pH Value

Date: 09/05/2001 at 02:24:28
From: Urvashi
Subject: pH value

Hi!

I would like to know the pH value of 0.1 N Sulphuric acid.

Please help! Thanks.

Date: 09/05/2001 at 15:59:48
From: Doctor Achilles
Subject: Re: pH value

Hi Urvashi,

Thanks for writing to Dr. Math.

Normally pH calculations can be very tedious.  pH is related to the 
concentration of protons in the following way:

  pH = -log[H+]

Where the log is base ten and [H+] is the concentration of H+ ions in 
moles/liter (or M).

With sulfuric acid, this gets especially complicated, because the 
formula is H2SO4, so there are two protons that can dissociate, and 
you have to find the sum total of protons that dissociate.

Fortunately, chemists have created Normality. Normality is similar to 
Molarity (moles/liter). There are two differences:

  1) Molarity can measure the anything, Normality can only be applied 
     to acids or bases.

  2) The Molarity of a solution of H2SO4 or of anything else, is just 
     the concentration of starting material in moles/liter.
     However, Normality is the concentration of H+ ions (for acids) or     
     of OH- ions (for bases) that you get out of the solution.

In other words, Normality normalizes for the fact that some acids and 
bases dissociate more than others, so the pH of a 1N solution of any 
acid is the same as a 1N solution of any other acid. A 1N solution of 
acid, by definition, has 1 mole/liter of H+ ions, and a 1N solution of 
base, by definition, has 1 mole/liter of OH- ions. In short, Normality 
measures the concentration of H+ ions or the concentration of OH- ions 
directly.

From here, you should be able to figure out the pH of a 0.1N solution 
of acid. Hope this helps. If you have other questions, or you'd like 
to talk about this some more, please let me know.

- Doctor Achilles, The Math Forum
  http://mathforum.org/dr.math/