Can You Discuss The Meaning Of Ph And [H+]?


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Naja Ellis Profile
Naja Ellis answered
I will discuss the definition of pH and the relationship with the hydrogen ion[H+] supplied by an acid.

By the way: [H+] = moles of [H+]/ 1liter.

Then I will give you one easy and one hard example why the pH equation cannot be your only way of determining whether a substance is an acid or base:

pH equation: PH = - log [H+]  where pH has a logarithmic relationship with the Molar concentration of [H+]  whether acidic or basic.

The pH scale ranges from 1 to 14, 1-7 an acidic solution, 7 - 14 a basic solution; by definition.

There is also the self-dissociation of water    H20 <> H+ + OH-   the KH2O = 1 x -14 = [H+][OH-], so in water alone we have a concentration of 1 x 10-7M [H+] and by definition is neutral because pH =-log[H+] and it is 7 right in the middle of the scale, so theoretically water has a pH of 7 and is neutral.

Ex.1 If you have a 1 x 10-3M HCl soln. Then we have pH = -log[1 x 10-3] = 3 equation says acidic, 3 is in the acidic range of the pH scale. It makes sense.

Ex. 2 What is the pH of a 1 x 10 -8 M HCl solution?
When you plug the value in the equation you get pH = -log[1 x 10-8] = 8 but wait a pH of 8? Isn't it an acid solution? Yes because we added acid to it....but pH on the pH scale says this solution is basic! Do you believe the equation or the pH scale? Are we missing something when dealing with such low amount of acid[H+]? Yes we are neglecting the Self-dissociation of water; what about its contrabution to the concentration of [H+]?

Concentration of HCL solution is 1 x 10-8M so it contribution is 1 x 10 -8
Cocentration contributed from H2O is [H+] = 1 x 10-7
lets add them together since they will coexist: [H+] becomes 1.1x 10-7 so
pH = -[H+] = -log[1.1 x -7] = 6.96 just barely acidic but according to the equation, scale, nd nature of solution; they all support an acidic solution.
This of course is an exception to very low conc. Of [H=].
thanked the writer.
Naja Ellis
Naja Ellis commented
The last entry in the end the is the notation [H=] should be corrected to read [H+]. Sorry.
Anonymous commented
Great answer! I had a doubt all the time!

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