Question

You have 750 mL of a solution that is composed of 9.11 grams of NH3 and 19.33 grams of Ammonium Chloride mixed well. Kb for ammonia = 1.8x10^-5. a.) Is this a buffer solution? Why or why not? b.) If so, what is the pH of this buffer solution? c.) How many mL of 6.00 M HCl can be added to this solution before the buffer is exhausted? d.) How many mL of 6.00 M NaOH can be added to this solution before the buffer is exhausted?

Answer 1

a) yes it is a buffer solution. because NH3-weakbase,NH4Cl-salt .combination of weakbase and its corresponding salt with strong acid is called basic buffer

b) pH = 14 - (pkb+log(NH4Cl/NH3))

pkb of NH3 = -log(1.8*10^-5) = 4.74

no of mol of NH3 = 9.11/17 = 0.536 mol

no of mol of NH4CL = 19.33/53.5 = 0.36 mol

      = 14 - (4.74+log(0.36/0.536))

      = 9.433

c) by the addition of HCl,pH decreases.so that maximum pH decreases up to (14-pkb)-1

        = 14-4.74-1

        = 8.26

pH = 14 - (pkb+log(NH4Cl+HCl/NH3-HCl))

8.26 = 14 - (4.74+log((0.36+x)/(0.536-x))

x = 0.454

volume of HCl can be added = n/M = 0.454/6 = 0.076 L

        = 76 ml

d) by the addition of NaOH,pH increases.so that maximum pH decreases up to (14-pkb)+1

        = (14-4.74)+1

        = 10.26

pH = 14 - (pkb+log(NH4Cl-NaOH/NH3+NaOH))

10.26 = 14 - (4.74+log((0.36-x)/(0.536+x))

x = 0.28

volume of NaOH can be added = n/M = 0.28/6 = 0.047 L

          = 47 ml