Answer to Question #160346 in Organic Chemistry for Stephanie Clark

Question

1)   Suppose the impurity in my sample of impure benzoic acid is salt, and in a 9.8 g sample there is actually 1 gram of salt. If I were to use 160 mLs of water in my recrystallization would it be likely for my recrystallized sample to contain any salt. The solubility of sodium chloride in water is .360 g/mL at 0 0 C and .380 g/mL at 100 0C. Show all your calculations. Remember to consider significant figures in your final answers. Label all units

Answer 1

Recrystallisation: Is a technique in which purification of chemicals is done by dissolving both desired compound and impurities in an appropriate solvent and either the desired compound or impurities can be removed leaving the other behind either by heating and then cooling the solution.

Impure benzoic aicd can also be purified using recrystallisation technique from water wherein benzoic acid is more soluble in hot water and less soluble in cold water. The usual yields in this process is approximately 65%.

Given,

Total wt of impure benzoic aicd = 9.8 g

Wt of salt impurity in the sample = 1 g

Solubility of salt (NaCl) in water 0.360g/mL at 0 ^oC and 0.380g/mL at 100 ^oC

Solubility of benzoic acid in water 1.7g / L or 0.280 g/mL at 0 ^oC and 56.31g/L or 9.29 g/mL at 100 ^oC

If solvent (water) quantity used for the recrystallisation process = 160 mL

that is salt is 0.360g/mL / 0.280 g/mL = 1.285 times more soluble at 0 ^oC than the desired compound so it is possible to avoid some slat but not completely.

Then will the recrystallised product contain the salt impurity? the answer is YES

Because the solubility of salt is very high even at 0 ^oC that is 0.36g/mL and the volume of taken for recrstallisation is considerably high (165 mL) which can completely dissolve both. As per my understanding removing salt completely may not be possible with this method.