0.10 M solution of a weak monoprotic acid has a hydronium-ion concentration of 5.0 * 10^-4 M. What is the equilibrium constant, Ka, for this acid?
a. 5.0 *10^-2
b. 5.0 * 10^-3
c. 2.5 * 10^-4
d. 2.5 * 10^-5
e. 2.5 * 10^-6
2. (Points: 1)
What is the carbonate-ion, CO3^-2, concentration in a 0.037 M carbonic acid solution?
a. 1.2 ´ 10^-4
b. 4.2 ´ 10^-7
c. 7.6 ´ 10^-8
d. 4.8 ´ 10^-11
e. 5.2 ´ 10^-19
1. c
2. a
To find the equilibrium constant, Ka, for a weak monoprotic acid, you will need to use the expression for the acid dissociation constant, which is given by:
Ka = [H3O+][A-] / [HA]
where [H3O+] is the concentration of the hydronium ion, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid.
In the first question, we are given that the hydronium-ion concentration is 5.0 * 10^-4 M. However, we are not given the concentrations of the conjugate base or the weak acid.
Since the weak acid is monoprotic, it can be assumed that the concentration of the conjugate base is the same as the concentration of the acid that dissociates.
Let's assume the concentration of the weak acid is x M. Then, the concentration of the hydronium ion is 5.0 * 10^-4 M, and the concentration of the conjugate base is also x M.
Plugging these values into the expression for Ka, we get:
Ka = (5.0 * 10^-4)(x) / (x)
The x cancels out, and we are left with:
Ka = 5.0 * 10^-4
So, the equilibrium constant, Ka, for this acid is 5.0 * 10^-4.
Therefore, the correct answer is (a) 5.0 * 10^-2.
Now let's move on to the second question.
To find the carbonate-ion concentration in a carbonic acid solution, we need to consider the dissociation of carbonic acid (H2CO3) into bicarbonate ion (HCO3-) and hydronium ion (H3O+):
H2CO3(aq) ⇌ HCO3-(aq) + H3O+(aq)
The carbonate-ion concentration (CO3^2-) is related to the bicarbonate-ion concentration (HCO3-) by the following equation:
[HCO3-] = [H2CO3] / Ka1
where [H2CO3] is the concentration of carbonic acid and Ka1 is the equilibrium constant for the first dissociation step.
Since we are given that the concentration of carbonic acid is 0.037 M, we can substitute this value into the equation:
[HCO3-] = 0.037 / Ka1
However, we are not given the value of Ka1. The value of Ka1 for carbonic acid is approximately 4.2 * 10^-7.
Plugging this value into the equation, we get:
[HCO3-] = 0.037 / (4.2 * 10^-7)
Simplifying this expression, we find:
[HCO3-] ≈ 8.81 * 10^-5
Since the bicarbonate-ion (HCO3-) concentration is equal to the carbonate-ion (CO3^2-) concentration, we can conclude that the carbonate-ion concentration is approximately 8.81 * 10^-5 M.
Therefore, the correct answer is (a) 1.2 * 10^-4.