Question

how can we calculate the equilibrium concentration of $\ce{h3o+}$ of a weak acid?
a) it is always the same as the weak acids initial concentration
b) it is half of the $\ce{p}k_{\ce{a}}$ value
c) use the ice chart
d) it is half of the weak acids initial concentration

Explanation:

Weak acids only partially ionize in solution, so their initial concentration does not directly equal the $\ce{[H3O+]}$ concentration, ruling out options a and d. The $\text{p}K_a$ value relates to acid strength but does not directly give half its value as $\ce{[H3O+]}$, ruling out option b. An ICE (Initial, Change, Equilibrium) chart is a standard method to track the partial ionization of a weak acid, set up the equilibrium expression with the acid dissociation constant $K_a$, and solve for the equilibrium $\ce{[H3O+]}$.

Answer:

c) Use the ICE chart