# Unit 4- Acid base calculations.

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• Created by: Jelly Tot
• Created on: 25-05-14 13:32

How to do the acid base calculations

and Buffered solution calculations.

Step by step guide on how to tackle the calculations...

Equations needed:

• pH = -log[H+]
• [H+] = 10^-pH
• In weak acids- [H+] = [OH-]
• Kc = [Products]/ [Reactants]
• n = cv

Method::

1) write out the equation for the reaction you have.

2) Write out an expression for Kc. (usually this value is given to you)

3) Rearrange your expression to find what you want. e.g [H+]= Kc x [HA}/ [A-]

4) Put your values into the equation. (Make sure they are concentrations and NOT moles!!!!!!!!!)

6) If you are trying to find a value for [H+] just use 10^-pH to find [H+]. You can do this when finding [OH-], but after finding [H+] you need to rearange your expression for Kc, to make it [OH-]= Kc x [HA] / [H+]

If you are using a weak acid (such as ethanoic acid) then the method is slightly different.

With weak acids, they only partially dissasociate, which means we can say that in a solution, [H+] = [OH-], so we can now re write our equation to say;

Kc= [H+]^2 / [HA], and we can find [H+] by saying;

[H+] = Kc x [HA] ; and then use this in pH= -log [H+] to find our pH.

Calcualtions for systems at equlibrillum.

When calculating concentrations or values for a system that has reached equlibrillum, we need to use a slightly