Most buffers are a mixture of a weak acid and its conjugate base. buffers resist changes in ph when small amounts of acid or base are added. the effective ph range of a buffer depends on the pka of the acid and the relative concentrations of the acid, ha, and its conjugate base, a−. the equation for the acid dissociation of a weak acid is: ha(aq) + h2o(l) ⇌ a−(aq) + h3o+(aq) the equation for the acid dissociation constant is: ka equals concentration of h3o+ times concentration of a− divided by concentration of ha rearrange the equation and then substitute in pka = −log(ka) and ph = −log(h3o+): ph equals pka plus log of the result of the concentration of a− divided by concentration of ha this expression is the henderson-hasselbalch equation and is useful for determining the ph of a buffer solution. a buffer solution has 0.620 m hf and 0.710 m f−. if 0.025 moles of hcl is added to 325 ml of the buffer solution, what is the ph after the addition? the pka of hydrofluoric acid is 3.16.