Purpose:

Determine the empirical formula, or preliminary formula of a compound formed by magnesium and oxygen.

Introduction:

When magnesium is ignited in the air, it combines with oxygen in the air to form magnesium oxide. According to the Law of Conservation of Mass, the total mass of all reactants or starting materials must be equal to the total mass of product(s) . In this case,

mass of magnesium + mass of oxygen = mass of magnesium oxide

In the experiment, the initial mass of magnesium is measured before it is burnt. Then the mass of product, magnesium oxide, is also measured. The difference of the two masses must be the mass of oxygen.

The number of moles of magnesium and number of moles of oxygen can be calculated. The ratio of the two moles is used as subscripts to determine the empirical formula of magnesium oxide.

The two most abundant molecules in air are oxygen and nitrogen. Magnesium primarily reacts with oxygen because it is more reactive than nitrogen. However, some of the magnesium reacts with nitrogen and forms a side product that can be converted to magnesium oxide by adding water.

Steps to calculate empirical formulas:

Step 1: find masses in grams of all elements that make up the compound

Step 2: convert all masses to moles

Step 3: find mole ratio by dividing all moles by the smallest, round to whole numbers

Step 4: the mole ratio is used as subscripts to write the empirical formula

Lab Procedure:

Watch the following video, record observations and data in Lab Report, perform calculations and determine the empirical formula of magnesium oxide.

Empirical formula of a compound lab (Links to an external site.)

Lab Report

Pre-lab questions:

Refer to “Steps to calculate empirical formulas” above.

1. A sample of 1.74 g of silver reacted with sulfur to form a compound that has a mass of 2.00 g. Find the empirical formula of this compound. Show work for full credit.

2. A sample of 2.29 g iron is burnt with chlorine to form a compound that has a final mass of 6.65 g. Determine the empirical formula of this compound. Show work for full credit.

Observations:

appearance of magnesium before heating: __________________________________________

(color? luster? physical state? etc. …)

observations during the heating: ______________________________________

appearance of product: ________________________________________________

Data: carry units

1) mass of empty crucible + lid _________________

2) mass of crucible + lid + magnesium before heating ______________

3) mass of crucible + lid + product after 1st heating _____________

4) mass of crucible + lid + product after final heating _____________

Calculations and results: carry units and use sig. fig. rules

1) mass of magnesium _____________

show work:

2) mass of product _____________

show work:

3) mass of oxygen _____________

show work:

4) moles of magnesium _____________

show work:

5) moles of oxygen _____________

show work:

6) ratio of moles of magnesium to moles of oxygen ____________

7) empirical formula of magnesium oxide ______________