Question

Titration of Buffer Solutions A buffer solution is prepared by placing 5.86 grams of sodium nitrite and 32.6 mL of a 4.90 M nitrous acid solution into a 500.0 mL volumetric flask and diluting to the calibration mark. If 11.47 mL of a 3.03 M solution of potassium hydroxide is added to the buffer, what is the final pH? The Ka for nitrous acid = 4.6 X 10-4. pH =

Answer 1

Given that,

Ka for nitrous acid = 4.6 X 10-4

The initial moles of HNO2 = 32.6 / 1000 x 4.90

= 0.15974 mol

The initial moles of NO2 = mass / molar mass

= 5.86 / 68.995

= 0.084934 mol

The moles of KOH added = 11.47 / 1000 x 3.03

= 0.03475 mol

The moles of HNO2 = 0.15974 mol - 0.03475 mol

= 0.12499 mol

The moles of NO2 = 0.084934 mol + 0.03475 mol

= 0.119684 mol

Let us consider ,

pH = pKa + log([NO2] / [HNO2]

= -log Ka + log(moles of NO2 / moles of HNO2)

= -log (4.6 X 10-4) + log(0.119684 / 0.12499)

= 3.33 - 0.0190

pH = 3.311