To determine the water of crystallisation in washing soda crystals

Safety

1. Read burette and pipette reading at meniscus level
2. Towards the end, add the solution drop by drop and swirl to avoid errors
3. Take away the funnel from the top of the burette when the HCl solution is poured
4. All apparatus rinsed with deionised water and their respective solutions
5. Take care of the pipette fillers by not letting the solution overshoot into the fillers
6. Put a plain paper underneath the volumetric flask to easily note the colour change

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Apparatus

        Instruments:

• 250cm3 conical flask
• 250cm3 volumetric flask
• 50.0cm3 burette
• 10.0cm3 pipette
• mass balance
• funnel
• dropper
• beakers

        Chemicals:

• washing soda crystals
• methyl orange indicator
• 0.1M HCl
• 0.1M anhydrous sodium carbonate solution
• deionised water

Diagram

Procedure

        Anhydrous sodium carbonate solution:

1. Fill up the burette with 50.0cm3 of 0.1M HCl solution and have the tap closed
2. Pipette 10.0cm3 of 0.1M anhydrous sodium carbonate solution and pour it into the conical flask
3. Add 4~5 drops of methyl orange indicator
4. Open the tap of the burette and let the HCl out into the conical flask until there is a colour change in the sodium carbonate solution
5. Read off the burette the volume that was needed for the colour change
6. Repeat this procedure at least three times

        Washing soda crystals:

1. Weigh exactly 2.00g of washing soda (sodium carbonate) crystals
2. Pour them into a volumetric flask carefully
3. Add deionised water to the volumetric flask until the 250cm3 mark.
4. Shake the solution until the washing soda is dissolved
5. Burette 50.0cm3 of 0.1M HCl solution and have the tap closed
6. Pipette 10.0cm3 of the washing soda solution prepared and pour it into the conical flask
7. Add 4~5 drops of methyl orange indicator
8. Titrate this solution using the HCl in the burette
9. Read off the burette the volume that was needed for the colour change
10. Repeat this procedure at least three times

Data Presentation

Colour change of the solution: orange-yellow → pink-red

        Anhydrous sodium carbonate solution:

Therefore, average titre = =18.467cm3 ≈ 18.5cm3

        Washing soda crystals:

Therefore, average titre =  = 5.233cm3 ≈ 5.2cm3

Data Processing

The systematic and random error values in our readings must be calculated. The uncertainties of the readings were given on each of the measuring instruments that we used.

Total percentage uncertainty is calculated by:

The 0.1M hydrochloric acid given in the laboratory was not exactly 0.1M. So first, we have to find out the actual molarity of the HCl solution.

        Equation:

        Ratio of mole of HCl : Na2CO3 = 2 : 1

Therefore, molarity of HCl =

The actual molarity of HCl = 0.108 M

Volume of HCl = dm3

Let M = molarity of washing soda solution prepared

Ratio of mole of HCl : Na2CO3 = 2 : 1

        

Mass of washing soda crystals in 250cm3 = 2.00g

Therefore, mass of crystals in 1.00dm3 = 8.00g

Mass = moles × RMM

RMM of=

RMM of =106

Therefore RMM of xH2O =

RMM of H2O = 18

Error in measurement:

        Percentage error (%) = 3.52%

        Therefore, 3.52% of 9.84

                

        Therefore the water of crystallisation = 9.84 ± 0.35

        Total % error =

        Systematic error = (Total % error) – (percentage random error)

= |1.6–3.52| = 1.92%

Conclusion

Washing soda is sodium carbonate and it contains 10 water molecules. Its formula is Na2CO3·10H2O. However Na2CO3·10H2O loses some water to the atmosphere. So the water present per molecule of sodium carbonate is less than 10 molecules. Hence we can write sodium carbonate crystals as Na2CO3·xH2O.

Doing the experiment, the value x for the water of crystallisation of sodium carbonate crystals was found to be 9.84. However, as x has to be an integer value, as it is the number of molecules of water per molecule of sodium carbonate, we can round it up to 10.

The error in the experiment is only 1.6%, which is fairly low. This means that the experimental value was quite close to the literature value. The reason it was slightly low was mostly because of experimental error. To reduce systematic errors, more care should be taken when taking readings. Random errors occur due to the distribution of the measurements above or below average value. Random errors are mostly caused due to the uncertainties in equipment.

Further experiments can be carried out by finding out the water of crystallisation of more common substances such as copper sulphate crystals.

Evaluation

The systematic errors can be reduced by:

• using more accurate equipment, for example, a more accurate mass balance
• error could also be caused when the human eye does not record the colour change early enough to close the tap of the burette

Random errors can be reduced by:

• taking the readings a few times and taking an average

Bibliography

Wah L.E., 2001, Effective Guide to ‘O’ Level Chemistry, Singapore

Chemsoc, http://www.chemsoc.org/word/learnnet/StandProcGCSE_pens.doc [Accessed 17 May 2005]

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