How do you calculate Keq from pKa values?

Question

Wyatt Atkins · Accepted Answer

Well, a #K_a# value is a measure of the following equilibrium.......

#HA(aq) + H_2O(l) rightleftharpoons H_3O^+ + A^-# #HA(aq) + H_2O(l) rightleftharpoons H_3O^+ + A^-# #K_a=([H_3O^+][A^-])/([HA(aq)])# Now #pK_a=-log_10K_a#...... And thus #K_a=10^(-pK_a)# And thus where #pK_a# is small (negative), this specifies a strong acid where the given equilibrium lies to the RIGHT............. Given #K_a=([H_3O^+][A^-])/([HA(aq)])# we can take #log_10# of both sides to give...... #log_10K_a=log_10[H_3O^+]+log_10{[[A^-]]/[[HA]]}# Regarding reorganization..... #underbrace(-log_10[H_3O^+])_(pH)=underbrace(-log_10K_a)_(pK_a)+log_10{[[A^-]]/[[HA]]}# And so. #pH=pK_a+log_10{[[A^-]]/[[HA]]}# This is a form of the #"buffer equation"#, which is used when a weak acid is mixed with its conjugate base in appreciable quantities. Protonation (or deprotonation) of the base or the acid moderates GROSS changes in #pH#, and solution #pH# remains tolerably close to the #pK_a# value.

Ellie Andrews · Answer

undefined To calculate the equilibrium constant (Keq) from pKa values, you can use the relationship:

$$ Keq = 10^{(pKa_{products} - pKa_{reactants})} $$

Where $ pKa_{products} $ represents the sum of the pKa values of the products, and $ pKa_{reactants} $ represents the sum of the pKa values of the reactants.