Question

1. The pH of a 0.50 M solution of ethylamine CH3CH2NH2 is 12.22. Calculate the Kb of ethylamine. Use “E” for scientific notation.
2. A 0.050 M solution of pyruvic acid, an intermediate in metabolism, has a pH of 1.93. Calculate Ka for pyruvic acid, a weak monoprotic acid. Use “E” for scientific notation.
3. Calculate the pH of a 0.17 M solution of HClO, with Ka1= 3.5x10.
4. At 25°C, 2.29E0 grams of sodium hydroxide is dissolved in enough water to make 500. mL of solution. Calculate ​[H3O+]. Do not enter units as part of your answer.

Answer 1

1)

concentration of CH3CH2NH2 = 0.50 M

pH = 12.22

pOH = 1.78

[OH-] = 0.0166 M = x

CH3CH2NH2   +   H2O   -----------> CH3CH2NH3+ +   OH-

    0.50                                                      0                       0

0.50 - x                                                    x                        x

Kb = x^2 / 0.50 - x

    = (0.0166)^2 / 0.50 - 0.0166

Kb = 5.70 x 10^-4

Kb = 5.7E-4

2)

pH = 1.93   , [H+] = 0.01175 M

HA    +   H2O    ----------> A-   +     H3O+

0.050                                  0              0

0.050-x                               x              x

Ka = x^2 / 0.050 - x

     = (0.01175)^2 / 0.050 - 0.01175

Ka = 3.6E-3

2)

HClO    ------------> H+ +   ClO-

0.17                         0          0

0.17-x                     x           x

Ka = x^2 / 0.17 - x

3.5 x 10^-8 = x^2 / 0.17 - x

x = 7.71x10^-5

[H+ ] = 7.71x10^-5 M

pH = 4.11