[Ericv_91]

Question: Calculate the volume of NaOH which can be added to 0.1L of 0.1M citric acid solution in order to prepare a buffer of pH 4. Since the required buffer of pH is closer to the second pKa of citric acid (pKa1=3.09, Pka2= 4.75), the conjugate pair that acts as a buffer is dihydrogen citrate (acid) and monohydrogen citrate (conjugate base). There are two parts to this calculation. First, calculate the amount of NaOH required to convert citric acid to dihydrogen citrate and in the second part, use the Hendersson-Hasselback equation to know how much extra NaOH is required to reach a pH of 4.

Answer: Well, acids and bases are my worst area of chemistry, so I'm really struggling with this. I tried to do an ice table to solve for the NaOH required to get the citric acid to dihydrogen citrate, but that didnt give me such a great number. Do I even need an ice table for that? Is there an easier way to solve that first problem? To my understanding, I need to remove one hydrogen atom from each citric acid molecule to get dihydrogen citrate. Is it just a 1-1 ratio (0.01mol of citrate therefore 0.01mol of NaOH will take 1 hydrogen away to make water and sodium and acid)?

Furthermore, I can't seem to understand how the Hasselbach equation will get me the rest of the volume of NaOH. I know I have the pH (4), pKa (4.75), [acid](0.1M?), but I already know the concentration of base (NaOH=2M), right? I don't need a different molarity, i just need more volume of the base.

Any insight on this question is greatly appreciated, I've thought about it for several hours and have come out with nothing but a headache and mass confusion.

Thanks,

Eric

[Borek]

Don't go for ICE table, it is useless here. Use Henderson-Hasselbalch equation to calculate required concentrations of acid and conjugate base. Assume neutralization is quantitative and use simple stoichiometry to calculate volumes of citric acid and NaOH required.