the partition coef of ethanoic acid between water and 2-methylpropan-1-ol

1. 20cm3 of aqueous ethanoic acid and 20cm3 of 2-methylpropan-1-ol

b). 25cm3 of aqueous ethanoic acid and 15cm3 of 2-methylpropan-1-ol

6. For each experiment, the ratio of the concentration of ethanoic acid in the aqueous layer to that in the 2-methylpropan-1-ol layer was calculated.

Result

Volume of 2-methylpropan-1-ol: 15 cm3 

Conclusion

The partition coefficient of ethanoic acid between water and 2-methylpropan-1-ol is :

K=

=1.154

Discussion

1. The ratio are respectively: a) 1.087  b) 1.200 c) 1.176

It is obvious that Distribution coefficient (Kd) is independent of masses of the 2 solvents used in the 3 trails of different volumes of solvents used but only temperature dependent. Hence, the distribution law does not hold when temperature is changed since the equilibrium constant is changed.

2. When temperature increases, the solubility of the two solvents increase. But the rate of the increase in solubility are not the same, it is expected that the partition coefficient varies with temperature.
3. The aim of titration is to find the concentration of the solvent, but not the total number of mole in the solvent. Therefore, the volumes of the aqueous and alcohol solution used in the titration must be known as accurately as possible in order to find accurately concentration. The aim of adding aqueous ethanoic acid and 2-methypropan-1-ol is only to leave the mixture to equilibrium and provide enough solvent for the titration. Therefore the amounts of aqueous ethanoic acid and 2-methypropan-1-ol need not be measured out accurately.

4. The following assumptions were made:

1. The temperature of the mixture remained constant throughout the experiment. This assumption was valid as it was felt (by hand) that the temperature of the separating funnel did not changed throughout the experiment.
2. Ethanoic acid, water and 2-methylpropan-1-ol are non-volatile and do not evaporate slowly. This assumption is not valid because there is a smell of alcohol over the separating funnel. That means that there are particles coming out from the mixture in the separating funnel.

c)  There is no dissociation and association in the ethanoic acid that means no dimerization in ethanoic acid. And that’s why the acid used is weak and its dissociation in water is negligible, such that
[acid]o = [acid]eqm and [salt]o = [salt]eqm 
The major error of the experiment is that the above assumption is not completely held.  In other words, the divergence of the results from accepted values is due to the dissociation of the acids in water.

d) The solution must not be concentrated one; otherwise, the viscosity between two layers makes the two solution become non-ideal. Any non-ideal solution cannot be used in the determination of the Partition Coefficient .

5. The distribution law does not seem to hold in partially miscible solvents (e.g. hexan-1-ol & water). But the basic strength of NaOH is not strong enough to neutralize 2-methylpropan-1-ol and only reacts with ethanoic acid in the experiment. Actually, Pure 2-methylpropan-1-ol (or methylpropan-1-ol) can act as an acid only when it reacts with a very reactive metal (e.g. Na). However, the reaction rate is slow.
6. Two successive extractions by using 25cm3 make a better interaction between two kinds of solutions; than one extraction during shaking. A better equilibrium is thus set up, resulting a higher extraction efficient.
7. In practice, we add little solvent to extract several times in order to obtain a higher yield. Partition can be also applied in chromatography to separate a mixture of solutes.

In laboratory, we often use partition applied to solvent extraction to purify a substance instead of by fractional distillation, since the boiling point for both substance and impurities may be so high that the substance may decompose at this temperature.