CHEMISTRY LAB
      DETERMINATION OF AN
      EMPIRICAL FORMULA

      INTRODUCTION:

      In this experiment, you will determine the empirical formula of a compound. In so doing you will gain a clear understanding of the difference between an empirical formula and a molecular formula.
      An empirical formula gives the simplest whole number ratio of the different atoms in a compound. The empirical formula does not necessarily indicate the exact number of atoms in a single molecule. This information is given by the molecular formula. For certain compounds, the empirical formula and the molecular formula are the same; for other compounds the empirical and molecular formulas are different. In all cases, the molecular formula is a simple multiple of the empirical formula. Consider the following examples.
      Experiments have shown that any sample of pure water contains two atoms of hydrogen for every atom of oxygen. The empirical formula for water is, therefore, H2O. Various molecular formulas; H2O, H4O2, H6O3, and so on, are possible. Each of these formulas expresses the same ratio of hydrogen and oxygen atoms as is expressed in the empirical formula. Scientists have shown, however, that each water molecule actually consists of two atoms of hydrogen bound to one atom of oxygen. Therefore, the molecular formula for this compound is H2O. For water, the empirical and molecular formulas are identical.
      For other compounds, the empirical and molecular formulas are different. Consider hydrogen peroxide. This compound contains one atom of hydrogen for each atom of oxygen, and its empirical formula is HO. There is, however, no stable molecule having this formula. In fact, it has been shown that individual hydrogen peroxide molecules contain two atoms of hydrogen and two atoms of oxygen. The molecular formula of this compound is, therefore, H2O2.
      In some cases, two or more different compounds share the same empirical formula. This is true of acetylene and benzene. Each of these compounds has the empirical formula CH. The molecular formula of acetylene, however, is C2H2, while that of benzene is C6H6.
      In this experiment, you will determine the empirical formula of magnesium oxide, a compound that is formed when magnesium metal reacts with oxygen gas. In determining this empirical formula, you will make use of the law of conservation of mass.

      According to this law, the total mass of the products of a chemical reaction must equal the total mass of the reactants.

      mass of Mg + mass of O2 = mass of MgxOy

      Therefore, knowing the mass of magnesium used and the mass of magnesium oxide produced in this reaction, you can determine the mass of oxygen used. This ratio between the number of moles of magnesium used and the number of moles of oxygen consumed can then be calculated and the empirical formula of magnesium oxide can be written on the basis of this ratio.

      MATERIALS:
      Safety goggles Crucible Magnesium ribbon, Mg
      Dark film or lenses Crucible lid Ring stand
      Wax marking pencil Crucible tongs Ring support
      Gas burner Clay triangle Desiccating jar
      Balance (to 0.01g)


      SAFETY:

      THIS LAB MUST BE DONE IN A FUME HOOD! The smoke given off, and the extreme heat must be contained safely. Safety goggles must be worn at all times, and a face shield might be added if available.
      Handle the crucible only with the tongs. There is a significant burn hazard associated with the handling of crucibles because a hot crucible looks exactly like a cold crucible.
      Remove the gas burner from beneath the crucible before using the crucible tongs to remove the crucible and its lid. Use the tongs to grasp the lid by its porcelain knob; the crucible should be grasped by its edge.

      PROCEDURE:

      DAY - 1

      1. Mark the top of your lid with your initials, using the wax pencil. Place the crucible on a clay triangle balanced on a ring support clamped to a ring stand. Light the gas burner and adjust it to give a clear blue flame (A yellow flame will deposit soot on the crucible and cause a large error in your data.). Place the burner under the crucible. Adjust the height of the ring support so that the bottom of the crucible is in the hottest part of the flame. Place the crucible lid slightly ajar on the crucible (The crucible lid should be large enough to fit loosely down over the crucible edge.).

      2. Heat the crucible so that its bottom glows red for five minutes. Remove the burner and allow the crucible and crucible lid to cool. This will take at least 10 minutes. CAUTION: The crucible gets extremely hot. Never touch it. Always use crucible tongs in handling this piece of equipment. When the crucible and lid are completely cool, use crucible tongs to transfer them to a balance. Do not place a hot crucible on the balance. Inaccurate mass readings and damage to the balance may result. Determine the mass of the empty crucible and lid to the nearest 0.01 g. Record this mass in the DATA TABLE.

      3. Coil a 25-cm length of magnesium ribbon and place it in the bottom of the crucible. Determine the combined mass of the crucible, lid, and the magnesium. Record this mass in the DATA TABLE.

      4. CAUTION: Do not look directly at the burning magnesium. The intense light may hurt your eyes. View the reaction through the dark film lenses.

      Place the crucible, without its lid, on the clay triangle. Heat the crucible strongly until the magnesium ignites.
      CAUTION: Be careful to keep the crucible at arm's length at all times. Do not inhale the "smoke" produced.
      When the magnesium begins to burn, Immediately place the cover on the crucible (using tongs) and remove the burner.

      5. After the reaction has subsided and "smoke" production has ceased, replace the burner and continue to heat the crucible. Every 2 or 3 minutes, remove the burner and check the progress of the reaction by using tongs to lift the lid of the crucible.
      CAUTION: Do not lean over the crucible.

      Then replace the lid and again apply heat. After 10 minutes of heating, remove the burner and check the product. When the reaction is completed, the magnesium should be completely converted to a light gray powder, magnesium oxide.

      If no ribbon-like material remains in the crucible, place the crucible in a desiccating jar to cool completely. If ribbon-like material remains, heat the covered crucible an additional 10 minutes, then place the crucible in the desiccating jar to cool. Be sure to completely seal the desiccating jar.

      DAY -2

      6. Remove your crucible from the desiccating jar and find the combined mass of the crucible, crucible lid, and magnesium oxide. Record on the DATA TABLE.

      DATA TABLE:


      Mass of crucible and lid. = _______ g


      Mass of the crucible, crucible lid, and the magnesium. = _______ g


      Mass of the crucible, crucible lid, and magnesium oxide. = _______ g


      CALCULATIONS:


      7. Determine the mass of magnesium ribbon used in the experiment by subtracting the mass of the crucible and lid from the mass of the crucible, lid, and magnesium.

      Mass of magnesium. = _______ g

      8. Determine the number of moles of magnesium used. Remember: mass / atomic weight = number of moles. the atomic weight of magnesium is 24.3 g / mole .

      Number of moles of magnesium. = _______ Mole

      9. Determine the mass of magnesium oxide that was formed by subtracting the mass of the mass of the crucible and lid from the mass of the crucible, lid, and magnesium oxide.

      Mass of magnesium oxide formed. = _______ g

      10. Determine the mass of oxygen that combined with the magnesium.

      Mass of oxygen = mass of magnesium oxide - mass of magnesium

      Mass of oxygen that combined with the magnesium. = _______ g

      11. Determine the number of moles of oxygen atoms that were used. This is elemental oxygen so use 16.0 g / mole for the atomic weight.

      Number of moles of oxygen atoms that were used. = _______ mole

      12. Calculate the ratio between moles of magnesium atoms used and moles of oxygen atoms used. Remember, this is a simple division. Divide the number of moles of Magnesium by the number of moles of oxygen. Round your answer to the nearest whole number, as we do not use part of an atom. This represents the moles (and also atoms) of magnesium. The moles (and also atoms) of oxygen, are represented by 1, because it was on the bottom of the division.

      Moles of Magnesium : Moles of Oxygen


      _______

      		 :


      ___1___


      13. Give the empirical formula for magnesium oxide that is indicated by your experimental data.


      Empirical formula of magnesium oxide. = __________


      Back to Chemistry Table of Contents
      Back to Lab Dad's Laboratory