Determine the relative formula mass and the molecular formula of succinic acid

(A) Preparation of a standard solution of succinic acid:

• Take a clean dry weighing bottle and weigh the bottle.
• Measure 1.65g of the solid succinic acid taken in the weighing bottle using a chemical balance (upto 2 decimal points).

• Dissolve the solid in a beaker completely stirring with a glass rod with approximately 100 cm3 of distilled water.
• Transfer the beaker contents into a volumetric flask of 250cm3.
• Wash (*3) beaker and funnel to ensure all the dissolved solid reaches the flask.
• Add distilled water to the volumetric flask directly until close to the mark.
• Use a neat and dry pipette to add the last few cm3 of water-allowing a good control of water.
• Seal the volumetric flask and shake it to ensure a homogeneous solution.
• Label the beaker with correct concentration in moles.dm -3

2. Titration:

Apparatus: Pipette, Burette, stand, Phenolphthalein indicator, conical flask, white tile, funnel and pipette filler.

•  Wear safety goggles, gloves and lab coat at all times during the titration.
• Rinse the pipette with standard NaOH solution provided.
• Using a pipette fitted with filler, measure a volume of 25cm3 of NaOH solution and transfer it into the conical flask.
• Rinse the inside edges of the conical flask with distilled water to wash all the NaOH into the bottom of the flask.
• Add three drops of phenolphthalein indicator to NaOH. The solution turns pink.
• Rinse a clean dry burette with acid solution
•  The burette is placed on the stand, making sure it is vertical and then fill with the acid solution using a funnel up to about 2cm3 above the 0.00cm3 level.
• Run acid through the tap to remove air. Keep the upper level of the acid exactly at the 0.00cm3 level without parallax error.
• Open the tap of the burette slowly so that the acid drips into the conical flask
• Observe  the conical flask constantly while stirring and observe the colour change (changes from pink to colourless)
• Towards the end point add acid  slowly drop by drop until a  single drop makes the alkali  completely colourless
• Repeat the titration until concordant results ( within ± 0.05 cm3) are obtained
• Record the initial reading and final reading  for each  titration and  the titration volume  is derived

Results:

An example of burette readings is given below:

Discussion:

 The gram concentration of the acid solution =1.65/250cm3 =6.6g.dm-3

Assume that n=3,  

Mr of succinic acid will be 132.

So the mass of the acid solution required to neutralize the standard alkali solution will be .00125*132=0.165g. Hence, the titration volume of solution will be

                               250/1.65*.165=25cm3

Similarly, if n=1

 Mr of succinic acid will be 104.

So the mass of the acid solution required to neutralize the standard alkali solution will be .00125*104=0.13g. Hence, the titration volume of solution will be

                               250/1.65 *.13=19.70cm3

Similarly, if n=2

 Mr of succinic acid will be 118.

So the mass of the acid solution required to neutralize the standard alkali solution will be .00125*118=0.148g. Hence, the titration volume of solution will be

                               250/1.65 *0.148=22.42 cm3

Similarly, if n=4

 Mr of succinic acid will be 146.

So the mass of the acid solution required to neutralize the standard alkali solution will be .00125*146=0.183g. Hence, the titration volume of solution will be

                               250/1.65 *0.183=27.72cm3

From the foregoing it can be inferred that if the titration volumes are 25cm3, 19.3cm3, 22.42 cm3 or 27.72 cm3 then the relative molecular mass will be 132, 104,118 or 146 respectively.

Conclusion:

In the above example of burette readings, we got the titre volume as 22.42cm3. This will correspond to the molecular mass of the acid to be 118 as per discussion above. Hence we can derive that the value of n is 2.And hence the molecular formula of the acid is derived as HOOC-(CH2)2 – COOH

Accuracy and limitation of the equipment:

1. The accuracy of the burette is only 1/20th of a cm3, i.e. 0.05 cm3
2.  There may be zero error for the weighing balance.
3. The perfect verticality of the burette may not be assured
4. The first appearance of the colorless mixture may be missed during the titration.

The assessment of the exact colour of the mixture may be hindered by poor or artificial light.

Hazards and Safety

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