Answer to Question #98582 in General Chemistry for Sara Noori

Question

In a question where the equilibrium constant K is given, how do you know if the K is a Kc or Kp?
For example:
Consider the reaction:

CH3COOH(aq)+H2O(l)⥫⥬=H3O+(aq)+CH3COO−(aq)K=1.8×10−5at25°C
If a solution initially contains 0.210 mol L−1 CH3COOH, what is the equilibrium concentration of H3O+ at 25 °C?

In this question, it gives you the K value, but is this K value the Kc or Kp? How would you determine this?

Answer 1

CH3COOH+H2O=CH3COO- + H3O+.

K=1.8x10-5

[H+]=[H3O+]

[H3O+]=✓[K x CH3COOH]=1.8x10-5 x 0.21 = 0.00194 mol/L.

This K is Kc, because these reactants is liquid.