Final answer:
To find the Ka value of lactic acid with a pH of 2.34, calculate the concentration of hydronium ions using 10^-pH and then use the equilibrium expression with the initial lactic acid concentration to get Ka = 4.57 x 10^-5.
Step-by-step explanation:
To calculate the acid dissociation constant (Ka) of lactic acid (HC3H5O3), we can use the pH of the solution and the concentration of the lactic acid provided. The pH formula is pH = -log[H3O+], where [H3O+] is the concentration of hydronium ions. To find [H3O+], we use the pH given: 2.34. So, [H3O+] = 10-2.34. The Ka expression for lactic acid, which has one acidic hydrogen, is Ka = [H3O+][C3H5O3-]/[HC3H5O3]. Since initially, the concentration of HC3H5O3 is 0.15 mol/L and lactic acid is a weak acid, we can make the assumption that the change in concentration due to ionization (x) is small compared to the initial concentration, thus the equilibrium concentration of HC3H5O3 is approximately 0.15 mol/L.
Using the [H3O+] and the initial concentration of lactic acid, we can solve for Ka: Ka = (10-2.34)2/(0.15 mol/L) = 4.57 x 10-5. Therefore, the Ka value for lactic acid is 4.57 x 10-5.