pKa Calculator

Calculate the logarithmic acid dissociation constant (pKa) from Ka. Essential for understanding weak acid buffering ranges.

Target / Measured Buffer pH

Concentration of Conjugate Base [A⁻]

Concentration of Weak Acid [HA]

Calculated pKa

4.82

Raw Acid Constant (Ka)1.5000e-5

Logarithmic Ratio log([A⁻]/[HA])0.18

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Reverse-Engineering pKa from Buffers

The pKa value is the universal language of organic chemistry and biochemistry. It allows scientists to instantly understand whether a molecule will be protonated or deprotonated at physiological pH.

While pKa is usually looked up in textbook tables, you can experimentally determine the exact pKa of an unknown acid by creating a Buffer Solution and measuring its pH.

The Reverse Henderson-Hasselbalch

The Henderson-Hasselbalch equation is traditionally used to find the pH of a buffer. However, if you already know the concentrations of the acid and its conjugate base that you mixed together, and you measure the resulting pH with a meter, you can algebraically rearrange the equation to solve for the unknown pKa.

The Equation

\begin{aligned} pK_a = pH - \log_{10}\left(\frac{[A⁻]}{[HA]}\right) \end{aligned}

Where:

pK_a

Logarithmic Acid Constant

pH=

Measured Buffer pH

[A⁻]=

Concentration of Conjugate Base

[HA]=

Concentration of Weak Acid

Example Calculation

You create a buffer by mixing a 0.1 M weak acid with 0.15 M of its conjugate base salt. The pH meter reads 5.0.

Find the Ratio [A⁻]/[HA]: $0.15 / 0.1 = 1.5$
Take the Logarithm of the Ratio: $\log_{10}(1.5) = 0.176$
Subtract from measured pH: $5.0 - 0.176$
Result: $4.82$

The calculated pKa of the unknown acid is $4.82$ . From this, you can also easily reverse the logarithm to find the raw Acid Dissociation Constant ( $K_a = 10^{-4.82} = 1.51 \times 10^{-5}$ ).

Frequently Asked Questions

This is a critical biological threshold. If the pH of the surrounding environment exactly equals the pKa of the acidic molecule, it means the acid is exactly 50% dissociated. The logarithmic ratio is log(1) = 0, so pKa = pH.

Buffer solutions are incredibly resistant to pH changes caused by environmental contamination (like dissolved CO2 from the air). Therefore, measuring the pKa in a buffer is often much more stable and accurate than measuring a pure, unbuffered acid solution.

Yes! In any acid-base reaction, the equilibrium will always favor the side with the weaker acid (the higher pKa). You can predict the outcome of organic reactions simply by comparing the pKa of the reactants and products.

Massively. Standard pKa values are measured in water. If you place the exact same acid in a non-polar organic solvent like DMSO, its pKa will change dramatically because the solvent cannot stabilize the resulting ions as effectively as water.

pH is a property of the solution (how many free protons are floating around). pKa is an inherent physical property of the molecule itself (how stubbornly it holds onto its protons).

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