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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: adi on September 18, 2022, 06:04:00 AM

Title: Why does the formula -log(M) work?
Post by: adi on September 18, 2022, 06:04:00 AM

Hi,

I am a little confused as to why the formula pH = -log(Molarity) work. Wouldn't 1M of a strong acid have a lower pH than 1M of a weak acid? Despite this, wouldn't using the formula (-log(Molarity)) give the same value for the pH?

Thanks in advance  ;D

Title: Re: Why does the formula -log(M) work?
Post by: Hunter2 on September 18, 2022, 10:27:52 AM

What is the difference of a week and a strong acid.
Strong acid  : dissociation HA = H + A^-  is 100% 1 M HA = 1 M H+, pH = -logH+
Weak acid HA = H+ +A^- is less then 100% 1 M HA = maybe 0,8 H+ and 0,2 M HA not dissociated.
For this we have the Ka or pKa value. Ka = ( H+ + A^-)/HA, pKa = - log Ka, pH = pKa + log( A-/HA)

Title: Re: Why does the formula -log(M) work?
Post by: Corribus on September 18, 2022, 10:55:48 AM

It is the molarity concentration of the proton after dissociation that determines pH, not the the molarity of the undissociated acid. If you start with 1 M of a strong acid and 1 M of a weak acid, the strong acid will fully dissociate (by definition) and the weak acid will partially dissociate. So [H+] for the strong acid will be 1 M and [H+] < 1 M for the weak acid.