What is the symbol used to represent the acid dissociation constant?

Ka

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To which type of acid (strong or weak) does the acid dissociation constant (Ka) apply?

Weak acids

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What is the expression for the acid dissociation constant (Ka) of a weak acid (HA)?

Ka ​=[HA][H+][A−]​

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What are the units of the acid dissociation constant?

mol dm-3

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What is the formula to calculate pKa from Ka?

pKa = -log10 Ka

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What is the relationship between the strength of a weak acid and its Ka value?

The stronger the weak acid, the larger the Ka value.


Conversely, the weaker the weak acid, the smaller the Ka value.

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Give two assumptions made when calculating the pH of a weak acid (HA) based on its concentration and its Ka value.

1The ionisation of water is negligible so the concentration of H+ ions produced by the ionisation of water molecules present in the solution is ignored, i.e.,

[H+]equilibrium ≈ [A-]equilibrium.

2The ionisation of the weak acid is so small that the concentration of undissociated HA molecules present at equilibrium is approximately the same as that of the original acid, i.e.,

[HA]equilibrium ≈ [HA]undissociated.

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Explain the situations when Ka assumptions become inaccurate.

  1. For stronger weak acids with Ka > 10-2 mol dm-3 and for very dilute solutions, the concentration of H+ ions becomes significant and there is a notable difference between [HA]equilibrium and [HA]undissociated.
  2. For very weak acids or very dilute solutions, the concentration of H+ ions from the dissociation of water will be significant and there is a notable difference between [H+]equilibrium and [A-]equilibrium.

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What is the relationship between the strength of a weak acid and its pKa value?

The stronger the weak acid, the smaller the pKa value.


Conversely, the weaker the weak acid, the larger the pKa value.

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What is the formula to calculate Ka from pKa?

Ka = 10-pKa

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