Question

1. If the Partition Coefficient for Benzoic Acid in one mL each of Water and Methylene Chloride is 2.10, is there more Benzoic Acid in the Water layer or the Methylene Chloride layer?

2. A) If you have one mL each of Water and Methylene Chloride in a vial, which layer is on the top?

B) If the same experiment is repeated with benzoic acid between ether and water, which layer would be the top and which layer would be the bottom.

3. If you start with 0.050 g of Benzoic acid, and add it to one mL each of Water and Methylene Chloride and find 0.035 g of Benzoic acid in the Methylene Chloride layer, what is the Partition Coefficient?

4. What is the purpose of adding anhydrous Sodium sulphate to the Dichlormethane layer in the above experiment.

5. The expected partition coefficient in the above experiment is 1.6. If a student determined the partition coefficient as 2.5, What is the possible source of error?

Answer 1

We have partition coefficient defined as, (ratio of concentration of solute in organic solvent and aqueous medium)

$K_d=\frac{[A]_{org}}{[A]_{aq}}$

From, the question the Partition Coefficient value is 2.10,

$\begin{aligned} K_d&=\frac{[A]_{org}}{[A]_{aq}}=2.10\\ [A]_{org}&=2.10\times [A]_{aq} \end{aligned}$

Therefore ,Methylene Chloride has more benzoic acid than water.

The density of Methylene Chloride is 1.32gm/cm³ at 20⁰C while that of water is 0.9982gm/cm³ so since Methlene Chloride is more dense than that of water water layer will be on top.

ether(Di ethyl ether) density is 0.7134 g/cm³ ,therefore it is lighter than water so the ether layer will stay at top.

Let m₁ and m is the mass of benzoic acid in Methylene Chloride and total mass of benzoic acid respectively.

$K_d=\frac{m_1/V_1}{(m-m_1)/V_2}$

From the question we have m₁=0.035 g and m=0.050 g and V₁ =1 ml and V₂=1 ml ,therefore

$K_d=\frac{0.035}{(0.050-0.035)}=2.333$

anhydrous Sodium sulphate is used as drying agent ,a chemical that is used to remove water from the organic solvent.Since solubility of Dichlormethane in water is around 25.6g/L at 15⁰C we can expect appreciable amount of water concentration in Dichlormethane.So anhydrous Sodium sulphate is used to remove water from the Dichlormethane.

The presence of water in Dichlormethane enhances the solubility of benzoic acid in Dichlormethane because of the hydrogen bonding interaction between water and benzoic acid which in turn increases the concentration of benzoic acid in Dichlormethane leading to higher partition coefficient.