Chemical formulas serve as a precise language in science, representing the elements and their proportions within a compound. These formulas are fundamental to understanding the composition of all substances. Magnesium oxide forms when magnesium metal combusts in the presence of oxygen. The empirical formula of magnesium oxide is MgO.
Understanding the Empirical Formula
The empirical formula represents the simplest, whole-number ratio of atoms of each element in a compound. This is distinct from the molecular formula, which shows the exact number of atoms of each element in a single molecule. For instance, hydrogen peroxide has a molecular formula of H2O2, but its empirical formula is HO because the atoms are present in a 1:1 ratio.
Magnesium oxide is an ionic compound, meaning it is formed by the electrostatic attraction between a metal cation (Mg2+) and a non-metal anion (O2-). Ionic compounds do not exist as discrete molecules but rather as vast, crystalline lattices with repeating units. Ionic compounds are always represented by their simplest ratio, which is known as the formula unit and is equivalent to the empirical formula.
The formula MgO indicates a 1:1 ratio of magnesium to oxygen atoms, reflecting the stable charge balance between the two ions. Magnesium, an alkaline earth metal, readily loses two electrons. Oxygen, a non-metal, readily gains two electrons to achieve a stable electron configuration. This chemical property dictates the 1:1 ratio seen in the formula.
Determining the Ratio: The Experimental Procedure
The ratio of magnesium to oxygen is determined through a classic laboratory procedure involving the combustion of magnesium ribbon. The first step involves accurately measuring the mass of a clean, empty porcelain crucible and its lid using a high-precision analytical balance. Next, a small piece of magnesium ribbon is cleaned and its mass is precisely measured before being placed inside the crucible.
The crucible, lid, and magnesium are then heated intensely over a Bunsen burner flame while the lid is kept slightly ajar. Keeping the lid partially open allows atmospheric oxygen to enter and react with the heated magnesium, forming the white solid product, magnesium oxide. If the lid were completely removed, a significant amount of the magnesium oxide product could escape as white smoke, leading to an inaccurate final mass measurement.
After the magnesium has completely reacted (indicated by the ribbon turning into a dull white powder), the crucible is heated strongly for a few minutes with the lid on tight. This ensures the reaction is complete and converts any magnesium nitride side product back to magnesium oxide. The crucible and its contents are then allowed to cool completely to room temperature to prevent air currents from affecting the final mass measurement.
Once cooled, the final mass of the crucible, lid, and the magnesium oxide product is recorded. The crucial raw data for the calculation are obtained by subtraction. The mass of the oxygen that reacted is found by subtracting the initial mass of the magnesium from the final mass of the magnesium oxide product. This difference represents the mass of oxygen incorporated into the compound.
Calculating the Final Formula
The raw mass data collected from the experiment must be converted into a mole ratio to find the empirical formula. The first step in this quantitative analysis is to convert the mass of magnesium and the mass of oxygen into moles. This conversion is achieved by dividing the measured mass of each element by its respective molar mass, found on the periodic table.
For example, if the mass of magnesium was 0.095 grams, dividing this by the atomic mass of magnesium (approximately 24.31 grams per mole) yields the moles of magnesium. Similarly, the calculated mass of oxygen (0.062 grams) is divided by the atomic mass of oxygen (16.00 grams per mole) to find the moles of oxygen. This step reveals the relative number of atoms of each element present in the compound.
The next step is to determine the simplest whole-number ratio. This is done by dividing the number of moles of both elements by the smaller of the two calculated mole values. When this calculation is performed using accurate experimental data for magnesium oxide, the resulting ratio for both magnesium and oxygen is very close to 1.
This near 1:1 result is rounded to the nearest whole number to establish the final empirical formula. A ratio of 1 mole of magnesium to 1 mole of oxygen confirms that the simplest whole-number ratio of atoms is one-to-one, leading directly to the empirical formula MgO.