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.
If a solution has a pH of 5, it must be an acid.
If a solution has a pH of 7, it is neutral.
If a solution has a pH of 9, it must be a base.
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