Seneca
Sign up

Weak Bases & Kb

Chemistry: IB Diploma Higher Level, topic 6.3.3

Weak Bases

Not all bases fully ionize in solution.

Weak bases

Weak bases

  • Weak bases behave similarly. They do not fully ionize in solution.
  • So, for weak acids and bases, the concentration of H+ or OH- ions is not just the concentration of the acid or base.
  • This makes it harder to calculate the concentrations in solution.
Kb

Kb

  • For the reaction of a base, B:
  • B(aq) + H2O(l) ⇌ HB+(aq) + OH−(aq)
  • We define a new constant as Kb = [OH−][HB+][B]\frac{[OH^-][HB^+]}{[B]}
  • B refers to the un-ionised base molecule, and HB+ refers to the cation remainder after gain of a proton.
pKb

pKb

  • Just like pH, we use pKb because Kb values vary a lot.
  • The definition of pKb is:
  • pKb = −log10(Kb)

Calculating the Concentration of Ions

We can use pKb along with pH to calculate the concentrations of varying ions in solution.

Converting pKb to Kb

Converting pKb to Kb

  • The definition of pKb is:
  • pKb = -log10(Kb)
  • This can be rearranged into:
  • Kb = 10-pKb
Calculating [OH-] from Kb

Calculating [OH-] from Kb

  • When you dissolve a weak monohydroxy (one OH- ion) base in solution, [OH-] is always equal to [BH+].
  • This means you can simplify Kb into:
  • Kb = [OH−]2[B]\frac{[OH^-]^2}{[B]}
  • But, [B] is the concentration of the base added ([B]0, minus the concentration of the ionized base), so we can simplify further to:
  • Kb = [OH−]2[B]0−[OH−]\frac{[OH^-]^2}{[B]_0 - [OH^-]}
  • If you know the values of Kb and [B]0, you can calculate the concentration of OH- ions.
Calculating [B] from Kb and pH

Calculating [B] from Kb and pH

  • First, use the definition of pH to calculate the [OH-].
  • Next, use [OH-] = [HB+] to simplify the equation for Kb.
  • Then, rearrange Kb to give [B]:
  • [B] = [OH−]2Kb\frac{[OH^-]^2}{K_b}

Test yourself on Weak Bases & Kb

  • KOH is a strong base, where would the equilibrium lie in the following equation?
    KOH ⇋ K+ + OH-

  • What is the correct mathematical expression for pKb?

  • PKb = -log10(Kb).
    What is the rearranged equation for Kb = ?

  • For a weak monohydroxide base, what is an expression for [HB]?

Practise this topic for free