Finding the Formula

Things were heating up in David Amburgey’s chem class last week as students used heat and measured mass to find the formula for a compound which has many common uses.

Here's a quick chem lesson from David:
The goal (of the lab) was to figure out a way to determine the formula for a compound.  For example, the formula for copper chloride is CuCl2 . The class learned that the formula is a ratio of the constituent elements. For example, CuCl2 is a 1:2 ratio of copper to chloride.  A ratio of what? A ratio of the moles (the amounts) of the elements. So, the class needed to make a compound, and then figure out the mole-mole ratio.  That would give the numbers in the formula.

The class started with magnesium metal and then heated to a very high temperature, where it reacted with oxygen in the surrounding air.  This created a compound that was heavier than just the magnesium alone. Students got the mass of the magnesium before the reaction, and the mass of the compound after the reaction, to determine the mass of the oxygen that bonded with the magnesium.  

After students got the masses of the elements separately, and the total mass for the compound, they were able to convert the masses into moles.  Calculating moles is a way that chemists measure amounts of elements on the periodic table. Then the class could set the moles of magnesium to the moles of oxygen to find the ratio. Most students got a 1:1, so the correct formula for magnesium oxide is MgO.  Students could verify if they were correct by looking at the periodic table and looking at the oxidation state of Mg and O.

Fun fact: MgO has many uses. It’s one of the components in Portland cement, it’s used extensively in soil and groundwater remediation, wastewater treatment, drinking water treatment, and air emissions treatment; it is also used for relief of heartburn and sour stomach!