Question

1. Construct the titration curve, for the titration of 50.0 mL of 0.125 M trimethylamine with 0.175 M HCl (aq).

A. Calculate the initial pH of the solution in the flask.

B. Calculate the pH after 15 mL of titrant is added.

C. Calculate the pH at the equivalence volume.

D. Calculate the pH after 45 mL of titrant is added.

E. Calculate the pH after 60 mL of titrant is added.

Please make sure to include the titration curve and show your work!

Answer 1

millimoles of trimethylamine = 50 x 0.125 = 6.25

trimethylamine pKb = 4.20

A. Calculate the initial pH of the solution in the flask.

pOH = 1/2 [pKb- logC]

       = 1/2 [4.20 - log0.125]

       = 2.55

pH +pOH =14

pH = 11.45

B. Calculate the pH after 15 mL of titrant is added.

millimoles of HCl = 15 x 0.175 = 2.625

base + acid ------------------> salt

6.25     2.625                        0     ----------------> initial

3.625    0                            2.625

buffer it is

pOH = pKb + log [salt/base]

pOH = 4.20 + log (2.625/3.625)

pOH = 4.06

pH = 9.94

C. Calculate the pH at the equivalence volume.

milimole of base= acid = 6.25

volume of base = 6.25 / 0.175 = 35.71ml

base + acid -------------> salt

salt only reamins its millimoles = 6.25

salt concentration = 6.25 / (50+35.71) = 0.073M

pH = 7- 1/2 [Pkb+ logC]

pH = 7 - 1/2 [4.20 + log 0.073]

pH = 5.47

D. Calculate the pH after 45 mL of titrant is added.

millimoles of acid = 0.175 x 45 = 7.875

base + acid ---------------> salt

6.25       7.875                   0

0              1.625                6.25

acid concentration = 1.625 / 95 = [H+]

                             = 0.0171 M

pH = -log [H+]

pH = 1.77

E. Calculate the pH after 60 mL of titrant is added

millimoles of acid = 0.175 x 60 = 10.5

base + acid ---------------> salt

6.25       10.5                0

0              4.25              6.25

acid concentration = [H+] = 4.25/ 110

                           [H+]   = 0.0386 M

pH = -log [H+]

pH = 1.41