O - pH calculations Flashcards
What is a neutral solution?
One in which [H+] = [OH-]
If [H+] is greater than [OH-] the solution is…
…acidic.
If [OH-] is greater than [H+] the solution is…
…alkaline.
What is the pH scale a measure of?
The hydrogen ion concentration.
For strong monoprotic acids, what is the relationship between [H+] and [acid]?
[H+] = [acid]
What does it mean if an acid is monoprotic?
Each mole of acid produces 1 mole of hydrogen ions. This means the H+ concentration is the same as the acid concentration.
What method can you use to find the concentration of an acid or base?
Titration.
How can you use titration to find the concentration of an acid or base?
1) Add a standard solution of base to a measured quantity of acid (or vice versa).
2) Pipettes and burettes are used so that you know precisely how much acid and base is used.
3) An indicator is often added to the acid or base to show you exactly when it’s been neutralised.
4) When you know how much base it takes to neutralise the acid, or vice versa, you can work out the concentration of the acid.
5) You can also use a standard solution of an acid to work out the concentration of a base, using titration.
Does water behave as an acid or a base?
Can behave as either.
How can water act as a base?
By accepting a proton.
How can water act as an acid?
By donating a proton.
How much does water dissociate?
Only a tiny amount.
In the water equilibrium, where does the equilibrium lie?
Well over to the left as water only dissociates a tiny amount.
What is the ionic product of water?
The Kc expression for water.
What is the symbol for the ionic product of water?
Kw
Write the equation/expression for Kw
Kw = [H+] [OH-]
What are the units for Kw?
mol^2 dm^-6
What is important to remember about Kw?
It always has the same value for an aqueous solution at a given temperature and changes as temperature changes.
What is the value for Kw at 298K (25^oC)?
1.00x10^-14 mol^2 dm^-6
What can you say about Kw of pure water?
In pure water, there is always one H+ ion for each OH- ion. So [H+] = [OH-]. That means if you are dealing with pure water, then you can say that Kw = [H+]^2.
What do you use Kw to find?
The pH of a strong base.
What is Ka?
The acid dissociation constant.
What do you have to use to find the pH of a weak acid?
Ka
Apart from assuming concentrations, what else can you assume when finding the pH of a weak acid?
That dissociation of the acid is much greater than dissociation of water. This means you can assume that all the H+ ions in solution come from the acid, so;
[H+(aq)] ≈[A-(aq)].
What are the units of Ka?
mol dm^-3
What is pKa calculated in the same way as?
Calculated from Ka in exactly the same way as pH is calculated from [H+] - and vice versa.
The smaller the pKa…
…the stronger the acid.
The bigger the pKa…
…the weaker the acid.
