Question

Determine the [H3O+] of a 0.100 M solution of benzoic acid.

Answer 1

Benzoic acid= C6H5COOH
Because benzoic acid is a weak acid, we need to do a hydrolysis reaction and an ICE table in order to determine how much it dissociates into H3O+ ions

C₆H₅COOH(aq) + H₂O(l) <--> C₆H₅COO^-(aq) + H₃O⁺(aq)

Initial 0.100M / 0 0

Change -x / +x +x

Equil. 0.100M-x / x x

We don't really care about H₂O because we create an ICE table to find the concentrations at equilibrium to use them in an equilibrium constant expressions which only includes aqueous solutions and gaseous solutions (not solids or liquids).

So now let's determine what our equilibrium expression is. The equilibrium expression is the concentrations of products at equilibrium raised to the power of their coefficients divided by the concentrations of reactants at equilibrium raised to the powers of their coefficients:

K for benzoic acid is 6.5*10^-5 (you can look these values up in the appendix of you textbook). Therefore we can substitute the K value, along with the equilibrium concentrations into the above equation, getting:

Because K for benzoic acid is soooo small, we can let (0.100M-x) approximately equal just 0.100M because subtracting that small of a value won't affect the overall answer (you can use this approximation generally speaking when K is smaller than 10^-4)

Therefore we get:

But from our ICE table we can see that x represents the concentration of H₃O⁺ at equilibrium so we have

[H₃O⁺]= 0.00255M

Let me know if you have any questions!