Question

If acetic acid is the only acid that vinegar contains (Ka=1.8×10−5), calculate the initial concentration of acetic acid in the vinegar.

Express your answer using two significant figures.

HC2H3O2 =

  M  

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Answer 1

A sample of vinegar has a pH of 2.9. Assuming that acetic acid is the only acid vinegar contains (Ka=1.8x10^-5) calculate the concentration of acetic acid

Equilibrium:

C2H3O2H <---> C2H3O2- + H+

Ka = [H+][C2H3O2-]/[C2H3O2H] = 1.8X10^-5

From pH = 2.9, [H+] = 10^-2.9 = 1.3X10^-3 M

Since [H+] = 1.3X10^-3 M, this must also equal [C2H3O2-] in the solution. So, plugging into the expression for Ka gives:

1.8X10^-5 = (1.3X10^-3)^2 / [C2H3O2H]

[C2H3O2H] = 0.088 M

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