titrations - pre internal assessment learning check

The assessment involves carrying out an acid base titration to work out the concentration of the unknown acid from the concentration of the known base, as well as some theoretical calculations involving empirical and molecular formulae and percentage composition.

KEy points / terms

Every titration needs to start with a balanced chemical equation!! This is so we know the mole ratio between the acid and the base.

What you accurately measure into the conical flask will depend on the titration, but for now we will assume it is the acid and we want to work out the concentration of the acid.

Use the styrette to accurately measure the acid's volume into the conical flask. Add indicator. We are going to work out the concentration of this.

The base is going into the burette, which should be rinsed first with distilled water, and then preferably with a little of the base.

Rememer you do not have to start from 0.00 mL - make sure you record the start and final volumes in your table.

The endpoint is when the colour changes. The colour should be faint and remember it will turn back to the intial colour in a short space of time due to CO2 in the air, which when dissolves in water is acidic (Carbonic acid). For our purposes we will say this is the equivalence point where the exact amount of acid and base have reacted.

After completion of the titration to work out out the concetration of the acid we need to use the balanced chemical equation and the the relationship between concentration (C), volume (V) and moles (n). Do not forget to use 3 significant figures and the correct units.

Set out your workings as below

Then carry out the titration and fill in the figures as you proceed, which should be in this order:

The volume of the base used to neutralise the acid. This is yout titre volume (an average of your titrations)

The moles of the base. You can now do this as you had the concentration and now have the volume. n = C x V

The moles of acid. This is worked out form the balanced chemical equation and the mole ratio, which will either be 1:1 or 2:1, ie if it is 1:1 the moles of acid = the moles of base; if it is 2:1 the moles of acid is double the moles of base

Now you can work out the concentration of the acid as you now have both moles and volume. C = n / V

Below is a worked through example. I have used sodium carbonate as the base with the known concentration, and I have then worked out the concentration of the hydrochloric acid.