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Home Q & A Board Science-Related Homework Help Chemistry (Moderator: Laser_3)
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fitzgeralds fitzgeralds
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11 years ago
C9H8O4(acetylsalicylic acid) is a weak monoprotic acid. To determine its acid dissociation constant a student dissolved 2.00g of C9H8O4 in .600L of water. What was the Ka value calculated if the pH of solution is 2.61??
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#1
11 years ago
For brevity call acetylsalalcylic acid HAs
Molar mass HAs=180.16g/mol
2gHAs in 600ml = 2/180.16*1000/600 = 0.0185M solution

[H+] = 10^-2.61
[H+] = 2.45*10^-3 = 0.00245

Being monprotic, HAs dissociates:
 HAs = H+  +   As-
and [H+] = [As-]

If [H+] = X,

Ka = [H+]*[As-] /[HAs]  because [H+] = [AS-] =X, we write
Ka = X²/[HAs] -X

[HAs] =  0.0185 - 0.00245
[HAs] = 0.01615

Ka = (0.00245)²/0.01615

Ka = 3.72*10^-4
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