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Embed code for: LAB: Percent Composition & Formula Mass
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We start this Lab Thurs 9/1. By class time, have the done in your notebook: Title, Date, Purpose, Pre-lab Qs. READ the whole procedure before coming to class.
Percent Composition & Formula Mass
In this experiment you analyze a compound of copper for its copper content. You will not know the identity of the copper compound except that it is a simple salt of copper such as CuSO4 or Cu(NO3)2·6H2O. The sample that will be given to you is called an unknown. In Experiment A you attempted to identify a metallic unknown by measuring its density. In this experiment, the parameter that is unknown is the percentage of copper in the solid copper salt given to you for analysis. The instructors do know this value and will be able to grade your work on how accurately you determined the percent copper and the formula mass of the compound. Unknowns are commonly used as a method of measuring one’s laboratory skills, and is commonly employed in chemistry courses.
You will determine the percent copper in a copper-containing compound by first dissolving a sample of precisely known mass in water. All of the unknowns are water-soluble, thus forming Cu+2(aq). For example, if your unknown were CuCl2, it would dissociate into ions in water:
CuCl2 Cu+2(aq) + 2 Cl-(aq)
The aqueous copper ion solution is then reacted with a molar excess of magnesium metal. The magnesium reduces the copper (II) ions to metallic copper:
Cu+2(aq) + Mg(s) Cu(s) + Mg+2(aq)
Magnesium is a good reducing agent and the reaction goes to completion. To insure complete displacement of copper from the solution, an excess of metallic magnesium will be used. After all the copper has been reduced, hydrochloric acid will be added to dissolve the excess magnesium. This reaction produces hydrogen gas:
Mg(s) + 2 H+(aq) Mg+2(aq) + H2(g)
Hydrochloric acid does not react with copper.
After the excess magnesium has been destroyed, the metallic copper, Cu(s), will be collected by filtration, washed, dried, and its mass determined.
You will then calculate the percent copper in the compound and use this value to calculate the formula mass of your copper-containing compound.
The percent copper in a sample of copper compound is given by the following relationship:
The formula mass, FM, of the compound is given by the formula:
All of the unknowns that have been used in this experiment contain only one copper atom per formula unit. Therefore, the number of moles of compound is equal to the number of moles of copper found in your experimental work today:
The number of moles of copper in your sample is equal to the mass of the copper that you will determine today divided by the atomic mass of copper (63.55g/mol):
Megan was given an unknown and was told that it was either CuCl2 or CuBr2. Calculate the percent copper in each of these salts. (Watch significant figures!)
In order to determine the correct identity of her unknown, Megan reacted her unknown with magnesium metal as per the experimental conditions discussed in the introduction. She used a sample of unknown that had mass of 0.7467 g which produced 0.2127 g copper metal. Determine the percent copper and identify the unknown. (Watch significant figures!)
The percent copper calculated in Question 1 did not exactly match the values determined in Question 2. Why is this so?
Justin measured out 1.1263 g of an unknown copper salt that was known to contain only one copper atom per formula unit. He determined that the sample contained 0.4480 g of copper. What is the formula mass of the unknown? (Watch significant figures!)
Later Justin learned that his unknown was either Cu(OH)2, CuSO4, or CuCO3. Which one is this best match? Defend your answer.
Safety glasses should be worn at all times. Today you will be using hydrochloric acid solutions which are strongly acidic and could cause chemical burns. Be careful when handling all solutions. If you do happen to get any chemical on you, wash the area thoroughly with water. The alcohol used is flammable.
You should work in pairs on this experiment. Your instructor will give you and your partner a test tube that contains an unknown copper compound. Record the unknown number on the report sheet. Place an empty weighing boat on the balance. Tare the balance. Remove the boat from the balance and tap about half the contents of the test tube onto into the boat.
IMPORTANT: Never pour chemicals
inside the balance chamber!
Determine the mass of the unknown and record the value on the report sheet. Transfer the unknown to a clean 250 mL beaker. Label the beaker “Trial 1” using a pencil. Repeat the procedure with a new boat and the remaining unknown. Transfer this material to a beaker labeled “Trial 2.”
Place a clean stirring rod in each beaker and leave them there throughout the experiment. Add approximately 50 mL of distilled water to each beaker by pouring it down the stirring rod in the beaker. Then add 2 mL (40 drops) of 6 M HCl (CAUTION! STRONG ACID!) Stir until the solid has dissolved. If solid remains after stirring, add 6 M HCl in 2 mL increments, stirring one minute after each addition of HCl, until the solution becomes clear.
Next, add about 0.7 g of magnesium turnings. (Because the magnesium is in excess, the exact mass of magnesium used is not important. You should use the top-loader balance to measure out this amount of magnesium.) Once the magnesium has been added, stir well. Continue to stir until the blue color of the Cu+2 has disappeared. There should now be a deposit of copper on the bottom of the beaker and the solution should be colorless. The evolution of hydrogen should be slow. If the solution turns cloudy during this time, add 6 M HCl drop by drop with stirring until the solution becomes clear.
You must now destroy the excess magnesium. Add 2 mL of 6 M HCl(aq) to each beaker and stir. Hydrogen should be evolved as the excess magnesium reacts with HCl. (Note: If hydrogen is not evolved, you probably did not have sufficient magnesium to displace all the copper. Consult your instructor.) When the evolution of hydrogen has become slow, add a little more HCl and continue to stir until the evolution of hydrogen has ceased and there are no visible pieces of magnesium left in the beaker.
Obtain two pieces of filter paper. Use a pencil to label each with your names and the words “Trial 1” or “Trial 2.”
Assemble a filter flask equipped with a Buchner funnel. Clamp the filter flask to a ring stand. Connect the thick-walled rubber tubing to an aspirator. (Aspirators are attached to selected faucets around the laboratory benches. When the water faucet is opened as far as possible, a good suction is created. This allows for the suction-filtration you will use to separate your product from the remaining water.) Your instructor will describe the proper use of this apparatus. Filter the solutions through the appropriately labeled filter papers. Use your wash bottle and the rubber policeman on the stirring rod to transfer all of the copper from the beakers onto the filter papers. Use approximately 50 mL of distilled water to wash the copper by pouring portions of the distilled water through the copper.
After the water has drained through the copper, disconnect the rubber hose from the aspirator before you turn off the water. (Chemists always turn off aspirators in this order: Tubing Before Water. If you turned off the water before disconnecting the tubing , the partial vacuum in the filter flask would suction water into the flask. This would be bad if the desired product were in the filter flask.) Temporarily remove the Buchner funnel and drain the acid solution into the Recovered Acids and Bases receptacle.
Reassemble the Buchner funnel and filter funnel. Wash the filter paper and copper with approximately 10 mL of ethanol (alcohol). After the ethanol has soaked the filter paper and copper for about 10 seconds, turn on the aspirator again until dry. Remove the tubing and then turn off the aspirator water. Repeat the washing with another 10 mL ethanol. Once the filter paper and copper are completely soaked, turn on the aspirator and continue to aspirate for about 5 minutes. Remove the filter paper from the funnel. The ethanol washings can be safely poured down the drain. Repeat the filtration and washing procedure for your second trial.
Place your two filter papers in the oven. After five or ten minutes, inspect them to see if they are completely dry. There should be no noticeable odor of alcohol present. If you can detect alcohol, dry them a little longer. Once the papers are dry, allow them to cool to room temperature. Tare a weighing boat on an analytical balance. Transfer the copper from one of the trials to the boat and determine the mass of the copper. Record the mass on the report sheet. Repeat for the other trial.
Calculate the percent copper and the formula mass of your unknown copper salt.
The two materials that you will need to recover are: acid solution and metallic copper. Use the Recovered Acids & Bases receptacle as instructed in the Procedure. Your metallic copper product should be given to your instructor along with your laboratory report sheet.
REPORT SHEET – a.k.a. – ***WRITE IT IN YOUR LAB NOTEBOOK***
The following should most definitely be in your lab report:
Trial 1 Data. Record the mass of the unknown used, mass of magnesium used, and mass of copper produced.
Trial 1 Calculations. Calculate the %Cu in your unknown and the formula mass of your unknown. Show all work and use significant figures.
Trial 2 Data. Record the data as in Trial 1.
Trial 2 Calculations. Perform the calculations as per Trial 1.
Data agreement. Do the results from Trial 1 agree with those from Trial 2? If so, you should average the results of the two trials. If not, describe any experimental mistake that may have occurred such as a spill or the addition of a wrong reagent, and what this may have done to affect the data. When there has been a mistake that you consider to seriously jeopardize the value of the data, you should discard the results from the flawed trial – but always explain your reasoning.
Possible Identity. Your unknown salt is among those listed below. Which do you think it may be? Explain why. It is possible that you cannot choose between two from the information you have collected. Can you justify eliminating all but one choice? Explain.
Salt Name and Formula
Copper (II) acetate hydrate, Cu(CH3COO)2·H2O
Copper (II) chloride dihydrate, CuCl2·2H2O
Copper (II) nitrate trihydrate, Cu(NO3)2·3H2O
Copper (II) sulfate pentahydrate, CuSO4·5H2O
Percent Composition and Formula Mass, 4
Retyped 2016ltration you will use to separate your product from the remaining water.) Your instructor will describe the proper use of this apparatus. Filter the solutions through the appropriately labeled filter papers. Use your wash bottle and the rubber policeman on the stirring rod to transfer all of the copper from the beakers onto the filter papers. Use approximately 50 mL of distilled water to wash the copper by pouring portions of the distilled water through the copper.
Reassemble the Buchner funnel and filter funnel. Wash the filter paper and copper with approximately 10 mL of ethanol (alcohol). After the ethanol has soaked the filter paper and copper for about 10 seconds, turn on the aspirator again until dry. Remove the tubing and then turn off the aspirator water. Repeat the washing with another 10 mL ethanol. Once the filter paper and copper are completely soaked, turn on the aspirator and continue to aspirate for about 5 minutes. Remove the filter pape