Introduction
The empirical formula of a compound is the lowest whole number ratio of atoms of each element present in the compound or the number of moles of each element present in the compound. One of the simplest ways of determining this ratio is to perform a direct combination reaction. You determine the mass of one of two elements present in the final compound, and react it with an excess of the other element. Any amount of the second element is removed, because it is a gas at the reaction conditions, while the compound formed remains as either a solid or a liquid with a very low vaporization, so that none of the compound is lost.
Then determine the mass of the compound formed (you must assume none of the first element is lost before reaction and none of the compound formed is lost). The difference in the masses is the mass of the second element that has reacted to form the compound. You now have the masses of each of the two elements that form the compound. The atom or mole ratio is obtained by dividing the mass of each element by its atomic mass. This gives two numbers. You want to reduce these two numbers to two smaller numbers where one is equal exactly to unity (one), and the other is a number larger than unity (one). Dividing each of the two numbers by the smaller of the two numbers does this.
Look at the value of the second number. If it is within one tenth of a whole number, round it off to that number; if it is off by more than one tenth look at the decimal value. If it is between 0.45 and 0.55 the fraction is ½ ; if the value is between 0.30 and 0.37 or between 0.64 and 0.70 the fraction is in 1/3's ; if the values are close to the fractions for ¼ or ¾ the fraction is ¼'s. Multiply each of the numbers by the denominator of the fractional value (2, 3, or 4 etc.). You now have the simplest whole number ratio for the compound. The molecular formula requires one additional piece of information that we do not do for ionic compounds and if our compond were a molecular one, we would then need the molecular mass which is an entirely different type of experiment.
Equipment
Crucible, crucible cover, crucible tongs, bunsen burner with rubber hose, ring stand, iron ring, clay-stem triangle, wire gauze.
Chemicals
Magnesium ribbon.
Magnesium Oxide
Procedure
Attach the iron ring to the ring stand and place the clay-stem triangle on the iron ring. Connect the rubber hose to the burner and connect the other end to the gas outlet on the Sink Island.
Obtain a crucible and crucibles cover. Gently loosen any solid in the crucible and discard in the wastebasket. The crucible should be dry. Wipe the crucible and crucibles cover with a paper towel to remove excessive fingerprints and moisture. Measure the mass of the crucible and its cover on a balance. Record the mass to the nearest 0.001 grams.
Obtain a piece of magnesium ribbon that is between 30 and 40 cm in length. Gently pull it between the folds of a paper towel to remove moisture and any loose magnesium oxide coating. Either, cut the magnesium ribbon into pieces about 1 cm in length and place them haphazardly in the crucible, or run the magnesium ribbon over a pencil or pen and between pieces of a paper towel, so that a coil is formed. Then pull out the magnesium in the middle of the coil and fit the magnesium into the crucible. The magnesium should fit loosely into the crucible, if it is tightly packed it will take much longer to finish the experiment.
Place the crucible on the clay-stem triangle so that it fits snuggly and will not be easily upset. Place the cover on the crucible. Light the burner (the instructor will demonstrate the method) and wave it under the crucible for a few seconds. Then you can place the burner under the crucible (again the instructor will show you the method to obtain the best results) and heat the crucible until the crucible start the turn a dull red color.
Carefully lift the cover to let some fresh air into the crucible. Also take a look inside to see the state of the magnesium ribbon. If the ribbon starts to turn red or white, replace the cover. A white smoke will form (this is the magnesium compound you are forming and want to keep inside the crucible). After 45 to 60 seconds, again lift the cover for a second or two and then replace the cover. Do this until there is no flaring up of the magnesium. When you can keep the cover off for a minute without any flaring up of the magnesium the reaction should be finished (about 30 minutes).
Remove the burner and turn off the gas. Let the crucible cool. If you wish, you can use the crucible tongs to move the crucible and its cover onto a wire gauze on the bench top. When the crucible is cool enough to hold in the palm of your hand (about 10 minutes),
determine the mass of the crucible, its cover and the magnesium product formed.
Using a plastic medicine dropper, carefully add water, drop by drop, to JUST moisten the solid residue inside the crucible - Do Not Add Too Much Water.
Place the crucible on the clay-stem triangle, place the cover on the crucible, re-light the burner and carefully reheat the crucible. After 4 to 5 minutes, carefully remove the cover and continue to heat for another 2 minutes or until the inside looks dry. Again cool the crucible and again determine the mass of the crucible, cover, and contents.
Repeat the heating, cooling, and weighing step. Compare the mass of these last two measurements. If they are within 0.010 grams you can stop, if not, repeat the heating, cooling and weighing until a near constant mass is obtained.
From the recorded data you should now be able to determine the empirical
formula of the compound formed.
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