Question

The active ingredient in aspirin is acetylsalicylic acid. A 2.51-g sample of acetylsalicylic acid required 27.36 mL of 0.5106 M NaOH for complete reaction. Addition of 13.68 mL of 0.5106 M HCl to the flask containing the aspirin and the sodium hydroxide produced a mixture with pH = 3.48. Determine the molar mass of acetylsalicylic acid and its Ka value. State any assumptions you must make to reach your answer.

Answer 1

Hey there!

mass of sample = 2.51 g of Aspirin

Number of moles of base  ( NaOH ) :

M*V = 27.36*0.5106 = 13.9700 mmol of base

M*V = 13.68*0.516 = 7.05888 mmol of acid

pKa calculation:

pH = pKa + log(A-/HA)

3.48 = pKa + log(7.05888 / (13.9700 -7.05888))

Pka =3.49 -  log(7.05888 / (13.9700 -7.05888))

pKa = 3.48

Molar mass calculation:

molar mass = mass/mol

(2.51)/(13.9700 *10⁻³) =

molar mass 180 g/mol approx

Hope that helps!