Question

Part A: How many grams of solid ammonium chloride should be added to 1.50 L of a 5.46×10^-2 M ammonia solution to prepare a buffer with a pH of 8.630?

Part B: How many grams of solid potassium nitrite should be added to 2.00 L of a 0.255 M nitrous acid solution to prepare a buffer with a pH of 2.353?  

Answer 1

Part A

It is a basic buffer

NH3 ---------> NH4Cl

pOH = 14 - pH = 5.37

pOH = pKb + log [salt] / [Base]

pKb of ammonia is 4.75

5.37 = 4.75 + log [salt] / [5.46*10^-2]

[salt] = 0.227 M = [NH4Cl]

Molariy = mass / molar mass*V in L

0.227 = Mass / 53.491*1.5

Mass of NH4Cl = 18.26 gms

Part B :

Acidic buffer

HNO2 -----------> NO2-

pH = pKa + log [salt] / [acid]

2.353 = 3.39 + log [salt] / [0.255]

[salt] = [KNO2] =0.0234 M

M = mass / Molarmass*V in L

Mass of KNO2 = 0.0234*85.1*2 = 3.985 gms