Chemistry 11 Lab #13

 

Lab #13: Empirical Formula for Zinc Iodide

Theory
The chemical formula for a compound, in the case of a covalent compound, tells us how many of each type of atom there are in each unit (molecule) of the substance.  In an ionic compound the chemical formula simply tells us the ratio in which the elements join together.  The latter is more correctly referred to as the “empirical formula.”  In this lab you will be find the empirical formula for zinc iodide.  Dry cleaning fluid has the chemical formula C₂Cl₄, so its empirical formula would be CCl₂.  In the case water the chemical formula is the same as the empirical formula.    

Procedure

1.     Mass a clean, dry 50 mL Erlenmeyer flask and a clean dry 50 mL beaker. Record the masses on the data table below.

2.     Measure out approximately 0.20 g of granular zinc and note the mass.  Measure out the approximately 0.20 g of iodine crystals.  Add both the iodine and the zinc to the flask.

3.     Add approximately 15 mL of methanol to the flask and swirl until the iodine is dissolved. The brown color of the liquid is due to the dissolved alcohol. Cover the flask with aluminum foil and gently heat the mixture for about 15 minutes on a hot plate. (This heating should be done with the plate near an open window to avoid the release of methanol vapor into the room.) The reaction is complete when the brown color has disappeared.

4.     Transfer the liquid (taking care not to pour out the unreacted zinc solid) into the beaker.  Rinse the flask with an additional 10 ml of methanol and transfer this liquid into the beaker as well. Heat both the beaker and the flask on the hot plate until the methanol has been evaporated and the flask and beaker are dry.

5.     The flask should still contain the unreacted zinc. The beaker will now hold the zinc iodide compound which had dissolved into the alcohol. Allow the flask and beaker to cool and then weigh each.  Make sure not to wait to long to weigh the zinc iodide product as it will quickly react with the moisture in the air.  Record the masses.

Calculations

1.     Determine the mass of zinc reacted by subtracting the mass of zinc remaining from the initial mass of zinc.

2.     Calculate the mass of zinc iodide that was produced in the reaction.

3.     Determine the number of moles of zinc and iodine that were used in the formation of the product.

4.     Produce a whole number ratio between zinc and iodine.(mol I reacted/mol Zn reacted)Use this to create an empirical formula for the compound.

5.     As a confirmation of the Law of Conservation of Mass, calculate the percent difference between the mass of the reactants used and the mass of the products formed.

Data and Calculation Table

Mass of Erlenmeyer flask		Mass of zinc iodide
Mass of beaker		Moles of zinc reacted
Mass of zinc initial		Moles of iodine reacted
Mass of iodine		Ratio mol I/mol Zn
Mass of zinc + flask final		Empirical formula of product
Mass of product + beaker		Total mass of reactants
Mass of zinc final		Percent Difference
Mass of zinc reacted