Chemistry 151 Last updated Dec Lab 6: Percent Composition and Empirical formulas

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1 Chemistry 151 Last updated Dec Lab 6: Percent Composition and Empirical formulas Introduction In this lab, you will experimentally determine the percent composition of a copper chloride, which you will then use to determine its empirical formula. To determine the percent composition, you will start with a solution containing a known mass of copper chloride. The copper chloride will react with zinc through a single displacement reaction, producing copper metal as a precipitate, as illustrated in the equation below 2CuCl x (aq) + xzn(s) xzncl 2 (aq) + 2Cu(s) The percent copper of the compound can be determined by dividing the mass of copper produced from the above reaction by the initial mass of the compound %Cu = mass of copper mass of copper chloride Since there are only two elements in the compound, subtracting the percent copper from 100 will give the percent chlorine %Cl = %Cu In lecture, you were taught how to determine empirical formulas from a given percent composition. However, experimental errors, even those beyond your control, can affect the accuracy of such calculations in this lab. For this reason, you will be using an alternate method for determining the identity of your copper chloride. The empirical formula will be one of the five choices below. You ll compare their percent compositions (which you ll determine as part of your pre-lab) to your experimentally determined mass percentage, choosing the formula with the closest match. Formula % Cu % Cl CuCl CuCl 2 Cu 2 Cl Cu 2 Cl 3 Cu 3 Cl 2 Observant students will notice that only two of these formulas actually give chloride a correct charge. However, the other formulas are possible by preparing a solution with the appropriate mixture of these two, so any of these five could potentially be your unknown.

2 Procedure Note: You will follow this procedure twice to give you two sets of data (see Data Sheets). In the interest of time, do both reactions simultaneously (e.g., you do one reaction while your partner does the other). 1. Place a clean evaporating dish on a hot plate and heat it for at least 10 minutes (continue with the rest of the procedure as you wait). Set the dish on a wire gauze to cool then record its mass. 2. Using a graduated cylinder, measure 25.0 ml of a copper chloride solution and transfer it to a small beaker ( ml). 3. Obtain about 4-5 g of zinc, clean it with sandpaper or steel wool to remove any dirt or zinc oxide (a white solid) that might have formed on the surface of the metal. 4. Carefully place the zinc in the copper chloride solution. Stir continuously to prevent the copper from sticking to the zinc. 5. Once the solution has turned colorless, add 10 drops of 10% aqueous hydrochloric acid to the solution and stir (this will help remove any trace impurities). 6. Remove the zinc from solution. If any copper is stuck to the zinc, carefully scrape it off and return it to solution. 7. Isolate the copper by decanting the solution into a second beaker. 8. Wash the copper with approximately 10 ml of deionized water, stirring the mixture, and decant the water into the second beaker. 9. Transfer the copper to the evaporating dish from step 1. Return the evaporating dish to the hot plate and begin heating the copper. Do not overheat your solid, as this may cause it to react with oxygen in the air to form copper oxide (a black solid). 10. When the copper appears dry, use a set of tongs to remove the evaporating dish from the hot plate and set it on a wire gauze. Let the dish cool for about a minute (it doesn t have to be room temperature) then reweigh it. Record the combined mass of the dish and copper 11. Return the evaporating dish to the hot plate and heat the copper for a few more minutes, then repeat step 10. If the mass of the dish and copper isn t within 0.1 g of the previously recorded mass, continue heating until it is. Waste Disposal Dispose of any used metals in the solid waste container. Solution waste should be disposed of in the waste hood.

3 Data Trial #1 Trial #2 1) Volume of solution, ml 2) Mass of copper chloride in solution, g (see label) 3) Mass of evaporating dish, g 4) Mass of evaporating dish + Cu, g Initial heating 2 nd heating 3 rd heating (if needed) 4 th heating (if needed) 5) Mass of copper, g 6) Percent copper in copper chloride, % Average 7) Percent chlorine in copper chloride, % Average 8) Empirical formula of copper chloride Show your work for the percentages of chlorine and copper from Trial #1.

4 Post-Lab Questions 1. Which reactant was the limiting reagent and which was the excess reagent? Explain. 2. How would the following experimental errors affect your percent Cu and percent Cl determinations (too high, too low, or no effect)? In each case, explain your answers. a) The zinc was dropped into the beaker, causing some of the solution to splash out. b) When the zinc was removed, there was copper stuck to it that wasn t returned to the solution. c) The zinc was removed from the solution before the solution turned colorless. d) The copper wasn t dried completely e) The copper was overheated during drying. 3. Using your experimentally derived percent composition (the average values) and the method taught in lecture, determine the empirical formula of the copper chloride.

5 Pre-Lab Questions 1. Define the following: a) Limiting reagent b) Excess reagent 2. Calculate the percent compositions of copper and chlorine in each of the formulas below (make sure to copy your answers to the table on the intro page). Formula % Cu % Cl Show your work for determining the percent composition of Cu 3 Cl 2 CuCl CuCl 2 Cu 2 Cl Cu 2 Cl 3 Cu 3 Cl 2 3. A g sample of a titanium bromide was found to contain g of titanium. a) Calculate the percent titanium in this compound. b) Calculate the percent bromide in this compound. 4. When performing the reaction in this lab, how will you know when the copper ion has been completely converted to copper metal? 5. In step 11, why is it important that the last two mass measurements be less than 0.1 g of each other?