Page 1 of 1

### pH of Weak Acid and Bases

Posted: Sun Dec 01, 2019 7:15 pm
Hello! How do we calculate the pH when the acid or base is weak?

### Re: pH of Weak Acid and Bases

Posted: Sun Dec 01, 2019 7:19 pm
I think you first calculate the pH using -log(10) of either [H+] or [OH-] and then determine if the acid or base is weak or strong. Closer to 7 would be a weak acid (since 1 is the strongest acid) and closer to 7 would be a weak base (since 10 is the strongest base)

### Re: pH of Weak Acid and Bases

Posted: Sun Dec 01, 2019 7:22 pm
We can find the pH of a weak acid or base using its Ka or Kb value. Use the equilibrium equation to solve for the concentration of H+. Then, take the -log of that concentration.

The equilibrium expression would look like this:
Ka= [H+][A-] / [HA]

### Re: pH of Weak Acid and Bases

Posted: Sun Dec 01, 2019 7:40 pm
Using the Ka value of the substance, which can be solved for if the chemical equation is known, you can also find the concentration of H+ ions and the take the -log of that.

### Re: pH of Weak Acid and Bases

Posted: Sun Dec 01, 2019 7:42 pm
As you would with a strong acid/base, you must take the -log of the concentration of hydronium or hydroxide, but you must multiply the equilibrium constant (Ka) first, which is defined as
Ka= [A-][H+] / [AH]