Acid Base : pH and Kw

pH Scale

H+ concentrations are typically very small for many solutions, and hence it is often difficult to easily know if a substance is an acid or a base.

So a scale, called a pH scale was invented to make it easy to tell!
pH is just a number that tells us whether a solution is an acid or a base.

(This is similar to giving a movie a review out of 10.)

The pH scale is actually a range from 0 to 14.
pH has no units.
Yeah, 14! Later, we will learn why.

Study the pH diagram.



This range is valid at 250C. Essentially, any solution with a pH from 0 to just under 7 is an acid solution.
A solution with a pH of 7 is neutral, which means it is neither acid nor base.
Any solution with a pH from just above 7 to 14, is a base solution.

pH Calculations

Notice that pH is about the H+ ions, and NOT the acid from where they come.
It is NOT pACID.

Example 1

Assume that a solution of H2SO4 acid has a concentration of 0,002mol.dm-3.
Calculate it's pH at 250C.


Solution 1
You would need to first determine the concentration of the actual H+ ions. H2SO4 is a strong acid, and is also DIPROTIC.
(for every ONE mole of acid that reacts, TWO mols of H+ are produced)



The concentration of the H+ is 0,004M.

Now to calculate the actual pH you use this equation:
Seems scary the first time you use this equation. But it is just a button on your calcuator called log. I am using the simplified form here.





Example 2

The H+ concentration in a solution is 3 x 10-9 M at 250C. Determine whether the solution is acidic or basic.

Solution 2


Since the pH > 7, the solution is basic according to the pH scale.


So how will you calculate the pH if you had a base? Bases produce OH- ions, not H+ ions.

Example 3

Calculate the pH of a solution of NaOH of concentration 0,02M.

Solution 3


Now you cannot use the OH- in the pH. So you have to convert it to a corresponding H+ ion concentration.
Use this formula to change OH- to H+


Scary? Nah!

Substitute the OH- value of 0,02M in this equation, and solve for the H+ ion concentration.



Now use the pH formula.





By the way, this formula :

is called the Kw and you will learn more about it later.
It is always equal to 1 x 10-14 at 250 C.

pH Calculations Backwards

Example 4

You have a solution of H2SO4 that has a pH of 1,2. Calculate the concentration of the acid.

Solution 4

Example 5

A solution of Ba(OH)2 has a pH of 11,4. Calculate the concentration of this barium hydroxide solution.

Solution 5


Please note that in scientific notation, this is also :
[OH-] = 2,5 x 10-3 M
Copyright ©
V. Gokal
academic@afrihost.co.za