EXPERIMENT 3: DETERMINATION OF AN EMPIRICAL FORMULA

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1 EXPERIMENT 3: DETERMINATION OF AN EMPIRICAL FORMULA PURPOSE To find the empirical formula of a product based on experimental data. To determine the percentage of water of hydration in an unknown salt. BACKGROUND EMPIRICAL FORMULA Atoms tend to combine in exact ratios. A molecule of water, H 2 O, has exactly two hydrogen atoms bound to one oxygen atom; methane, CH 4, has four hydrogens for every carbon. It is impossible to have formulas such as H 2.43 O 6.8 because it is impossible to have fractions of atoms in a chemical compound. An empirical formula is one which gives the simplest whole-number ratio of atoms in a compound. In this experiment, the empirical formula of magnesium oxide will be determined by burning magnesium metal, or causing the magnesium to combine with oxygen in the air. Mg (s) + O 2(g) reactants Mg? O?(s) product (ash) Magnesium can also combine with nitrogen from the atmosphere.

2 Name: Date PRE-LAB QUESTIONS 1. Define: a) empirical formula b) molecular formula 2. Why is it necessary to heat the crucible before performing this experiment? 3. Why is it necessary to cool the crucible before measuring its mass? 4. If you can t look directly at the burning magnesium how are you going to know that it ignited? 5. When you are washing the ash, why should you heat the crucible gently by moving the Bunsen burner slowly back and forth? 6. How does spattering would affect your results? 7. If you have evaporated off all the liquid, why do you need to strongly heat the contents one last time? 8. If you were burning aluminum metal (combining it with oxygen) rather than magnesium metal, what empirical formula might you expect?

3 MATERIALS Crucible & lid Crucible tongs Clay triangle Ring stand Bunsen burner Four-beam balance Magnesium ribbon Hydrated salt PROCEDURE 1. Clean and dry a porcelain crucible and lid. Place the crucible on a clay triangle, place the lid on the crucible. Heat the crucible, cautiously at first with a very small flame to avoid cracking it. Gradually increase the flame until you have the tip of the blue inner cone touching the bottom of the crucible. Continue heating for 3 minutes. Allow the crucible to cool to room temperature. When you can hold the crucible in your hand, accurately determine the mass of the crucible and lid. Record the mass of the crucible and lid together. 2. Obtain about 25 cm of magnesium ribbon. Measure and record the mass of the crucible, lid and magnesium together. 3. Place the crucible in a clay triangle over your Bunsen burner. Heat the uncovered crucible until the magnesium ignites, and then cover the crucible with the lid leaving a small gap for air to enter. ***DO NOT LOOK DIRECTLY AT THE BURNING MAGNESIUM*** 4. When you see only grey ash and no more magnesium ribbon, the reaction is complete. Turn off your burner and allow the crucible to cool to room temperature. 5. Once the crucible has cooled, use a medicine dropper to add enough water to the crucible to just cover the contents. 6. Holding the Bunsen burner in your hand, gently heat the contents of the uncovered crucible by moving the burner slowly back and forth. Avoid spattering. Observe the odor of the vapor given off by wafting it towards you nose. Record observations. 7. When all the liquid has evaporated repeat steps 5 and When all the liquid has boiled off a second time, strongly heat the uncovered crucible for 5 minutes. 9. Allow the crucible to cool and determine the mass of the crucible, lid and ash together and record this in your notebook.

4 Name: Date DETERMINATION OF AN EMPIRICAL FORMULA CALCULATIONS AND RESULTS You will be guided through the Part A calculations by the instructions in italics. FORMULA OF MAGNESIUM OXIDE 1. Mass of Mg (s) reactant: 2. Mass of ash (magnesium oxide) product: 3. Moles of Mg (s) reactant: convert (1) to moles 4. Moles of Mg in product: 5. Mass of Mg in product: 6. Mass of O in product: 7. Moles of O in product: convert (6) to moles 8. Ratio of O to Mg in product: 9. Consider the ratio in (8) and decide what the simplest whole-number ratio of Mg atoms to O atoms would be. Empirical formula of product: POST-LAB QUESTIONS 1. What other compound is formed when magnesium is heated in air? 2. What happened to this other compound? Write a balanced equation for this reaction.

5 3. Based on your knowledge of the charges of both magnesium and oxygen ions, what do you think SHOULD be the formula of magnesium oxide? Did this match your experimental determination? If not, why?

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