Question

What is the Henderson Hasselbach equation? Use this equation to explain the relationship between pH and dissociation constants.

 

Answer 1

It is an equation useful for calculating the pH of a buffer solution or the concentration of the acid and base

It is given as

pH = pK_a + log ([A^-]/[HA])

where: [A^-] = molar concentration of a conjugate base

[HA] = molar concentration of a undissociated weak acid (M)

The equation can be rewritten to solve for pOH:

pOH = pK_b + log ([HB⁺]/[ B ])

[HB⁺] = molar concentration of the conjugate base (M)

[ B ] = molar concentration of a weak base (M)

If you know either pH or pKa you can solve for the other value

pH is the sum of the pKa value and the log of the concentration of the conjugate base divided by the concentration of the weak acid.

At half the equivalence point:

pH = pKa

It's worth noting sometimes this equation is written for the K_a value rather than pKa, so you should know the relationship:

pKa = -logK_a