Question on Acid Base Equilibrium

In the acid – base equilibrium equilibrium NaCN + Na₂HPO₄ = Na₃PO₄ + HCN, if pK_a of Na₂HPO₄ and HCN are 12.3 and 9.3 respectively which side (forward / backward) of the equilibrium is favoured?

Answer:

From the pK_a it is evident that HCN is a stronger acid than Na₂HPO₄. As Na₂HPO₄ is a weaker acid, its conjugate base Na₃PO₄ will be stronger than NaCN (conjugate base of HCN). Both the species on the product side are stronger and hence are relatively unstable than the reactants. So the equilibrium is favoured in the backward direction.