Experiment #5. Empirical Formula

Transcription

1 Experiment #5. Empirical Formula Goal To experimentally determine the empirical formula of magnesium oxide based on reaction stoichiometry. Introduction The molecular formula (usually shortened to simply "formula") of a compound gives the number of atoms in each molecule of that compound. For example, the formula for glucose, C 6 H 12 O 6,indicates that there are 6 atoms of C, 12 atoms of H, and 6 atoms of O in one molecule of glucose. Unfortunately, experiments to determine the formula of a compound do not always reveal how many atoms are in each molecule, but rather give us only sufficient information to calculatethe simplest ratio of atoms or moles in the substance. The simplest ratio is called the empirical formula. For hydrogen peroxide, H 2 O 2, the molecular formula shows there are 2 H and 2 O, but its simplest ratio is 1 H to 1 O. Thus the empirical formula is HO. For glucose, the empirical formula is CH 2 O. For some compounds the empirical formula is also the molecular formula (NaCl or H 2 O). In this experiment you will determine the empirical formula of magnesium oxide. The experiment is straightforward. A sample of magnesium is weighed and burned in air. (Air is not pure oxygen, so some side reactions will occur.) The unwanted product will be reacted away and magnesium oxide will be the final product. The final product will be weighed and the ratio of moles of magnesium to moles of oxygen will be calculated to determine the empirical formula. Using Bunsen Burners Bunsen burners are a common heat source in chemistry labs. When using a bunsen burners, first check that your hose has no cracks. Connect one end to the gas outlet on your lab bench and the other to the gas inlet on your bunsen burner. You may need to twist the hose to get the burner to stand up straight. Gas flow is controlled at the outlet. Additionally, some bunsen burners have a knob under the base that also controls gas flow. To light a bunsen burner, first, light a match, second, have your partner turn on the gas. This way gas is not flowing into the room if you cannot immediately light the match. Bring the match up from under the barrel putting a match directly into the flame may blow it out. Matches may also blow out if the gas flow is too high. Any time the match blows out, turn off the gas, and begin again. Once the flame is lit, adjust the amount of of oxygen by rotating the air inlet holes. An efficiently burning flame should have no yellow, a clear blue outer cone and a brighter blue inner cone. The hottest part of the flame is the inner tip of the cone. Each time you heat a crucible in this lab you will be asked to heat gently for one minute before heating strongly. If glass or ceramics are heated or cooled too quickly, they may crack. You may have experienced this with pyrex pans they cannot be heated on the stovetop and they may shatter if cooled too quickly in the sink with water. To heat gently, the crucible should be in the clear blue outer flame. To heat strongly move the crucible base to the tip of the inner cone. You may do this by carefully lowering your ring clamp, or by increasing the gas flow (assuming you don t have it very high to start).

2 Laboratory Activity Equipment ring stand iron ring clay triangle crucible and cover crucible tongs Bunsen burner and burner hose wire gauze pad sand paper analytical top-loader balance matches deionized water and dropper Chemicals Mg (magnesium ribbon) Safety Hazards Caution scorching hot crucibles look the same cool crucibles. When determining if a crucible is still hot, slowly bring the back of your fingers towards it. If you sense any heat radiating from the crucible, do not touch, wait! Procedure 1. Obtain a clean, dry crucible and a cover, and inspect them for cracks. Weigh the crucible and its cover separately and record the masses the data sheet. If you think that either the crucible or lid is cracked, give it to your instructor. Do not use wet crucibles and do not wash the crucible or lid. 2. Obtain a pre-cut piece of magnesium ribbon. Rub the magnesium ribbon with the sandpaper to remove any oxide coating. Cut the magnesium ribbon into small pieces and place them into the crucible. Weigh the magnesium, crucible and cover on the same analytical balance and record the mass. 3. Set up your apparatus: attach a ring clamp to your ring stand, and place a clay triangle on top. You may want to verify that the triangle will hold your crucible with out falling though. Connect your bunsen burner and be sure it is steadily under your appartus. Have a gauze pad nearby to place hot objects on. Iron Ring Ring Stand Clay Triangle Crucible Bunsen Burner 4. Heat the covered crucible gently for one minute and then strongly. The bottom of the crucible will glow red when it is heated strongly. Periodically lift the cover slightly with the crucible tongs to let in air (mostly nitrogen and oxygen gas). Put the cover back on after the magnesium begins to glow brightly and bursts into flames. Continue to heat the crucible strongly, frequently lifting the cover. When the magnesium stops producing smoke and fire, remove the lid and continue heating for 5 additional minutes. During this first heating, some hot A magnesium will react with nitrogen in air to form magnesium nitride. Most of the B magnesium reacts with oxygen to form magnesium oxide. 5. Pull the Bunsen burner away from the crucible and allow the crucible to cool for 2 minutes. Add 8 drops of deionized water with a pippette. Cover the crucible and reheat gently for one minute, then strongly for 5 minutes. C The magnesium oxide will react with water to form magnesium hydroxide. D The magnesium nitride will react with water to form ammonia gas (NH 3 ) and magnesium hydroxide. The ammonia gas evaporates leaving just magnesium hydroxide. E The magnesium hydroxide decomposes upon heating to form magnesium oxide and water vapor. By this process, all the original magnesium ends up as magnesium oxide. 6. Let the crucible cool to room temperature. Measure the mass of the crucible and cover and record as the first mass on your data sheet on the same balance used earlier. Reheat the covered crucible for 5 minutes, cool and measure the mass (the second mass). Check with your instructor to see if your two weights are close enough. If the second weight is not very close to the first weight, reheat until a constant weight is obtained. When the weight is constant, record it.

3 Waste Disposal magnesium oxide product - scrape into the regular waste. Do not wash out the crucible or cover. unheated magnesium return to the setup tray. Calculations: Subtract the mass of magnesium from the mass of the magnesium oxide to find the mass of oxygen in the product. Using these masses, follow the instructions in the example below to determine the empirical formula of your product. Example: grams of lead was allowed to react with excess sulfur and grams of a lead sulfide product was obtained. The following table summarizes the calculations that were performed: Grams Moles (divide by molar mass of element) Find the simplest whole number ratio of moles (Divide each by the smallest number of moles.) lead (Pb) g Pb g Pb x 1 mol Pb = mol Pb 207 g Pb mol = mol Sulfur(S) g product g Pb = g S g S x 1 mol S = mol S 32.1 g S mol = mol ratio of moles (round to the nearest whole number) 1 2 EMPIRICAL FORMULA: PbS 2 Percent Yield: We were given an actual yield of g of PbS 2 for this experiment. Did 100% of the lead react to make PbS 2, or like most experiments, was the percent yield less that 100%? To determine the percent yield, first we determine the theoretical yield. The theoretical yield is the calculated mass of PbS 2 that could have been produced from the reaction of grams of Pb with excess S. Start with a balanced equation: Pb(s) + 2S(s) PbS 2 (s) Then use stoichiometry to determine the maximum amount of lead (II) sulfide that can be formed (the theoretical yield): g Pb 1 mole Pb 1 mole PbS g PbS 2 207g Pb 1 mole Pb 1 mole PbS 2 = g PbS 2 Percent Yield = Actual yield. x 100 % theoretical yield Percent Yield = (2.485 g / g) x 100 = 98.4 % yield.

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5 Name Team Name CHM111 Lab Empirical Formula of a Compound Grading Rubric Criteria Points possible Points earned Lab Performance Printed lab handout and rubric was brought to lab 3 Safety and proper waste disposal procedures observed 2 Followed procedure correctly without depending too much on instructor or lab partner 3 Work space and glassware was cleaned up 1 Lab Report Data was recorded with correct units and significant figures 1 Calculations are correct; work is shown in detail with units 1 Question 1 (Equations have correct formulas, phases, and are balanced) 5 Question 2 (works shown clearly with units) 1 Question 3 (works shown clearly with units) 1 Question 4 1 Total 20 Subject to other additional penalties as per the instructor

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7 Empirical Formula: Data Sheet Name Mass of crucible Mass of cover Mass of crucible and cover Mass of magnesium, crucible and cover Mass of magnesium First mass of magnesium oxide, crucible and cover Second mass of magnesium oxide, crucible and cover Mass of magnesium oxide (actual yield) Mass of oxygen in magnesium oxide Calculations: Show all your work clearly and completely Magnesium (Mg) Oxygen (O) Grams (show how you calculated from data above) Moles Simplest Ratio Empirical formula of magnesium oxide based on experimental results: Actual empirical formula of magnesium oxide: Report Page 1 of 2

8 Empirical Formula: Post lab questions Name 1. Write the five balanced chemical equations for the reactions that occurred in this experiment. (see steps 4 & 5, write correct formulas and phases for all reactions.) A. B. C. D. E. 2. a) Which reagent was in excess in this lab - magnesium or oxygen? b) Calculate the theoretical yield of magnesium oxide in your experiment based on the limiting reagent. (use the actual formula of magnesium oxide) 3. Calculate the percent yield of magnesium oxide. 4. The magnesium in a student s reaction did not reacted completely. How would this impact the following: (Be specific in your answers) actual yield theoretical yield calculated empirical formula Report Page 2 of 2