# What mass of potassium lactate is needed to 300.0 mL of 0.238 M Lactic acid solution to make a solution with a pH of 4.00? pKa (lactic acid) =3.86

Upon attempt, I've yielded an answer of 11.91g. My calculations do not take 300 mL Lactic acid solution into account, and now I somewhat know the error is at least partially due to the fact that my working out is independent on the the quantity of solution. All help is highly appreciated.

Here is my working out:

Note:

• my molar mass of potassium lactate is incorrect in the final step, none the less the answer is still incorrect.

Using the Henderson Hasselbalch equation pH = pKa + log [base]/[acid].
4.0 = 3.86 + log [lactate]/[lactic acid].
log [lactate]/[lactic acid] = 0.14.
[lactate][lactic acid] = 1.38
moles lactic acid = 0.300 L x 0.238 mole/L = 0.0714 moles.
moles lactate = 1.38 x 0.0714 = 0.09853 moles
grams lactate = 0.09853 moles x 128.2 g/mole = 12.63 grams = 12.6 grams (to 3 significant figures)
NOTE: this assumes no change in volume upon addition of the potassium lactate.

• I find it incredible that there is a down vote with no explanation. It appears the answer is complete and accurate, and if not, some comment should appear with the down vote. This site essentially SUCKS. – Dr. J. May 26 '18 at 1:09
• I am relatively new to the site and I agree with you. Your post was indeed accurate and helpful. Perhaps the community gets bothered by students asking questions they're genuinely having trouble with. – Dogukan Kayhan May 26 '18 at 2:11