Calculate the pH of a 75.0 ml solution containing 0.85 M lactic acid after the addition of 0.0225 moles NaL (assume a negligible volume change.)
Use the Henderson-Hasselbalch equation.
Lactic acid is the acid.
sodium lactate is the base.
DrBob--How do I determine the Pka value?
How do I figure out the Pka value?
Watch.
pH = -log(H^+)
pOH = -log(OH^-)
pKa = -log Ka
pKb = -log Kb
Do you get it? p means -log.
Thanks Dr. Bob!
kkm
To calculate the pH of the solution, we need to consider the dissociation of lactic acid.
Lactic acid (HC3H5O3) is a weak acid that partially dissociates in water, forming H+ ions and C3H5O3- ions.
The balanced equation for the dissociation of lactic acid is:
HC3H5O3 ⇌ H+ + C3H5O3-
First, let's calculate the initial concentration of lactic acid (HC3H5O3) in the solution:
Initial concentration = number of moles / volume of solution
Given that the volume of the solution is 75.0 ml and the concentration of lactic acid is 0.85 M, we can calculate the number of moles of lactic acid:
Number of moles = concentration × volume
Number of moles = 0.85 mol/L × 0.075 L = 0.06375 moles
Since lactic acid is a weak acid, we are assuming it does not fully dissociate, so the initial concentration of lactic acid will be equal to the number of moles.
Initial concentration of lactic acid = 0.06375 moles / 0.075 L = 0.85 M
Now, let's consider the addition of NaL (sodium lactate). NaL dissociates in water to yield Na+ ions and C3H5O3- ions.
The balanced equation for the dissociation of NaL is:
NaL ⇌ Na+ + C3H5O3-
NaL contributes additional C3H5O3- ions to the solution, which reacts with H+ ions from lactic acid to form lactic acid molecules. This reaction is known as the hydrolysis of the salt.
Since the volume change is assumed to be negligible, the total volume of the solution remains 75.0 ml.
Given that 0.0225 moles of NaL is added to the solution, the concentration of the C3H5O3- ions in the solution will increase.
Concentration of C3H5O3- ions = number of moles / volume of solution
Concentration of C3H5O3- ions = 0.0225 moles / 0.075 L = 0.3 M
Now, we need to calculate the new concentration of lactic acid (HC3H5O3) in the solution after the addition of NaL.
Since the C3H5O3- ions react with H+ ions from lactic acid, the decrease in the concentration of lactic acid will be equal to the increase in the concentration of C3H5O3- ions.
Change in concentration of lactic acid = -0.3 M (due to hydrolysis of NaL)
New concentration of lactic acid = Initial concentration - Change in concentration
New concentration of lactic acid = 0.85 M - 0.3 M = 0.55 M
Now, we can calculate the pH of the solution using the concentration of lactic acid.
pH = -log[H+]
Since the concentration of H+ ions is equal to the concentration of lactic acid, we can directly calculate the pH:
pH = -log(0.55) = 0.259
Therefore, the pH of the solution containing 0.85 M lactic acid after the addition of 0.0225 moles NaL is approximately 0.259.