HC9H7O4(aq) + H2O(l) ⇄ H3O+(aq) + C9H7O4-(aq) The molecular formula of acetylsalicylic acid, also kn — Acids and Bases Chemistry Question
Problem Context
HC9H7O4(aq) + H2O(l) ⇄ H3O+(aq) + C9H7O4-(aq)
The molecular formula of acetylsalicylic acid, also known as aspirin, is HC9H7O4. The dissociation of HC9H7O4(aq) is represented by the equation above. The pH of 0.0100 M HC9H7O4(aq) is measured to be 2.78.
Model Answer
Ka = [H3O+][C9H7O4-] / [HC9H7O4]
1 point is earned for the correct expression.
Model Answer
[H3O+] = 10^-pH = 10^-2.78 = 1.66×10^-3 M
[H3O+] = [C9H7O4-] = 1.66×10^-3 M
[HC9H7O4] = 0.0100 M − 1.66×10^-3 M
Ka = (1.66×10^-3)^2 / (0.0100 − 1.66×10^-3) = 3.3×10^-4
1 point is earned for the correct [H3O+]. 1 point is earned for the value of Ka.
Model Answer
pH = pKa + log([C9H7O4-]/[HC9H7O4]) = -log(3.3×10^-4) + 0 = 3.48
1 point is earned for a pH consistent with the Ka calculated in part (b).