Question

3a)

What is the pH of a sln with [H+] = 1x10-8?

b)

What is the pH of a sln with [OH-] = 1x10-8?

c)

If the pH is 7.99, what is the pOH, and what are the [H+] and [OH-]?

d)

If the pH of a 0.0500 M weak monoprotic acid solution is 4.57, what is the Ka and pKa of this acid?

Answer 1

a)

use:

pH = -log [H+]

= -log (1*10^-8)

= 8

Answer: 8

b)

use:

pOH = -log [OH-]

= -log (1*10^-8)

= 8

use:

PH = 14 - pOH

= 14 - 8

= 6

Answer: 6

c)

POH = 14 - pH

= 14 - 7.99

= 6.01

use:

pH = -log [H3O+]

7.99 = -log [H3O+]

[H3O+] = 1.023*10^-8 M

use:

pOH = -log [OH-]

6.01 = -log [OH-]

[OH-] = 9.772*10^-7 M

d)

use:

pH = -log [H+]

= -log (1*10^-8)

= 8

use:

pOH = -log [OH-]

= -log (1*10^-8)

= 8

use:

PH = 14 - pOH

= 14 - 8

= 6

use:

pH = -log [H3O+]

4.57 = -log [H3O+]

[H3O+] = 2.692*10^-5 M

HA dissociates as:

HA -----> H+ + A-

5*10^-2 0 0

5*10^-2-x x x

Ka = [H+][A-]/[HA]

Ka = x*x/(c-x)

Ka = 2.692*10^-5*2.692*10^-5/(0.05-2.692*10^-5)

Ka = 1.45*10^-8

use:

pKa = -log Ka

= -log (1.45*10^-8)

= 7.84