Determining an Empirical Formula
The law of definite proportions states that when two elements form a binary ionic compound they always combine in a simple, whole number ratio (e.g. 2 cations to 1 anion, or 2 cations to 3 anions). The empirical formula is simply the element symbols with subscripts written to indicate how many atoms of each element are combined in this whole number ratio.
It is easy for us to miss this simplicity when we look at a chemical reaction because we are limited to measuring the masses of the reactants and don’t see the number of atoms involved in the reaction. The fact that atoms of different elements have different masses only makes it more difficult to see the simplicity.
For example, if we combine hydrogen and carbon to form methane, we might find that a 24 g sample of methane includes 6 g of hydrogen and 18 grams of carbon. That gives us a mass ratio of:
It would be easy if this meant there were 3 atoms of carbon for every atom of hydrogen, but it does not work that way. To find the number of atoms involved, one needs to consider the masses of the individual atoms and decide what combinations will produce the same ratio as in the experiment. In this case we can try combining 1 atom of carbon with 1 atom of hydrogen, 1 atom of carbon with 2 atoms of hydrogen, 2 atoms of carbon with 1 atom of hydrogen, etc. until we find the right ratio of masses:
Nope; Closer; Bingo!
So methane is a combination carbon and hydrogen in a 1:4 ratio of atoms, which means the empirical formula for methane is CH4.
To find the empirical formula for the magnesium oxide that is formed when magnesium is burned in air.
Crucible and cover Crucible tongs
· Do NOT touch a hot crucible with your fingers.
Be sure to use tongs every time you handle a hot crucible.
· Use your hand to waft the gas given off by the burning material.
Avoid directly inhaling reaction product gases.
· Do NOT place any magnesium ribbon in an open flame.
1. Clean a crucible and cover. Dry them by heating them in the hottest part of your burner flame for 3 minutes. Allow them to cool. Measure and record the mass of the crucible (without the cover).
2. Cut a 70 cm length of magnesium ribbon into 1 cm pieces. Place the pieces in the crucible, then measure and record the mass of the crucible with the magnesium.
3. Cover the crucible and place it in the clay triangle. Heat gently for 2 minutes then lift the crucible cover with your tongs. Wait until the magnesium begins to burn then put the cover back on the crucible. Continue heating and briefly lift the cover off the crucible every 10 seconds or so. Continue until the magnesium no longer burns when you lift the lid.
4. Remove lid and heat strongly (bottom of crucible should glow orange) for 10 minutes – stir sample occasionally using metal spatula.
5. Turn off the burner, cover the crucible and allow the contents to cool. When the crucible is cool enough to touch, remove the cover and examine the contents. If any unreacted magnesium remains, replace the cover at a slight tilt and reheat the crucible strongly for several minutes.
6. Put the cover all the way on and allow to cool completely. After making sure that all of the magnesium has reacted, use a dropper pipette to add enough water (10 – 15 drops) to the crucible so as to just cover the contents. As you do so, wash any material that may have spattered onto the inside of the cover into the crucible. As you add drops of water, waft some of the vapors that may come off.
7. Pick up the burner by its base and move the burner back and forth slowly across the crucible to gently heat the contents. Avoid spattering. Waft some of the vapors given off by the heated crucible toward your nose so you can smell their odors. Record your observations of the scent. Continue heating until all of the water is driven off.
8. Repeat steps 6 and 7.
9. When all of the liquid has boiled away for a second time, strongly heat the uncovered crucible for 10 minutes.
10. Turn off the burner and allow the crucible and its contents to cool. Once they are cool, measure and record the combined mass of the crucible and its contents consisting of magnesium and oxygen.
Based on the data you recorded, calculate the mass of magnesium you used and the mass of oxygen that reacted with it. Convert the grams of magnesium and grams of oxygen determined in the product to moles. Next perform the appropriate calculation steps to determine the empirical formula. Be sure to show your work.
1. Determine the empirical formula for magnesium oxide by finding the combination of atoms that will match the mass ratio you found in your data.
2. Write an equation for the combination of magnesium and oxygen to form magnesium oxide. Use the common ions formed by magnesium and oxygen and be sure that the charges are balanced for the ionic compound. Does the empirical formula that you found this way match the one that you found using your data? If not, explain (error analysis).
3. How is the chemical composition of carbon monoxide (CO) similar to that of carbon dioxide (CO2)? How is it different? Name the law that explains this phenomenon.
4. 23.92 grams of aluminum will combine with 21.62 grams of oxygen to form alumina (aluminum oxide). What is the empirical formula of this compound?
5. 26.58 grams of potassium combine with 3.17 grams of nitrogen to form potassium nitride. What is the empirical formula of this compound?
6. A sample of 1.28 grams of sulfur combines with oxygen to form 3.20 grams of compound. What is the empirical formula of this compound?