Question

An analytical chemist is titrating 118.8 mL of a 0.2500 M solution of nitrous acid (HNO) with a 0.6300 M solution of KOH. The pk of nitrous acid is 3.35. Calculate the pH of the acid solution after the chemist has added 21.98 ml of the KOH solution to it. Note for advanced students: you may assume the final volume equals the initial volume of the solution plus the volume of KOH solution added. Round your answer to 2 decimal places. pH-

Answer 1

pKa of acid = 3.35

HNO2 + KOH ---------------------> KNO2 + H2O

118.8x0.25=29.7 21.98x0.63=13.85 0 0 initial mmoles

15.85 0 13.85 ----------equilibrium

thus the solution is a buffer whose pH is given by Hendersen equation as

pH = pKa + log [conjugate base]/[acid]

= 3.35 + log [13.85/15.85]

= 3.29