Question

1) Both HC2O4− and HS− are amphoteric.
Write an equation to show how HC2O4− can act as a base with HS− acting as an acid.
Write an equation to show how HC2O4− can act as an acid with HS− acting as a base.

2) If Kb for NX3 is 1.5×10−6, what is the pOH of a 0.175 M aqueous solution of NX3?
If Kb for NX3 is 1.5×10−6, what is the percent ionization of a 0.325 M aqueous solution of NX3?

3) Caffeine (C8H10N4O2) is a weak base with a pKb of 10.4. Calculate the pH of a solution containing a caffeine concentration of 420 mg/L .

Answer 1

1)

Write an equation to show how HC2O4− can act as a base with HS− acting as an acid.

HC2O4- + H2O    ---------->   H2C2O4 +   OH-

HS-   +   H2O   ----------->   S2-   +   H3O+

Write an equation to show how HC2O4− can act as an acid with HS− acting as a base.

HC2O4- + H2O   ----------> C2O42-   +   H3O+

HS-   + H2O   ----------->   H2S   +     OH-

2)

pKb = -log Kb = -log (1.5 x 10^-6)

       = 5.82

pOH = 1/2 (pKb - log C)

       = 1/2 (5.82 - log 0.175)

pOH = 3.29

b)

[OH-] = sqrt (Kb x C) = sqrt (1.5 x 10^-6 x 0.325)

         = 6.98 x 10^-4 M

% ionization = ([OH-] / C ) x 100

                     = (6.98 x 10^-4 / 0.325) x 100

                       = 0.22 %

3)

pH = 7.47