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March 29, 2017

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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?

  • Chemistry - ,

    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.

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