Determine the pH during the titration of 15.00ml of 0.200mHF with 0.15M KOH at t

Question

Determine the pH during the titration of 15.00ml of 0.200mHF with 0.15M KOH at the following points

Ka = 6.8 x 10-4

pKa=______________

Kb( conjugate base) = __________

a) 0.00ml of KOH added milli moles acid initially=______ milli Moles of base added=_______ What part of the titration curve are you in? __________ pH = ____________

b) 10.00mL KOH added moles acid initially=______ Moles of base added=_______ What part of the titration curve are you in? __________ pH = ___________

c) 20.00ml of KOH added moles acid initially =______ Moles of base added=_______ What part of the titration curve are you in? __________ pH = ____________

d) 30.00ml KOH added moles acid initially =______ Moles of base added=_______ What part of the titration curve are you in? __________ pH = ____________

Explanation / Answer

Determine the pH during the titration of 15.00ml of 0.200mHF with 0.15M KOH at the following points

Ka = 6.8 x 10-4

pKa= - log Ka

= - log 6.8 x 10-4

= 3.17

Kb( conjugate base) = Kw/ Ka

= 1.0 x 10- 14 /6.8 x 10-4

=1.47x 10-11

a) 0.00ml of KOH added

milli moles acid initially= molarity * volume in L

= 0.200 * 0.015

= 0.003 moles

= 3 mmoles

milli Moles of base added=0

What part of the titration curve are you in? initial part

pH =

HF + H2O <==> H3O+ + F-
Initial . . . . . .0.2 . . . . . . . . . . . . .0 . . . . . 0
Change . . . . .-x . . . . . . . . . . . . .x . . . . . x
At Equil. . .0.2-x . . . . . . . . . . . . .x . . . . ..x

Ka = [H3O+][F-] / [HF]

6.8 x 10-4 = (x)(x) / (0.20-x)

Here .the -x term will be small compared to 0.20 so we ignore it.


x^2 / 0.20 = 6.8 x 10^-4
x^2 = 1.4 x 10^-4
x = 1.2 x 10^-2 = [H3O+]

pH = -log [H3O+]

= -log (1.2 x 10^-2)

= 1.93

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