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An unknown monoprotic acid has a pH of 2.87. Is this acid weak or strong if 22.32 mL of 0.112 M NaOH is required to titrate 25.00 mL of the acid? If the acid is weak, calculate its Ka.

Answer is Ka = 1.8 x 10^-5, but I don't know how to get there. Any help would be appreciated. Thanks!

1 answer

the initial acid molarity is
... [HA] = .112 * (22.32 / 25.00) = 0.100

[H+] = 10^(-2.87) = .00135 ... this is also [A-]

Ka = [H+][A-] / [HA] = .00135^2 / (.100 - .00135)

remember ... the initial molarity changes because of the dissociation
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