Experiment 3: The relative molecular mass of citric acid


To find the relative molecular mass of a sample of citric acid by back titration


Citric acid is a tribasic acid which would present problems if a direct titration method were to be employed. The principle of back titration is introduced to demonstrate the possibility of finding the Mr of citric acid by non direct means.


Measure accurately about 5 g of the acid.

Make up a solution containing approximately 40g NaOH (accurately measured to 2 decimal places) in 1 litre of water

Use a pipette to deliver 50cm3 of this standard sodium hydroxide solution into a 250ml conical flask containing the citric acid.

Dilute the resultant solution to 250cm3 in a volumetric flask

Titrate 25cm3 aliquots against the standard potassium hydrogen phthalate using phenolphthalein indicator.

Repeat the titration until concordant results are obtained.

Treatment of results

Calculate the average titre (use only the concordant results).

Calculate the moles of sodium hydroxide remaining in the volumetric flask.

Consequently calculate the number of moles of sodium hydroxide used up in the reaction.

If citric acid is assumed to have three replaceable hydrogens calculate the number of moles of citric acid in the 5 g sample

Work out the Mr of citric acid

Experimental report

The following factors must be considered: