The pH of an 0.0870 M solution of weak acid A is 2.20. The pH of an 0.0830 M solution of weak acid B is 4.07. The pH of an 0.0890 M solution of weak acid C is 5.13. Identify the three acids from among those listed below (the Ka values are listed in Appendix E of your textbook).
A H3BO3
B CH3CO2H
C HClO
D HCO2H
E H2S
F C6H5CO2H
G HNO2
H HCN
I HF
J None of the above
I'm not exactly sure what to do with the molarities... Any help please?
We will call weak acid A as HA.
pH = 2.20; therefore, H^+ = 6.31E-3
...........HA ==-> H^+ + A^-
I........0.0870.....0.....0
C...........-x.......x.....x
E......0.0870-x.....x......x
You know x = 6.31E-3; therefore,
Ka = (H^+)(A^-)/(HA)
Ka = (6.31E-3)(6.31E-3)/(0.08069) = ?
Now look up the Ka values in Appendix E of your text and find the acid with the value you have above. That will identify the acid. Textbooks differ in what they print for the Ka/Kb values for weak acids and weak bases but I suspect acid A is HNO2.
To identify the three weak acids from the given data, we need to compare the pH values of the solutions with the Ka (acid dissociation constant) values of the potential weak acids listed in Appendix E of the textbook.
The pH of a solution is related to the concentration of hydrogen ions (H+). In general, a lower pH value corresponds to a higher concentration of H+ ions, indicating a stronger acid.
Here's how you can solve the problem step by step:
1. Calculate the pKa values for each weak acid.
- pKa = -log10(Ka), where Ka is the acid dissociation constant.
2. Compare the pKa values with the given pH values for each solution.
- If the pH is closer to the pKa value, it suggests that the weak acid is responsible for the solution's acidity.
Let's calculate the pKa values for each weak acid:
A: H3BO3 (boric acid)
- No given pH value.
B: CH3CO2H (acetic acid)
- pKa = -log10(Ka) = -log10(1.8 * 10^-5) = 4.74
C: HClO (hypochlorous acid)
- No given pH value.
D: HCO2H (formic acid)
- No given pH value.
E: H2S (hydrosulfuric acid)
- No given pH value.
F: C6H5CO2H (benzoic acid)
- No given pH value.
G: HNO2 (nitrous acid)
- No given pH value.
H: HCN (hydrocyanic acid)
- No given pH value.
I: HF (hydrofluoric acid)
- No given pH value.
Now let's compare the pKa values with the given pH values:
pH = 2.20:
- The closest pKa value is for acetic acid (pKa = 4.74), but the difference is significant. So, it is not acid B.
pH = 4.07:
- The closest pKa value is for acetic acid (pKa = 4.74), and the difference is relatively small. So, it is likely to be acid B.
pH = 5.13:
- None of the given weak acids have a pKa value close to this pH. So, it is not any of the weak acids provided.
From the analysis, the three weak acids that match the given pH values are:
- Acid B (CH3CO2H) for the pH value of 4.07.
- None of the provided weak acids for the pH values 2.20 and 5.13.
Therefore, the answer would be J (None of the above) for the pH values 2.20 and 5.13.