Question

A 25.00mL of a 0.500M trimethylamine ((C2H5)3N) solution is titrated against 0.625M HCl. Answer the following questions or calculate the pH after the addition of each of the following volumes of HCl solution. If it is the equivalence point or the midpoint, note it. (Kb=5.6x10^-4)

a. 0.00mL HCl

b. 10.00 mL HCl

c. 20.00mL HCl

d.40.00mL HCl

e. 60.00mL HCl

f. At what pH would this make a good buffer?

g. Select an appropriate indicator for this titration.

(NEED HELP PLEASE!!! If possible can you work it out fully so i can know how to do it)

Answer 1

First let's calculate the equivalence point:
Va = 25 * 0.50 / 0.625 = 20 mL

Now, I'll do til c, so you can try to do d and e.

At 0 mL, means that we only have the base in solution:
r: (C₂H₅)₃N + H₂O <-------> (C₂H₅)₃NH⁺ + OH^-   Kb = 5.6x10^-4
i: 0.5 0 0
e: 0.5-x x x

5.6x10^-4 = x²/0.5-x --> Kb is small so we can approximate 0.5-x to 0.5 only:
5.6x10^-4 * 0.5 = x²
x = [OH^-] = 0.0167 M
pOH = -log(0.0167) = 1.78
pH = 14-1.78 = 12.22

At 10 mL of acid added, we are at the half equivalence point, and in this point pH = pKa so:
pKb = (-log5.6x10^-4) = 3.25
pKa = 14-3.25 = 10.75 = pH

At 20 mL, we are at the equivalence point and the moles of acid are the same moles of base:
moles A = moles B = 0.5 * 0.025 = 0.0125 moles
Concentration = 0.0125 / 0.045 = 0.2778 M

At the equivalence point, we also know that the moles of base are consumed and now, the moles of acid are in excess, therefore, the reaction taking place here is:

r: (C₂H₅)₃NH⁺ + H₂O <-------> (C₂H₅)₃N + H⁺   Ka = 1x10^-14/5.6x10^-4 = 1.79x10^-11
i: 0.2778 0 0
e: 0.2778-x x x

1.79x10^-11 = x²/0.2778-x
1.79x10^-11 * 0.2778 = x²
x = [H⁺] = 2.23x10^-6 M
pH = -log(2.23x10^-6) = 5.65

At 40 mL, we are actually at double equivalence point, so the pH will be the same as pKb, in this case:
pH = 3.25

Now, try to do at 60 mL (3 times the equivalence point, maybe 1/3 of pKa ?)

A great buffer will be made when the pH is near the pKa of solution, so at 10 it would be great.

As the pH at equivalence point is 5.65, the best indicator could be alizarine red, (pH range 4.6 - 6.0)

Hope this helps