Question

There are many opportunities during are crystallization to lose crystals. How could you improve your percent recovery? Explain.

Answer 1

In an recrystalization experiment, the percentage recovery is often less than 100 %. This is due to following reasons.

(1) The substance to be recrystallized has some solubility even at low temperature. Hence, the mother liquor contains some quantity of it which decreases the percentage yield.

(2) Some substance remains stick to the walls of the flask and the filter paper and cannot be recovered. This reduces the percentage recovery.

Some suggestions are:

1. Limit the number of transfers. Every time you transfer a component to another container, you lose some of the compound.

2. Add more of one reactant. If your reaction is an equilibrium reaction, use a large excess of the cheapest reactant.

3. Remove a product as fast as it forms. Removal of product forces the position of equilibrium to the right and increases the yield.

4. If your reactants are gases and you have fewer moles of gaseous products, increase the pressure.

5. Give the reaction time. If you stop a slow reaction before it has had time to complete, you will have a low yield.

6. Add a catalyst. It won't change the yield, but the reaction will be faster. You will be less likely to stop the reaction before it has finished.