Titration Experiment

Seite 1

Method

Fill a burette with 50cm^3 of acid.

Add 50cm^3 of alkali solution containing a few drops of an indicator (methyl orange or phenolphthalein) to a conical flask.

Use the burette to add the acid to the alkali, do this slow and give the flask a regular swirl.

Add this at a drop every time when you see the alkali begin to neutralise.

The indicator changes colour when the alkali has been neutralised - phenolphthalein is pink in alkalis but colourless in acids, and methyl orange is yellow in alkalis but red in acids.

Record the amount of acid needed to neutralise the alkali solution, then repeat this twice over to give a more reliable answer.

Take the mean of these results.

Seite 2

Titration Calculations

Example 1: If you are asked to calculate the concentration in moles per dm^3.

You start with 25cm^3 of sodium hydroxide in your flask, with a concentration of 0.1 moles per dm^3. You then find from your titration experiment that it takes 30cm^3 of sulphuric acid with an unknown concentration to neutralise the sodium hydroxide. You must work out the concentration of the acid in moles per dm^3.

1) Work out the number of moles in the given substance using the formula: no. moles = conc x vol

Hence, 0.1mole/dm^3 x (25 x 1000)dm^3 because 1000cm^3 = 1dm^3, therefore there are 0.0025 moles of NaOH.

2) Write down the balanced equation of the reaction, and work out how many moles of the unknown stuff you must have had.

Hence, 2NaOH + H2SO4 = Na2SO4 + 2H2O - from this equation you can see that for every two moles of sodium hydroxide, there was just one mole of sulphuric acid. Therefore if you had 0.0025 moles of sodium hydroxide, you must have had 0.0025 / 2 = 0.00125 moles of sulphuric acid. Usually, the equation will form the ration 1:1 of reactants to product.

3) Work out the concentration of the unknown stuff using the formula: conc = no. moles / vol

Hence, 0.00125 mol / (30 / 1000) dm^3 = 0.041666... mol/dm^3

Seite 3

Titration Calculations

Example 2: If they ask for the concentration in grams per dm^3.

They might ask you to find the concentration of the acid in grams per cubic decimetre (g/l), if they do, you just need another formula triangle. The basis of which is: mass (g) = no. moles x relative formula mass

1) Work out the relative formula mass for the acid (you should be give the relative atomic masses, eg. H=1, S= 32 and O=16.

Hence, H2SO4 = (1 x 2) + 32 + (16 x 4) = 98

2) Convert the concentration in moles into concentration in grams.

Hence, in 1dm^3: mass = no. moles x relative formula mass = 0.041666... x 98 = 4.08333...g therefore, the concentration in g/dm^3 = 4.08