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Using Kw

Chemistry: IB Diploma Higher Level, topic 6.2.3

Use of Kw

Kw can be used to calculate the concentration of hydroxide ions given the pH.

The value of Kw

The value of Kw

  • At room temperature (25°C), it just so happens that Kw is around 1 × 10-14.
  • As the concentration of hydroxide ions must equal the concentration of hydrogen ions for pure water, we know the concentration of both must be 1 × 10-7.
  • If the [H+] is 1 × 10-7, then the pH is 7.
  • This is why we say a neutral solution has a pH of 7.
  • A neutral solution is one with the same concentration of hydroxide ions and hydrogen ions.
Use of Kw

Use of Kw

  • The Kw is a constant at a specific temperature.
  • This means that at a specific temperature, we can relate the [H+] to the concentration of [OH–].
  • If we know the pH of a solution, we can calculate the concentration of [OH–].
Example

Example

  • Say we dissolve 1 mole of HCl into a liter of water:
  • The concentration of H+ ions is 1 moldm-3.
  • We can rearrange the ion product equation from:
  • Kw = 1 ×10-14 = [H3O+(aq)][OH-(aq)] → [OH-(aq)] = 1×10−141molL−1\frac{1 \times10^{-14}}{1 mol L^{-1}}
  • So we can calculate that [OH-(aq)] = 1 ×10-14.
  • The [OH-(aq)] is much lower in an acidic solution than in a neutral solution.
Ka & Kb

Ka & Kb

  • We will cover acid and base dissociation constants in a subsequent section.
  • An important and useful link between Kw and these two constants is:
  • Kw = Ka x Kb

Test yourself on Using Kw

  • The value of Kw is determined to be 2 x 10-14. From the below options, what temperature is the solution most likely to be at?

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