PH and the Acid Dissociation Constant

[Soaring Crane]

1. What is the hydronium ion concentration and the pH for an aqueous solution of NH3 that has a hydroxide ion concentration of 2.25E-3 M?

a. 4.44 x 10-11 M, 3.65
b. 4.44 x 10-11 M, 10.35
c. 4.44 x 10-12 M, 2.65
d. 4.44 x 10-12 M, 11.35

[OH] = 2.25E-3
pOH = -log(2.25E-3) = 2.6478
-log[H3O+] = 14 - 2.6478 = 11.352 = pH
[H3O+] = antilog(-11.352] = 4.44E-12


2. Determine the acid dissociation constant for a 0.010 M nitrous acid solution that has a pH of 2.70. Nitrous acid is a weak monoprotic acid and the equilibrium equation of interest is HNO2 + H2O <-> H3O+ + NO2-.

a. 8.0 x 10-3
b. 2.0 x 10-3
c. 5.0 x 10-4
d. 4.0 x 10-4

K_a = [NO2-][H30+]/[HNO2]

pH = -log[H3O+]
2.70 = -log[H3O+]
[H3O+] = antilog(-2.70) = 0.001995 M

K_a = [0.001995 M]^2/[0.010 M - 0.001995 M] = 4.97E-4

Thanks.

 

[Borek]

2xOK

 

[Blueshift5]

Your calculations for both questions are correct. The hydronium ion concentration for the first question is indeed 4.44 x 10^-12 M and the pH is 11.35. This is because the concentration of hydroxide ions is inversely proportional to the concentration of hydronium ions in an aqueous solution.

For the second question, the acid dissociation constant for a 0.010 M nitrous acid solution with a pH of 2.70 is 4.97 x 10^-4. This indicates that nitrous acid is a weak acid, as its K_a value is relatively small. This means that only a small fraction of the acid molecules dissociate into hydronium and nitrite ions in solution.

Understanding the pH and acid dissociation constant is important in many areas of science, including biochemistry, environmental science, and chemical engineering. These concepts help us understand the behavior of acids and bases in solution and their impact on various processes and systems.