# How do you calculate pH from acid dissociation constant?

##### 1 Answer

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 #x# in the denominator because it is a small number compared with 0.2. Solving this equation gives** * See note below about what to do if you do not want to ignore this...

pH =

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.