Question

Construct the titration curve, for the titration of 50.0 mL of 0.125 M trimethylamine with 0.175 M HCl (aq). (Use graph paper and show all your work for full CREDIT)

Calculate the initial pH of the solution in the flask?

Calculate the pH of the solution mixture after 15 mL of titrant is added?

Calculate the pH at the equivalence volume?

Calculate the pH at of the mixture after 45 mL of titrant is added?

Calculate the pH of the mixture after 60 mL of titrant is added?

Answer 1

trimethylamine , Kb = 6.3 x 10^-5

pKb = 4.20

Calculate the initial pH of the solution in the flask

pOH = 1/2 [pKb - log C]

pOH = 1/2 [4.20 -log 0.125]

pOH = 2.55

pH + pOH = 14

pH = 11.45

Calculate the pH of the solution mixture after 15 mL of titrant is added

millimoles of base = 50 x 0.125 = 6.25

millimoles of acid = 0.175 x 15 = 2.625

B + HCl ------------------> salt

6.25 2.625 0

3.625 0 2.625

pOH = 4.20 + log (2.625 / 3.625)

pOH = 4.06

pH + pOH =14

pH = 9.94

Calculate the pH at the equivalence volume?

at this point : millimoles of acid = millimoles of base

0.125 x 50 = 0.175 x V

V = 35.7 mL

volume - 35.7 mL

Calculate the pH at of the mixture after 45 mL of titrant is added?

millimoles of HCl = 0.175x 45 = 7.875

B + HCl ----------------------> salt

6.25 7.875 0

0 1.625 6.25

strong acid remains

[H+] = 1.625 / (50 +45) = 0.0171 M

pH = 1.77

Calculate the pH of the mixture after 60 mL of titrant is added?

millimoles of HCl = 0.175x 45 = 10.5

B + HCl ----------------------> salt

6.25 10.5 0

0 4.25 6.25

strong acid remains

[H+] = 4.25 / (50 +60) = 0.0386 M

pH = 1.41