Calculations with pH
The most imporatnt reason to use p-values is that we often apply concentrations of very diluted solutions.
It is much easier to say:
pH = 6 than [H3O+] = 10-6 mol/l.
Please don't ever forget tjat a high p-value always automatically means a very small corresponding real value.
pOH = 9 (a rather high value) means a low concentration of OH--ions:
pOH = 9 → [OH-] = 10-9 mol/l
Saying of writing something about concentrations, the unit mol/l (mol per liter) may not fail; using the p-value, there is no need for any unit.
But mind you: the p-value is based upon the well known unit mol/l
To remember: in water with a normal temperature (say 20 - 25ºC): pH + pOH = pKW = 14
So, as soon as you know pH, you also know pOH.
An aqueous solution is called NEUTRAL when pH = pOH, at whatever temperature. That is the most important criterium for a neutral solution.
It is similar to say: the concentrations of H3O+ and OH- are equal.
Adding acid to a solution means that the pH-value becomes lower and the pOH-value increases.
Adding a base means a higher pH and a lower pOH.
The values of pH can also be broken number, like 3.4 and 10.7 e.d.
This can difficult the mathematical calculations.
For example: if the pH=3.5, then the concentration [H3O+] equals 10-3,5mol/l.
But often we do not accept broken exponents.
You have to see and understand immediately in this example that the concentration must be between the values 10-3 and 10-4mol/l (because the pH is between 3 en 4).
A calculator of course immediately gives you the solution, but even without the machine, you must be able to execute the calculation:
pH = 3.5 = 4 – 0.5 → -log[H3O+] = 4 - 0.5 → [H3O+] = 3 x 10-4 mol/l.
Calculations with KA and KBand with pKA with pKB
Calculate the pH of the following solutions:
- 0,1M HAc
- 0,1M NH3
- 0,1M HCl
0.1M HAc means: 0.1 mol acetic acid (CH3COOH) was dissolved in water with an final volume of 1 liter.
A part of the acid molecules dissociate in H+ and Ac-.
The amount of H+ (in water H3O+) determines the value of the pH (=-log[H3O+]).
We must know this amount of H+, just like the strength/weakness of the acid, or: the KA
We consult the table to see that KA = 10-4 or pKA = 4
We know that HAc is a weak acid. The value of x will be small compared to the [HAc]
or: x can be neglected.
So: 0.03 mol HAc dissociated into ions → [H3O+] = 0.03 = 3 x 10-2 mol/l → pH = 2-log3 = 1.5
The weaker an acid, the smaller KA
The stronger an acid, the bigger KA
you can check this statement in table I.
Strong acids and bases in the table do not have a K-value.