Hi, I'm doing a lab with acid-base indicators and I'm utterly confused. The lab procedure goes like this:

Question

1) Make your own indicator.
2) set up 12 test tubes and pout a little pH solution into tube.
3) add 4 drops of indicator into each test tube.
4) Estimate the equilibrium constant for the dissociation of the acid form.
In my case, I made the common red cabbage indicator and concluded that the pH is around 5. The thing I'm not understanding is the equilibrium constant. Can someone explain or show me in steps? Thanks a bunch!!!