Calculate and apply the dissociation constant (Ka) and pKa (pKa = -log10 (Ka)) to determine the difference between strong and weak acids

Calculation of pKa for strong and weak acids:

  • For HCl, which is a strong acid, the Ka value is 1.3 x 106. Hence,
    pKa = -log10(Ka)
    = -log10(1.3 x 106)
    = -6.1
  • For HCOOH, which is a weak acid, the Ka value is 1.8 x 10-4. Hence,
    pKa = -log10(Ka)
    = -log10(1.8 x 10-4)
    = 3.7
  • Strong acids have a higher Ka value because they dissociate completely in aqueous solution. Since pKa is the negative logarithm of Ka, it tends to be lower. The opposite scenario is for weak acids which have a lower Ka value and a higher pKa value.