Two drops of phenolphthalein indicator were added to the chloroacetic acid solution to enable us to determine when the solution is neutralized. Phenolphthalein is pink in basic and neutral solutions and colorless in acidic solutions.
When 20.8 cm3 � 0.1cm3 of sodium hydroxide were titrated into the flask containing 10.00cm3 � 0.06 cm3 chloracetic acid and two drops of phenolphthalein indicator, the color of the solution changed from colorless to pink, which indicates that the solution has been neutralized.
Data Processing and Presentation
Number of moles of sodium hydroxide reacted
Conversion factor
1 cm3 = 10-3 dm3
20.8 cm3 = 20.8 * 10-3 dm3 = 2.08 * 10-2 dm3
Number of moles (n) = concentration (c) * volume (v)
= (2.08 * 10-2) (0.10)
= 2.1 * 10-3 moles of sodium hydroxide reacted
Number of moles of chloroacetic acid reacted
Chloroacetic acid (ClCH2COOH) reacts with sodium hydroxide (NaOH) to produce sodium chloroacetate (CClH2COONa) and water (H2O) according to the following equation:
ClCH2COOH (aq) + NaOH (aq) �CClH2COONa (aq) + H2O (l)
From the equation we see that:
1 mole of ClCH2COOH � 1 mole of NaOH
x � 2.1 * 10-3 mole NaOH
Number of moles of ClCH2COOH reacted = x = 1 * 2.1 * 10-3 = 2.1 * 10-3 mol
Number of moles of chloracetic acid in 100 cm3 chloroacetic acid solution
2.1* 10-3 moles of ClCH2COOH � 10.00 cm3 of ClCH2COOH
x � 100.00 cm3 of ClCH2COOH
Number of moles of ClCH2COOH in 100.00 cm3 solution = x = 2.1*10-3 * 100.00 = 2.1 * 10-2 mol
10.00
Mass of Chloroacetic acid reacted
Mass of
chloroacetic acid reacted = (mass of beaker + chloroacetic acid) – (mass of beaker)
= (47.80) -(45.80)
= 2.00g
Experimental relative molecular mass of chloroacetic acid
number of moles (n) = mass (m)
relative molecular mass(Mr)
n = m (multiply by Mr)
Mr
(Mr) (n) = m (divide by n)
Mr = m = 2.
00 = 95 gmol-1
n 2.1 * 10-2
Theoretical relative molecular mass of chloroacetic acid
Theoretical relative molecular mass (Mr) of
chloroacetic acid (ClCH2COOH) = Mr Cl + 2(Mr C) +3(Mr H) +2(Mr O)
= (35.45) + 2(12.01) +3(1.01) +2(16.00)
= 94.50 gmol-1
Percentage Error
Percentage error = theoretical value-experimental value *100%
theoretical value
= 94.50 – 95 *100% = 0.5%
94.50
Conclusion and Evaluation
In this experiment, the experimental relative molecular mass of chloroacetic acid (ClCH2COOH) was found to be 95 gmol-1. The experiment was quite successful as the experimental molecular mass of chloroacetic acid is quite close to the theoretical molecular mass of chlorocetic acid (94.50 gmol-1). This is indicated by the small percentage error in the result (0.5%). However, there were sources of error in the experiment.
Sources of error
- Human parallax error in taking volume measurements: the meniscus might not have been viewed at right angles.
- Human parallax error in taking mass measurements: the scales of the triple beam balance might not have been viewed at right angles.
- The end point might not have been reached. This means that a greater volume and therefore a greater number of moles of sodium hydroxide were needed to completely neutralize the 2.00g of chloroacetic acid. Since the reaction between chloroacetic acid and sodium hydroxide is a one to one reaction, a greater number of moles of sodium hydroxide means a greater number of moles of chloroacetic acid. The relative molecular mass was found by dividing the mass of chlorocetic acid reacted over the number of moles of chloroacetic acid reacted. Thus if the number of moles of chloroacetic acid in the 2.00g was larger, the relative molecular mass would have been smaller and therefore closer to the theoretical value.
- The beaker might not have been cleaned properly. Thus impurities may have affected mass measurements by slightly increasing the mass of the chloroacetic acid.
- The uncertainty of the triple beam balance (�0.05g) might have affected mass measurements.
- Volume measuring equipment such as the burette and pipettes might not have been cleaned and dried properly. Impurities and traces of water might thus have slightly increased volume measurements.
Improvements
- Using a more accurate balance: e.g. a triple beam balance.
- Making sure that the equipment is properly cleaned before conducting the experiment in order to get rid of any impurities.
- Repeating the experiment once or twice and taking average results. This is likely to reduce the effects of random error.
- Making sure that the scale of the triple beam balance is viewed at right angles to minimize the possibility of parallax error occurring.
- Making sure that the meniscus in the pipettes and burette is viewed at right angles to minimize the possibility of parallax error occurring.
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