Question

If a 20mL sample of milk has a 0.027M concentration of lactic acid, what would the pH be? what would the concentration and pH be in one litre?

Answer 1

Let's call lactic acid HL.
.........HL ==> H^+ + L^-
I.....0.027M....0.....0
C.......-x......x.....x
E....0.027-x....x.....x

Look up Ka for Lactic acid and substitute the E line above into the Ka expression and solve for x = (H^+), then convert that to pH.

Remember that the concentration is measured in molarity which is M = mols/L. So it makes no difference what the volume is, mols/L is mols/L is mols/L.