Hydrate Lab

Due to water’s polar structure, ions in some compounds attract and form bonds with water molecules, forming hydrates. A hydrate is a salt that has water molecules trapped within its crystals. Every hydrate has a certain number of water molecules weakly bonded to the salt as follows: salt • number of water molecules Anhydrous salts are salts that can form hydrates but which have had all the water driven off, usually by heat. By heating the Copper (II) sulphate hydrate until its color changes from blue to white, the compound can be decomposed into CuSO4, a white crystal, and H2O gas, represented as follows:

CuSO4 • xH2O(s) ←⎯→ CuSO4(s) + xH2O(g) Because of the mass conservation law and the fixed proportion of water molecules, the mole of H2O can be calculated by calculating the mass difference of the substance before and after the reaction:

Mole of H2O = (Mass of substance before the reaction) (Mass

In order to determine the value of X, the hydrate is heated on a burner to undergo decomposition reaction to be decomposed into CuSO4 and water vapor. Water vapor is evaporated during the reaction, leaving CuSO4 crystals, which is supposed to be white, in remain. By weighing the mass of CuSO4 and the mass difference of substance before and after the reaction, the mole of CuSO4 and H2O can be calculated. The value of X can thus be determined by calculating the mole ratio of CuSO4 and H2O.

In the lab, through calculation, the value of X is determined to equal to 5.361211229, which is close to 5. Therefore, the hydrate is probably CuSO4•5H2O. However, the percentage of error, 7.224%, is not small enough, and the crystals turn out to have a yellowish green color instead of to be white. This can be eliminated by decreasing the amount of hydrate, avoid touching the hydrate with other substances during the reaction, and increasing the intensity of the flame and the time of