Chem question with Ka?

Question

lactic acid is a waste product that accumulates in muscle tissue during exterion, leading to pain and a feeling of fatigue. In a 0.100M aqueous solution, lactic acid is 3.7% dissociated. Calculate the value of Ka for the acid.

Answer 1

OK, I'm going use some symbols here, so let me define those:

HLa = un-ionized lactic acid
La- = lactate ion
H+ = protons

Ka for this weak acid can be written as:

Ka = [H+][La-]/[HLa]

Since the acid is 3.7% ionized, that means that if you had 0.100 mole of lactic acid in water, 0.0037 moles would be ionized and 0.0923 moles would still be present as HLa.

Since that proportion is ionized, the concentration of hydrogen ions (protons) = concentration of lactate ions = 0.0037 mol/L

So, just plug those concentrations into the equation for Ka and solve.