Calculate the mass of potassium acid phthalate (HKC8H4O4) that will require about 15 mL of the approximately 0.1 M NaOH solution. Measure approximately this mass on a tared piece of weighing paper and transfer it to a clean, labeled Erlenmeyer flask. Similarly, prepare all three samples, recording the masses for each. Dissolve the KHP in about 50 mL of R. O. water and add 2 drops of phenolphthalein.