What are pKa and pKb in acids and bases?

Question

17677 views around the world

You can reuse this answer
Creative Commons License

Answer 1 · 2014-12-20T19:46:09

#"p"K_"a"# and #"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_"a"#, the stronger the acid.

For most weak acids, #K_"a"# ranges from #10^-2# to #10^-14#.

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

The operator #"p"# means "take the negative logarithm of".

So #"p"K_"a" = -logK_"a"#.

For most weak acids, #"p"K_"a"# ranges from 2 to 14.

Thus, the smaller the value of #"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_"b"#, the stronger the base.

For most weak acids, #K_"b"# ranges from #10^-2# to #10^-13#.

#"p"K_"b" = -logK_"b"#.

For most weak acids, #"p"K_"a"# ranges from 2 to 13.

The smaller the value of #"p"K_"b"# , the stronger the base.

Here's a video on #"p"K_"a"# and #"p"K_"b"#.