Enough of a monoprotic acid is dissolved in water to produce a .0167 M solution.

Question

Enough of a monoprotic acid is dissolved in water to produce a .0167 M solution. The pH of the resulting solution is 6.03. calcualte the Ka for the acid.

OK! Im trying again to get help with this problem. I had another one like it and the way that everyone has been solving it worked for those numbers. But on this one the typical way isnt working and I dont understand how to apply what the bold red letters are saying. Please help. I have also tried 5.11x10^-11 and that was wrong as well.

Enough of a monoprotic acid is dissolved in water to produce a .0167 M solution. The pH of the resulting solution is 6.03. Calculate the Ka for the acid.

Explanation / Answer

From the given pH = 6.03, [H3O+]+ = 10-6.03 = 9.332*10-7 M

actual value of H+ because of the addition of [H+]acid + [H+]H2O = 9.332*10-7 M

             Thus - [H+]acid = (9.332*10-7 M - 10-7 ) = 8.332*10-7 M

Thus -

               Ka = [H3O+][A-] / [HA] = (8.332*10-7)(8.332*10-7) / 0.0167 = 4.15*10-11

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