Question

What mass of ammonium chloride should be added to 2.55 L of a 0.150 M NH3 in order to obtain a buffer with a pH of 9.60?

Answer 1

According to Henderson's Equation,

pOH = 14 - pH = pK_b + log ([salt] / [base] )

   14 - 9.60 = - log K_b + log ([salt] / [base] )

   4.4 = 4.76 + log ([salt]/0.150)

log ([salt]/0.150) = -0.36

      ([salt]/0.150) = 10 ^-0.36

                         = 0.44

                [salt] = 0.44x0.150

                        = 0.066 M

Given Volume of the solution , V = 2.55 L

We know that Molarity , M = number of moles / volume of solution in L

                                0.066 = n / 2.55 L

                                      n = 0.168 moles

Molar mass of NH₄Cl = At.mass of N + (4xAt.mass of H ) + At.mass of Cl

                                = 14+(4x1) + 35.5

                                = 53.5 g/mol

So mass of NH₄Cl added , m = number of moles x molar mass

                                           = 0.168 mol x 53.5 (g/mol)

                                           = 8.988 g