[Prodan]

Hey everyone,

I can't seem to get the final answer to this question, I've gotten 2.9mL but the answer is 12.8mL....

Calculate the volume of 1M NaOH solution in mL which must be added to the above 0.1M citric acid solution in order to prepare a buffer of pH 4.0

The pKa values are pKa1 - 3.05, pKa2 - 4.40, pKa3 - 5.80

(note from question Since the required buffer pH is closest to the second pKa of citric acid (pKa2), the conjugate pair tat acts as a buffer is dihydrogen citrate (acid) and monohydrogen citrate ( conjugate base) Remember that you must add enough NaOH to first theoretically convert all the citrx acid to the first ionized species (monosodium citrate). Then use the HEnderson-Hassel-bach equation to determine how much of the monosodium citrate (the weak acid in your pH 4.0 buffer) must be converted to disodium citrate (the conjugate base in your pH4.0 buffer)

I used an ICE table initially and then followed with the Henderson-Hasselbach equation, I think I may have just put in the values, if anyone gets the answer could they give me some hints as how I go about doing it? What is your mind set when you tackle these types of questions?

Thanks

Kevin

[Yggdrasil]

Can you show your calculations.  It might be easier to determine what errors you made if we can see your work.

Also, to solve the question, we need to know the volume of the citric acid solution that we are starting with.

[Prodan]

Oh I'm sorry it was 100ml Citric acid solution, kind of key information I left out..
Alright so What I did was set up my ICE table
C6H8O7     +    OH- =     C6H7O7 +   H2O
0.01                x               0   
0.01 - x          -x               +x
0.01 - x           0               +x

pH = pKa +log(C6H7O7)/(C6H8O7)

4 = 4.40 + log (x)/(0.01-x)

10^4-4.40 =(x)/(0.01-x)
After some rearranging and what not
2.847E-3 = x
Amount of NaOH = 2.847E-3 mol NaOH x 1 Litre/ 1M x 1000mL / Litre
= 2.847mL NaOH

Now the answer is 12.8, so I know I need to add enough NaOH to convert the citric acid to the first ionized species, but I dont know how to do that.... I know its around 10 ml for that answer but I need help thinking about how to get that answer.

[Borek]

No need for ICE table, do exactly as told - calculate first how much base is needed for neutralization of the first proton, then use Henderson-Hasselbalch for second proton.

[Prodan]

Thank you so much! Wow I completely overlooked that question.....as I usually do.