - (-0.602) = 4.40

The acid's dissociation constant is $K_a = 10^{-4.40} = 4.0 \\times 10^{-5}$ .

Tips and pitfalls

Use activity coefficients for strong acids or concentrated solutions; otherwise $K_a$ derived from concentration may be misleading.
Remember that pKa decreases as acid strength increases; a drop of 1 unit means a tenfold stronger acid.
When dealing with polyprotic acids, compute a separate pKa for each dissociation step.
Any experimental error in the ratio $[A^-]/[HA]$ is amplified by the logarithm; measure concentrations carefully.

References and further reading

Frequently Asked Questions

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Calculator info

Version:: v1.0.0
Last reviewed:: Nov 15, 2025
Sensitivity:: low
References:: 1

Last updated: November 19, 2025

pKa calculator explained

Use it to convert literature $K_a$ tables into pKa values for buffer design, or to back-calculate $K_a$ from titration data.

How the conversion works

Two direct relationships:

pK_a = -\\log_{10} K_a

and

pK_a = pH - \\log_{10}\\left(\\frac{[A^-]}{[HA]}\\right)

The calculator keeps both forms synchronized so you can switch between them instantly.

Units and conversions

Quantity	Units	Notes
$K_a$	dimensionless (activity-based)	Enter the dissociation constant.
$[A^-]$ , $[HA]$	mol/L	Any consistent concentration works because only their ratio appears.
pH, pKa	dimensionless	Outputs of the calculation.

Worked examples

Convert $K_a$ to pKa
Acetic acid has $K_a = 1.8 \\times 10^{-5}$ .

pK_a = -\\log_{10}(1.8 \\times 10^{-5}) = 4.74

Entering $K_a$ into the calculator returns the same result.

Determine pKa from buffer measurements
A buffer has $pH = 3.80$ with $[A^-]/[HA] = 0.25$ .

pK_a = 3.80 - \\log_{10}(0.25) = 3.80 - (-0.602) = 4.40

The acid's dissociation constant is $K_a = 10^{-4.40} = 4.0 \\times 10^{-5}$ .

Tips and pitfalls

Use activity coefficients for strong acids or concentrated solutions; otherwise $K_a$ derived from concentration may be misleading.
Remember that pKa decreases as acid strength increases; a drop of 1 unit means a tenfold stronger acid.
When dealing with polyprotic acids, compute a separate pKa for each dissociation step.
Any experimental error in the ratio $[A^-]/[HA]$ is amplified by the logarithm; measure concentrations carefully.

Quick Summary

Calculator Tool

How this is calculated

What these terms mean

Conjugate base concentration

K==a==

Weak acid concentration

pH

pK==a==

pK==a==

When to use this calculator

pKa calculator explained

How the conversion works

Units and conversions

Worked examples

Tips and pitfalls

References and further reading

Frequently Asked Questions

Related Calculators

Henderson Hasselbalch

Hydrogen ion concentration

Beer-Lambert law

Molarity

Mole

Neutralization

Was this calculator helpful?

Found an error?

Calculator info

pKa calculator explained

How the conversion works

Units and conversions

Worked examples

Tips and pitfalls

References and further reading

Frequently Asked Questions

Related Calculators

Henderson Hasselbalch

Hydrogen ion concentration

Beer-Lambert law

Molarity

Mole

Neutralization

Was this calculator helpful?

Found an error?

Calculator info