The following questions refer to the following acids and bases.
A) Hydrazoic acid HN3 Ka = 1.9 x 10^-5
B) Hydrofluoric acid HF Ka = 6.8 x 10 ^-4
C) Nitrous Acid HNO2 Ka = 4.5 x 10^-4
D) Phenol C6H5OH Ka = 1.3 x 10^-10
E) Aniline C6H5NH2 Ka = 4.3 x 10^-10
1) which compound is best when preparing a buffer of pH = 9.5?
2) which .20M solution will have the lowest pH?
3) Which .10M solution will have the lowest percentage ionization?
4) rank the compounds in order of increasing acid strength, lowest to highest.
A) D<E<A<C<B
B) E<D<A<C<B
C) D< E< A< B< C
D) B< C< A< E< D
E) None of these
If you include explanations that be great!
Zach, this is about a 20 question post which I've done several times in the last day or so. All of these questions are similar. Can you summarize what it is you don't understand about these. We need to get at the root so you can do these yourself.
How exactly do you get pH from Ka?
You don't get pH from Ka UNLESS you have the concentration of the acid. In this case you don't have that so you can't get a straight pH. If you are referring to the first question, which is
The following questions refer to the following acids and bases.
A) Hydrazoic acid HN3 Ka = 1.9 x 10^-5
B) Hydrofluoric acid HF Ka = 6.8 x 10 ^-4
C) Nitrous Acid HNO2 Ka = 4.5 x 10^-4
D) Phenol C6H5OH Ka = 1.3 x 10^-10
E) Aniline C6H5NH2 Ka = 4.3 x 10^-10
1) which compound is best when preparing a buffer of pH = 9.5?
You answer that by remembering that the Henderson-Hasselbalch equation is
pH = pKa + log (base/acid)
When preparing a buffer you want the pH of the desired buffer to be close to pKa. That's because when (base)/(acid) = 1, then log 1 = 0 and pH = pKa.
You get pKa by pKa = -log Ka. So you find the pKa of each of the acids and pick the one closest to the pH of the buffer you want to prepare.
To answer these questions, we need to understand the concepts of pH, acidic strength, and percentage ionization.
1) To prepare a buffer solution of pH 9.5, we want to choose a compound that can act both as a weak acid and its conjugate base. A suitable buffer solution will be formed when the weak acid and its conjugate base are present in roughly equal concentrations.
Looking at the given compounds, we can check their pKa values (negative logarithm of the acid dissociation constant, Ka) to determine their strength as acids. The lower the pKa, the stronger the acid. The compound with the pKa closest to 9.5 will be the best choice for preparing a buffer of pH 9.5.
Calculating the pKa values for the given compounds:
A) pKa = -log(1.9 x 10^-5) = 4.72
B) pKa = -log(6.8 x 10^-4) = 3.17
C) pKa = -log(4.5 x 10^-4) = 3.35
D) pKa = -log(1.3 x 10^-10) = 9.89
E) pKa = -log(4.3 x 10^-10) = 9.37
From the calculations, we can see that compound E (Aniline) has the pKa closest to 9.5. Therefore, Aniline (C6H5NH2) is the best compound for preparing a buffer solution of pH 9.5.
2) To determine which compound will have the lowest pH for a 0.20 M solution, we can refer to the given Ka values. The higher the Ka value, the stronger the acid and the lower the pH.
Calculating the pH values for each given compound at a concentration of 0.20 M:
A) pH = -log(1.9 x 10^-5 * 0.20) ≈ 4.68
B) pH = -log(6.8 x 10^-4 * 0.20) ≈ 2.57
C) pH = -log(4.5 x 10^-4 * 0.20) ≈ 2.65
D) pH = -log(1.3 x 10^-10 * 0.20) ≈ 9.81
E) pH = -log(4.3 x 10^-10 * 0.20) ≈ 9.29
From the calculations, we can see that compound E (Aniline) will have the lowest pH for a 0.20 M solution.
3) To determine which compound will have the lowest percentage ionization for a 0.10 M solution, we need to compare their Ka values. The lower the Ka value, the weaker the acid and the lower the percentage ionization.
Calculating the percentage ionization for each given compound at a concentration of 0.10 M:
A) Percentage Ionization = (Ka / [A]) * 100 = (1.9 x 10^-5 / 0.10) * 100 ≈ 0.019%
B) Percentage Ionization = (Ka / [A]) * 100 = (6.8 x 10^-4 / 0.10) * 100 = 0.68%
C) Percentage Ionization = (Ka / [A]) * 100 = (4.5 x 10^-4 / 0.10) * 100 = 0.45%
D) Percentage Ionization = (Ka / [A]) * 100 = (1.3 x 10^-10 / 0.10) * 100 ≈ 1.3 x 10^-7%
E) Percentage Ionization = (Ka / [A]) * 100 = (4.3 x 10^-10 / 0.10) * 100 ≈ 4.3 x 10^-7%
From the calculations, we can see that compound A (Hydrazoic acid) will have the lowest percentage ionization for a 0.10 M solution.
4) To rank the compounds in order of increasing acid strength, lowest to highest, we can use their pKa values. The lower the pKa value, the higher the acid strength.
Ranking the compounds from lowest to highest acid strength:
A) D < E < A < C < B