Acid Base Titration

Question

How many milligrams of potassium hydrogen phthalate (KHP, C8H5KO4, FW=204.23 g/mol) should be weighed into a flask to standardize approximately 0.05 M NaOH if you wish to use at around 20 mL of base for the titration?

Answer 1

Let us first look at the balanced equation for titrating NaOH with KHP

NaOH + KHP ==>NaKP + H₂O (NaKP = sodium potassium phthalate)

moles NaOH = 20 ml x 1 L / 1000 ml x 0.05 mol / L = 0.0010 mols NaOH

moles KHP needed = 0.0010 mols

mass KHP needed = 0.0010 mols KHP x 204.23 g / mol x 1000 mg / g = 204 mg KHP needed (3 sig.figs)

But since the 20 ml of base is only 1 sig.fig., we should report the answer as 200 mg (1 s.f.)