Hydrated zinc sulfate has the formula ZnSO4•yH2O where y is an integer. Hydrated zinc sulfate decomposes when heated, losing only its water of crystallisation and becoming anhydrous. You will determine the value of y in ZnSO4•yH2O by heating the hydrated salt until it becomes anhydrous. FA 4 is hydrated zinc sulfate, ZnSO4•yH2O.

1. (a)Method • Weigh the crucible with its lid. Record the mass in the space for Results. • Add between 3.20 g and 3.40 g of FA 4 to the crucible. • Weigh the crucible with its lid and FA 4. Record the mass. • Place the crucible on the pipeclay triangle. Gently heat the crucible and contents for approximately 2 minutes with the lid on. • Remove the lid. Heat the crucible and contents strongly for approximately 4 minutes. • Replace the lid and leave the crucible and residue to cool for at least 5 minutes. You may wish to begin work on Question 3 while the crucible is cooling. • Weigh the crucible with its lid and its contents. Record the mass. • Remove the lid. Heat the crucible strongly for approximately 3 minutes. • Replace the lid and leave the crucible and residue to cool for at least 5 minutes. • Weigh the crucible with its lid and its contents. Record the mass. • Calculate the mass of FA 4 used and the mass of residue obtained. Record the masses. Results[5]
2. (b)(i)Calculations Calculate the amount, in mol, of anhydrous zinc sulfate residue formed in the decomposition of FA 4. Calculate the amount, in mol, of water of crystallisation lost. amount of ZnSO4 = ................................... mol amount of H2O = ................................... mol [2][2]
3. (b)(ii)Calculate the value of y in the formula ZnSO4•yH2O. Show your working. y = ................................... [2][2]
4. (c)A student suggests using this thermal decomposition method to investigate the number of moles of water of crystallisation in hydrated ethanedioic acid. The teacher says that this method is unsuitable. Suggest why this method is unsuitable. [1][1]