Question

Which of the following indicators would be most appropriate to determine the equivalence point for a titration of a weak base (25 mL 0.10 M morpholine Kb=3.1 x 10^-6) with 0.10 M HCL. Phenolthalein pH=9 Bromthymol blue pH=7 Methyl Red pH=5 Phenol Red pH=8

My TA needs me to show how to calculate the pH for this titration.

Answer 1

we must first identify the pH of the weak base iun equivalence point...

since this is a weak base, the most suitable is a low pH since it will hydrolyze

short answer --> Methyl red...

proof:

let B = base of morpholine

B + H2O <-> HB+ + OH-

after neutralization fiwh H+

OH- + H+ = H2O

so

HB+ is left

it will hydrolyze with water as follows:

HB+ + H2O <-> H3O+ + B

this is acidic, since H3O+ is present

so

Ka [H3O+][B]/[HB+]

Ka = Kw/Kb = (10^-14)/(3.1*10^-6) = 3.225806*10^-9

in the equivalence point

Vtotal = V1+V2

V1 = 25 mL

voluem from acid = 25 mL as well

[HB+] = M1V1/(V1+V2) = 25*0.1 //(25+25) = 0.05 M

so

[H3O+] = [B] = x

[HB+] = M-x = 0.05 - x

so

3.225806*10^-9 = x*x/(0.05-x)

x = 1.269*10^-5

since x = [H3O+] = 1.269*10^-5

pH = -log(x) = -log(1.269*10^-5) = 4.89653

acidic, near the methyl red point, as expected