How do you calculate pH from acid dissociation constant?
For a weak acid, set up the equilibrium expression for dissociation to ions in solution, then solve this equation for the hydronium ion concentration. The pH can be calculated directly from
Example: The pH of 0.2 M acetic acid (HOAc)
If the acid is weak, then only a small concentration,
where we have ignored
NOTE: If you don't simpify 0.2-x = 0.2, you need to use the quadratic equation to solve for the pH. Here is a video which discusses how to do this.