What Is Ka and pKa in Chemistry?

This page explains what Ka and pKa are, how they relate to acid strength, and how to use them in calculations. You enter an acid dissociation constant or its pKa value, and the calculator converts between the two and estimates the degree of dissociation for a given concentration. You get the converted value and the percent ionization, which helps you compare acids and predict behavior in solution.

Formula

Ka is the acid dissociation constant, defined by the equilibrium of a weak acid in water:

HA ⇌ H� + A�

Ka = [H�][A�] / [HA]

where [H�] is the hydrogen ion concentration, [A�] is the conjugate base concentration, and [HA] is the undissociated acid concentration, all in mol/L (molar). pKa is simply the negative base-10 logarithm of Ka:

pKa = −log�₀(Ka)

The calculator uses the first expression to convert Ka to pKa. For percent ionization, it assumes a weak acid where x (the amount dissociated) is small compared to the initial concentration, so [H�] ≈ √(Ka × C), and percent ionization = ([H�] / C) × 100.

Worked Example

Using the default values: Ka = 1.8 × 10�� (acetic acid) and concentration = 0.1 mol/L.

1. Calculate pKa: pKa = −log�₀(1.8 × 10��) = 4.74 (rounded to two decimals).
2. Estimate [H�]: [H�] = √(Ka × C) = √(1.8 × 10�� × 0.1) = √(1.8 × 10��) = 1.34 × 10�³ mol/L.
3. Percent ionization = (1.34 × 10�³ / 0.1) × 100 = 1.34%.

The calculator shows pKa = 4.74 and percent ionized = 1.34%, confirming the steps above.

What Ka and pKa Actually Mean

Ka is an equilibrium constant that measures how completely an acid donates a proton (H�) to water. A large Ka (greater than 1) means the acid dissociates almost fully – that is a strong acid. A small Ka (much less than 1) means only a tiny fraction of acid molecules lose a proton – that is a weak acid. For example, hydrochloric acid (HCl) has a Ka in the millions (effectively infinite), while acetic acid (vinegar) has Ka = 1.8 × 10��.

pKa is just a more convenient way to express those very small or very large numbers. Instead of writing 1.8 × 10��, you write 4.74. The lower the pKa, the stronger the acid. Water has pKa = 14, ammonia has pKa ≈ 38, and typical weak acids fall between 3 and 7. Strong acids like HCl have negative pKa values (about −7).

The two are inversely related: a high Ka gives a low pKa, and a low Ka gives a high pKa. A difference of one pKa unit means a tenfold difference in Ka.

How to Choose Between Ka and pKa

Chemists use pKa more often than Ka because it gives a manageable scale. Comparing Ka values like 1.8 × 10�� and 1.4 × 10�� is possible, but comparing pKa values of 4.74 and 3.85 is easier. pKa also appears directly in the Henderson–Hasselbalch equation for buffer pH:

pH = pKa + log([A�]/[HA])

That equation is used to prepare buffers, predict the protonation state of a molecule at a given pH, and design drug molecules. Ka is more common in fundamental equilibrium calculations, while pKa is the standard in biochemistry, organic chemistry, and pharmacology.

When you need to compare acid strengths quickly, use pKa. When you need to compute concentrations of H� or A� from equilibrium, use Ka directly.

Common Mistakes and Misconceptions

• Confusing Ka with acidity: Ka is a constant at a given temperature; it does not change with concentration. The pH of a solution changes with concentration, but Ka does not.
• Using pKa = pH: pKa is a property of the acid; pH is a property of the solution. They are equal only when [Aâ�»] = [HA] (half-neutralized).
• Forgetting units: Ka has units of mol/L, but pKa is dimensionless because you take the logarithm of a number (in practice, you drop the units).
• Applying the approximation blindly: The percent ionization formula works only when Ka is small and concentration is not extremely dilute. If Ka is greater than about 10â�»Â³ or the concentration is below 10â�»â�´ M, you should solve the quadratic equation instead.
• Ignoring temperature: Ka values are temperature-dependent. A value measured at 25°C may be different at 37°C.

Real-World Applications

pKa is essential in biology and medicine. Amino acids have two or more pKa values; the pKa of the side chain determines whether it is charged at physiological pH (7.4). That affects protein folding and enzyme activity. Drug molecules are often weak acids or bases; their pKa determines whether they exist in a charged or uncharged form in the body, which influences absorption across cell membranes.

In environmental chemistry, pKa helps predict whether a pollutant will dissolve in water or bind to soil. In food science, it explains why acids like citric acid (pKa ≈ 3.1) taste sour and why buffers keep processed foods at a stable pH.

For everyday chemistry, the pKa of an acid tells you whether it will react with baking soda (sodium bicarbonate, pKa of carbonic acid ≈ 6.4) or with a metal. Acetic acid (pKa 4.74) fizzes with baking soda, while boric acid (pKa 9.2) does not.

Limits of the Formula

The percent ionization formula given here assumes a monoprotic weak acid (one proton) and that the concentration is high enough that the small-x approximation holds. It does not work for polyprotic acids like phosphoric acid, which has three Ka values. It also fails for very dilute solutions (below about 10â�»â�µ M) where water’s autoionization contributes significantly to [Hâ�º]. In those cases, you need to solve the full equilibrium including water.

For strong acids, Ka is so large that percent ionization is essentially 100%, and the formula would give a nonsensical result. Use the concentration directly for strong acids.

Always check the temperature and the ionic strength of the solution. Ka values are thermodynamic constants at infinite dilution; in real solutions with high salt concentrations, the effective (apparent) Ka changes.

Frequently Asked Questions

What is the difference between Ka and pKa?

Ka is the acid dissociation constant, a number that can be very large or very small. pKa is the negative logarithm of Ka, which compresses that range into a more convenient scale. Lower pKa means stronger acid; higher Ka means stronger acid.

Why do we use pKa instead of Ka?

Because pKa values are easier to compare and use in equations like the Henderson–Hasselbalch. A pKa difference of one unit is a tenfold difference in Ka, so you can quickly gauge relative strength without handling scientific notation.

How do I convert Ka to pKa?

Take the negative base-10 logarithm: pKa = −log�₀(Ka). For Ka = 1.8 × 10��, pKa = 4.74. To convert back, use Ka = 10^(−pKa).

What does a high pKa mean?

A high pKa (e.g., 10 or more) means the acid is very weak – it does not donate protons easily. For example, phenol has pKa ≈ 10, so it is a much weaker acid than acetic acid (pKa 4.74).

Can pKa be negative?

Yes. Strong acids like HCl have pKa around −7, and sulfuric acid’s first proton has pKa ≈ −3. A negative pKa means Ka is greater than 1, indicating essentially complete dissociation.

Does pKa change with concentration?

No. pKa is a constant for a given acid at a given temperature. Concentration changes pH, not pKa.

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in ka | Ka value | - | 1.8e-5 | 0
in conc | Acid concentration | mol/L | 0.1 | 0
out pka | pKa | - | -log10(ka) | 2
out percent_ion | Percent ionized | % | (sqrt(ka * conc) / conc) * 100 | 2