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 ï¿½ 0.1cm3 of sodium hydroxide were titrated into the flask containing 10.00cm3 ï¿½ 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.
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
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) ï¿½CClH2COONa (aq) + H2O (l)
From the equation we see that:
1 mole of ClCH2COOH ï¿½ 1 mole of NaOH
x ï¿½ 2.1 * 10-3 mole NaOH
Number of moles of ClCH2COOH reacted = x = 1 * 2.1 * 10-3 = 2.1 * 10-3 mol
2.1* 10-3 moles of ClCH2COOH ï¿½ 10.00 cm3 of ClCH2COOH
x ï¿½ 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
chloroacetic acid reacted = (mass of beaker + chloroacetic acid) – (mass of beaker)
= (47.80) -(45.80)
number of moles (n) = mass (m)
relative molecular mass(Mr)
n = m (multiply by Mr)
(Mr) (n) = m (divide by n)
Mr = m = 2.
00 = 95 gmol-1
n 2.1 * 10-2
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 = theoretical value-experimental value *100%
= 94.50 – 95 *100% = 0.5%
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.
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