pKb Calculator

The pKb (negative log of the base dissociation constant) measures how strongly a base ionizes in water — the lower the pKb, the stronger the base. Select your calculation type — convert Kb to pKb, pKb to Kb, or between all four constants — then enter your known values (Kb, pKb, Ka, or pKa) and temperature. The pKb Calculator returns your primary pKb result alongside secondary outputs for Kb, pKa, and Ka.

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Enter the base dissociation constant in scientific notation if needed

Negative logarithm of Kb

For conversion calculations

Negative logarithm of Ka

°C

Calculations assume 25°C (Kw = 1.0 × 10⁻¹⁴)

Results

pKb

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Kb

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pKa

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Ka

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Frequently Asked Questions

What is the relationship between pKa and pKb?

At 25°C, pKa + pKb = 14 for any conjugate acid-base pair. This relationship comes from the water ionization constant (Kw = 1.0 × 10⁻¹⁴).

How do you calculate pKb from Kb?

pKb is calculated as the negative logarithm of Kb: pKb = -log(Kb). For example, if Kb = 1.8 × 10⁻⁵, then pKb = -log(1.8 × 10⁻⁵) = 4.74.

What does a higher pKb value indicate?

A higher pKb value indicates a weaker base. Since pKb = -log(Kb), a larger pKb means a smaller Kb, which corresponds to less base dissociation.

How are Ka and Kb related?

For a conjugate acid-base pair, Ka × Kb = Kw = 1.0 × 10⁻¹⁴ at 25°C. This means that knowing one constant allows you to calculate the other.

Can pKb be negative?

Yes, pKb can be negative for very strong bases where Kb > 1. However, most weak bases have positive pKb values between 1 and 14.

How does temperature affect these calculations?

Temperature affects the water ionization constant (Kw). At 25°C, Kw = 1.0 × 10⁻¹⁴, but it changes with temperature, affecting the pKa + pKb = 14 relationship.