Question

Calculate the mass in grams of NH4Cl that must be added to 500. mL of a 0.81-M solution of NH3 to prepare a pH = 8.70 buffer. (The pKa for NH3 is 9.25.)

Answer 1

Solution :-

Using the Henderson equation lets calculate the molarity of the NH4Cl needed to achieve the desired pH

pH= pka + log ([base]/[acid])

8.70 = 9.25 + log ([0.81]/[acid])

8.70 – 9.25 = log ([0.81]/[acid])

-0.55 = log ([0.81]/[acid])

Antilog [-0.55] = 0.81 / [acid]

0.2818 = 0.81/[acid]

[acid ] = 0.81 /0.2818

           = 2.874 M

Now lets find the moles of NH4Cl using the molarity and volume

Moles of NH4Cl = molarity   * volume in liter

                              = 2.874 mol per L * 0.500 L

                             = 1.437 mol

Now lets find the mass of NH4Cl

Mass = moles * molar mass

          = 1.437 mol * 53.491 g per mol

         = 76.9 g NH4Cl

So the mass of NH4Cl needed is 76.9 g