pH of Weak Acid and Bases

Jedrick Zablan 3L
Posts: 68
Joined: Sat Aug 17, 2019 12:17 am

pH of Weak Acid and Bases

Hello! How do we calculate the pH when the acid or base is weak?

alex_4l
Posts: 82
Joined: Thu Sep 26, 2019 12:18 am

Re: pH of Weak Acid and Bases

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)

Posts: 94
Joined: Thu Jul 25, 2019 12:15 am

Re: pH of Weak Acid and Bases

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]

Posts: 54
Joined: Sat Jul 20, 2019 12:17 am

Re: pH of Weak Acid and Bases

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.

Lauren Stack 1C
Posts: 80
Joined: Sat Aug 17, 2019 12:18 am

Re: pH of Weak Acid and Bases

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]

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