Question

1. What is the pH of pure water at 40.0C if the Kw at this temperature...

1. What is the pH of pure water at 40.0C if the Kw at this temperature is 2.92X10^-14?

2. Determine the pH of a 0.227M C5H5N solution at 25C. The Kb of C5H5N is 1.7X10^-9

Please show work.

Homework Answers

Answer #1

1)

pH = -log(H+)

by definition

and

Kw = [H+][OH-]

always

due to stoichiomerty = [H+] = [OH-] for pure water

2.92*10^-14 = Kw

2.92*10^-14 = [H+][Oh-]

assume x = [H+] = [OH-]

2.92*10^-14 = x*x

x = sqrt(2.92*10^-14) = 1.70880*10^-7 M

pH = -log(x) = -log(1.70880*10^-7) = 6.76730876465

2)

C5H5N + H2O <--> C5H5NH+ + OH-

Kb = [C5H5NH+][OH-]/[C5H5N]

assume in equilbirium that

[C5H5NH+] = x = [OH-]

and

[C5H5N] = M-x = 0.227-x

(1.7*10^-9) = x*x/(0.227x)

x = 1.964*10^-5

[OH-] = 1.964*10^-5

pOH = -log(OH-) = -log(1.964*10^-5) = 4.706858

pH = 14-pOH = 14-4.706858

pH = 9.293142

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