Determination of a Chemical Formula

Transcription

1 1 Determination of a Chemical Formula Introduction Molar Ratios Elements combine in fixed ratios to form compounds. For example, consider the compound TiCl 4 (titanium chloride). Each molecule of TiCl 4 is composed of 1 titanium and 4 chlorine atoms. Therefore, 1 mole of titanium will combine with 4 moles of chlorine to produce 1 mole of TiCl 4. The molar ratio of titanium to chlorine in the compound is 1 to 4. This can be expressed as a fraction: moles of Ti in compound = = 0.25 moles of Cl in compound 4 Alternatively, we can think about the molar ratio of chlorine to titanium in TiCl 4, which is 4 to 1. This can also be expressed as a fraction: moles of Cl in compound = = 4.0 moles of Ti in compound 1 Suppose you don t know the formula of a particular compound. If you know the molar ratio of two elements in the compound, you can use that ratio to determine the simplest possible formula for the compound. For example, suppose you determined the following ratio in the lab for a compound that is composed of nitrogen and hydrogen: moles of H in a sample of the compound mol = = moles of N in a sample of the compound mol 1 This data could also be expressed as follows, with the moles of N over moles of H instead: moles of N in a sample of the compound mol = = moles of H in a sample of the compound mol 3 From either of these ratios (H over N, or N over H) you know that there are approximately 3 H s for every 1 N in the compound. Therefore, the simplest possible formula for the compound is NH 3. In this experiment you will determine the chemical formula of a compound that is composed of only copper (Cu) and chlorine (Cl). You will not know the formula of this compound ahead of time, but you will be able to determine its formula by determining the number of moles of copper that combine with a given number of moles of chlorine. You will calculate the moles or copper and chlorine from the masses (grams) of copper and chlorine in the compound, which are easily measured in the lab. The prelab sheet provides an example of exactly how this will be done, but uses a compound containing hydrogen and oxygen instead of copper and chlorine.

2 2 Determination of the Chemical Formula of a Compound Containing Copper and Chlorine You will be given a sample of a compound that is known to be composed only of copper and chlorine. More specifically, it is composed of copper ions and chloride ions. To determine the formula of the compound, the first step will be to determine the number of grams of copper and grams of chlorine in the sample of the compound. To determine the masses (in grams) of copper and chlorine in the sample, you will need to separate the copper from the chlorine. This separation will be accomplished by carrying out a chemical reaction in which all of the copper ions in the compound undergo a reaction in which they are converted to solid copper metal. The copper metal can then be filtered out of solution and its mass determined. Below is the reaction that will be carried out to convert the copper ions to copper metal (the reaction is not balanced): Cu ions + Cl - + Al metal Cu metal + Cl - + Al ions from from aluminum will appear compound compound foil as brown particles For the reaction, the compound is dissolved in water and mixed with aluminum metal (pieces of aluminum foil). The copper ions from the compound react with the aluminum foil. Notice that the copper ions are converted to solid copper metal, and the aluminum metal is converted to aluminum ions. Also notice that the chloride ions from the compound do not react they remain unchanged. After the reaction is complete, the brown particles of copper metal that have formed can be filtered out of the solution. The particles will then be dried and their mass determined. Based on that mass, you will be able to determine the formula of the compound, using the calculation steps outlined on the prelab. Conversion of Mass to Moles If you do not remember how to convert a mass (in grams) to moles, use the example below as a guide. Convert grams of zinc to moles of zinc. The molar mass of zinc (65.38 g/mol) can be obtained from the periodic table g of zinc 1 mole of zinc = moles of zinc g Therefore, g of zinc is equivalent to moles of zinc.

3 3 Determination of a Chemical Formula Prelab Name Suppose a certain compound contains only hydrogen and oxygen. Analysis of a 5.00 g sample of this compound revealed that the sample contained 4.44 g of oxygen. How many grams of hydrogen does the sample contain? Show your work here. Convert the grams of oxygen (4.44 g) to moles of oxygen. Show your work below and include units on each number in the calculation. (From the periodic table, oxygen s molar mass is 16 g/mol) Convert the grams of hydrogen (that you calculated first) to moles of hydrogen. Show your work below and include units on each number in the calculation. (From the periodic table, hydrogen s molar mass is 1 g/mol) Now that you know the number of moles of oxygen (O) and the number of moles of hydrogen (H) the 5.00 g sample contained, what is the H to O molar ratio of this compound? Based on this H to O molar ratio, what is the simplest formula of this compound? In today s experiment you will begin with a compound containing only copper and chlorine, and determine the grams of copper in a sample of the compound. Then you will do the same calculations as above in order to determine the simplest formula for the copper/chlorine compound.

4 4 Determination of a Chemical Formula Procedure 1. Weigh out approximately 2.5 g of the copper chloride compound and record the exact mass (all digits from the reading on the balance) on the report sheet. Place it in a 150 ml beaker and add 40 ml of purified water. Stir until the compound is completely dissolved be patient as this may take a few minutes. 2. To the beaker add approximately 0.5 g of aluminum foil which has been cut into small squares. Use a stirring rod to push the foil into the solution if necessary. The reaction should begin to occur immediately and should be obvious. Wait until the reaction ceases and the solution becomes less cloudy (~10 minutes; when most of the bubbling has stopped). While waiting, obtain a Buchner funnel and filter flask from the stockroom. 3. Add 10 ml of 6 M HCl to dissolve the excess aluminum foil that remains. Wait until this reaction ceases, at which point there should be no tiny particles of aluminum remaining. If you can still see particles of aluminum in the solution, ask the instructor for help. You should be able to see a large quantity of brown solid that has precipitated to the bottom of the beaker this is the copper metal (in very fine particles). 4. Obtain 30 ml of acetone in a beaker and set it aside for the next step. Fill your wash bottle with distilled water and set it aside. Place the Buchner funnel into the top of the filter flask, and clamp the neck of the flask to a ring stand so that it doesn t tip over. Then set up a vacuum filtration apparatus by connecting the arm of the filter flask to a water aspirator using one of the heavy hoses from the cabinet under your lab bench. Place a piece of filter paper into the Buchner funnel, making sure that all the holes are covered. Wet the filter paper so that it sticks in place by squirting it with distilled water from your wash bottle. Ask the instructor to check your setup before proceeding with vacuum filtration. 5. Turn on the water aspirator to create a vacuum in the flask this will draw liquid from the Buchner funnel into the flask, and filter out any solid particles in the process. Pour your reaction mixture into the Buchner funnel in order to filter out the solid copper. You must try to get ALL copper particles into the Buchner funnel. If any copper remains stuck to the sides of the beaker, use as much distilled water from your wash bottle as necessary to wash the particles into the funnel. Wash the copper in the Buchner funnel with three additional portions of distilled water (~10 ml each) in order to rinse away any chloride or aluminum ions from the reaction mixture that may remain stuck to the copper.

5 5 6. Break the vacuum by detaching the hose from the arm of the filter flask. Pour about 1/3 of the acetone into the Buchner funnel to cover the copper; allow it to drip through by gravity. Acetone will wash much of the water away from the copper, speeding up the subsequent process of drying the copper. Water evaporates slowly, but acetone evaporates much more quickly. Therefore, you will only have to wait a short period of time for the acetone to evaporate before weighing the dry copper. When all the acetone has dripped out of the Buchner funnel, pour another portion of the remaining acetone into the funnel to cover the copper, and allow it to drip through. Then repeat this process one more time with the last of the acetone. When the last portion of acetone has dripped out of the funnel, re-attach the hose to the arm of the filter flask (be sure the aspirator is turned on to create a vacuum). Allow the vacuum to draw air through the copper in the funnel for a few minutes in order to help the acetone evaporate. 7. Detach the hose from the arm of the filter flask and carefully remove the Buchner funnel from the top of the flask. Transfer the copper to a clean, dry evaporating dish, being careful not to lose any particles of copper in the process. Scrape as much copper as possible off of the filter paper and into the evaporating dish. The filter paper can then be discarded. Label the dish with your name by attaching a piece of tape to the edge. Put the dish in the oven for 15 minutes to dry the copper thoroughly (the oven should be on setting 5, at 110 o C). 8. The liquid in the filter flask can be discarded down the drain. Wash the Buchner funnel and flask and return them to the stock room. Check the copper after it has been in the oven for 15 minutes. When it is dry, it should seem powdery, rather than sticky, when stirred with a glass stirring rod. 9. Allow the dry copper to cool. Then weigh it and record its exact mass on the report sheet. Ask the instructor how to dispose of the copper. Complete the rest of the calculations on the report sheet in order to determine the formula of the compound.

6 6 Determination of a Chemical Formula Report Sheet Name Data 1. Mass of original sample of compound g 2. Mass of copper obtained g Calculations 3. Mass of chlorine in original sample g 4. Moles of copper obtained mol 5. Moles of chlorine in original sample mol 6. Molar ratio of chlorine to copper: moles of Cl / 1 mole of Cu Record the exact number from the calculation. 7. Simplest formula for the compound:

7 7 Determination of a Chemical Formula Postlab Name 1. Circle one answer for each of the following: If you did not let the copper dry thoroughly, how will the apparent mass of copper (line #2) be affected? too high or too low How will the calculated mass of chlorine (line #3) be affected? too high or too low How will the calculated moles of copper (line #4) be affected? too high or too low How will the calculated moles of chlorine (line #5) be affected? too high or too low How will mole ratio of chlorine to copper (line #6) be affected? too many moles of Cl for one mole of Cu or too few moles of Cl for one mole of Cu Finally, based on the answers you circled above, will the formula (line #7) come out correctly for the compound even if you failed to let all the water evaporate from the copper? YES or NO 2. What was the purpose of washing the copper residue with acetone? 3. What was the purpose of adding 6 M HCl in step 3 of the procedure? If you skipped this step with HCl, how would the apparent mass of copper (line #2) be affected? too high or too low How would the calculated mass of chlorine (line #3) be affected? too high or too low