Lactic acid, CH
3
CHOHCO
2
H, is found in sour milk, in sauerkraut, and in muscles after
activity.
K
a
for lactic acid = 1.4 x 10
-4
.
a.
If 2.75 g of NaCH
3
CHOHCO
2
, sodium lactate, is added to 5.00 x 10
2
mL of 0.100 M
lactic acid, what is the pH of the resulting buffer solution?
b.
Is the final pH lower or higher than the pH of the lactic acid solution?
Your posting leaves a little to be desired. It's quite difficult to interpret. For this problem, use the Henderson-Hasselbalch. Substitute the numbers and solve for pH.
To determine the pH of the resulting buffer solution, we need to understand how sodium lactate, NaCH3CHOHCO2, acts as a buffer. A buffer is a solution that can resist changes in its pH when small amounts of an acid or base are added.
a. To calculate the pH of the resulting buffer solution, we need to consider the Henderson-Hasselbalch equation:
pH = pKa + log([salt]/[acid])
In this case, the salt is NaCH3CHOHCO2 (sodium lactate) and the acid is lactic acid (CH3CHOHCO2H).
1. Calculate the amount of lactic acid in moles:
- Convert the volume of lactic acid solution from mL to liters:
5.00 x 10^2 mL = 5.00 x 10^2 / 1000 = 0.500 L
- Calculate the number of moles of lactic acid using the molarity:
moles of lactic acid = (0.100 mol/L) * (0.500 L)
2. Calculate the amount of sodium lactate in moles:
- Convert the mass of sodium lactate from grams to moles:
moles of sodium lactate = (2.75 g) / (molar mass of sodium lactate)
3. Calculate the molar ratio of sodium lactate to lactic acid:
- Divide the moles of sodium lactate by the moles of lactic acid to get the ratio.
4. Substitute the values into the Henderson-Hasselbalch equation:
pH = pKa + log([salt]/[acid])
- The pKa for lactic acid is given as 1.4 x 10^-4.
- Plug in the molar ratio from step 3 for [salt]/[acid].
5. Calculate the pH using a calculator or software.
b. To determine whether the final pH is lower or higher than the pH of the lactic acid solution, compare the pH calculated in step 4 with the pH of the original lactic acid solution. If the pH of the resulting buffer solution is lower than the initial lactic acid solution, the final pH is lower; if it is higher, the final pH is higher.