• For the titration, it is necessary to dilute the Sodium Hydroxide so that it is an easier concentration to titrate. Using a 25cm3 pipette, washed out with some of the NaOH solution, transfer 25cm3 of the solution to a 250 cm3 volumetric flask, and fill to the line with distilled water. The alkali will now have a concentration of 0.1 mol dm-3.
• In the titration, the NaOH will be in the burette, and will be titrated into HCl in a conical flask:
NaOH (aq) + HCl (aq) NaCl (aq) + H2O (l)
• The reaction is 1:1 between NaOH and HCl, so a solution of 0.2 mol dm-3 should be used.
• Rinse a 25cm3 pipette out with some of this solution, and then transfer 25cm3 of it to a clean conical flask.
• Add 3 drops of phenolphthalein.
• Wash a burette with some of the diluted alkali and then fill the burette to the top of the graduations.
• Titrate the alkali into the acid until the indicator in the flask has turned a pink colour.
• Repeat the process until two titres are obtained within 0.1cm3 of each other, as well as a rough titre.
e.g. Average titre = 24.80 cm3.
Moles of acid used = 5x10-3
Acid neutralised by 24.80 cm3 NaOH
Therefore concentration of NaOH = (5x10-3 x 1000)/ 24.8 =0.2016129…
Diluted by factor of 10, so: 0.2016129…x10 = 2.01 mol dm-3.
Method 2- H of neutralisation
• Transfer 25cm3 of NaOH into a polystyrene cup with a lid, and put the cup into a plastic beaker.
• Take the temperature of the contents of the cup using a thermometer accurate to 0.1 cm3.
• Add excess HCl and replace the lid of the cup.
• Stir the contents constantly until the maximum temperature has been reached.
• Record the temperature change.
• Use the formula q= -(mc t) to work out the energy change of the quantities used.
• Work out the number of moles that reacted, then find the energy change that would have occurred had 1 mole been used.
• This gives H of neutralisation.
• Use this to work out the concentration of NaOH.