# What are pKa and pKb in acids and bases?

Dec 20, 2014

$\text{p"K_"a}$ and $\text{p"K_"b}$ are measures of the strengths of acids and bases, respectively

Acids

When you dissolve an acid in water, it undergoes an equilibrium reaction with the water in an.

HA + H₂O ⇌ H₃O⁺ + A⁻

The value of the equilibrium constant is given by

K_"a" = (["H"_3"O"^+]["A"^-]]/["HA"]

The greater the value of ${K}_{\text{a}}$, the stronger the acid.

For most weak acids, ${K}_{\text{a}}$ ranges from ${10}^{-} 2$ to ${10}^{-} 14$.

We convert these exponential numbers into a normal range by taking their negative logarithm.

The operator $\text{p}$ means "take the negative logarithm of".

So $\text{p"K_"a" = -logK_"a}$.

For most weak acids, $\text{p"K_"a}$ ranges from 2 to 14.

Thus, the smaller the value of $\text{p"K_"a}$ , the stronger the acid.

Bases

When you dissolve a base in water, it reacts with the water in an equilibrium reaction.

B + H₂O ⇌ BH⁺ + OH⁻

The value of the equilibrium constant is given by

K_"b" = (["BH"^+]["OH"^-]]/["B"]

The greater the value of ${K}_{\text{b}}$, the stronger the base.

For most weak acids, ${K}_{\text{b}}$ ranges from ${10}^{-} 2$ to ${10}^{-} 13$.

$\text{p"K_"b" = -logK_"b}$.

For most weak acids, $\text{p"K_"a}$ ranges from 2 to 13.

The smaller the value of $\text{p"K_"b}$ , the stronger the base.

Here's a video on $\text{p"K_"a}$ and $\text{p"K_"b}$.