1.2: Experiment_602_Empirical Formula of MgO 1_4_2

Student Name		Laboratory Date: Date Report Submitted:	___________________________
Student ID		Experiment Number and Title	Experiment 602: Empirical Formula

Experiment 602: Empirical Formula

Section 1: Purpose and Summary

• Determine the empirical formula of magnesium oxide.
• Calculate the mass of oxygen using the weighing-by-difference method.
• Calculate the moles of a sample from its mass.

In this experiment, students will conduct the reaction between magnesium and oxygen gas. Students will determine the mass of the magnesium sample before and after the reaction, as well as the mass of magnesium and oxygen in the product. Students will learn how to convert mass to moles of a given sample and determine the empirical formula of a substance from mass and mole data.

Section 2: Safety Precautions and Waste Disposal

Safety Precautions:

Do not look directly at the burning magnesium ribbon. The flame is bright enough to damage your eyes. Use of eye protection is required for all experimental procedures.

A hot crucible will break if placed directly on a cold surface. Set hot crucibles onto wire screens to cool.

A hot crucible will break if splashed with water directly. Let the crucibles cool before adding water.

Waste Disposal:

The solid product from the reaction can be disposed of in the regular waste bin in the lab.

Section 3: Procedure

Part 1: Preparation of the crucible

Heat a clean and dry porcelain crucible with lid (on a clay triangle supported on a ring stand) directly over a Bunsen burner flame for about 5 minutes. This will evaporate water and burn off any greasy organic impurities, ensuring the crucible is clean and dry. Here is what the setup should look like:
Turn off the burner and allow the crucible to cool. Leave the crucible resting on the clay triangle. DO NOT put a hot crucible on the lab bench.
When the crucible is cool to touch, weigh the crucible and lid on the digital balance. It is important to use a balance that measures to 0.0001 grams. If you use the wrong balance, you will have to repeat this experiment.	Mass of empty crucible and lid: (a)________________ grams

Part 2: Preparation of magnesium sample

Obtain a strip or two of magnesium ribbon, approximately 6.0 to 7.0 cm in length. NOTE: Magnesium has a shiny silver-white surface, but may lose its shine by reacting with oxygen in the air, forming a magnesium oxide coating. If it is not shiny, polish it slightly with steel wool to remove any oxide coating. Use sufficient magnesium strips to mass between 0.4 and 0.6 grams on an analytical balance (a balance that measures to 0.0001 g)
Fold or loosely coil the magnesium ribbon(s) to make sure they sit entirely on the bottom of the crucible.
Place the coiled magnesium ribbon into the crucible, cover it, and weigh it on the digital balance. Record mass.	Mass of crucible, magnesium ribbon, and lid: (b)________________ grams

Part 3: Heating the magnesium sample

Place the crucible containing the magnesium ribbon back into the clay triangle. Put the lid close by on the lab bench, with crucible tongs to handle it later.
Light the Bunsen burner and start heating the crucible and magnesium ribbon. As soon as the magnesium ribbon starts to burn, replace the lid (using crucible tongs) to extinguish the flame. An unbalanced equation of magnesium burning in air is the following: \(\ce{Mg(s) + O2(g) -> Mg_{x}O_{y}(s) + Mg3N2(s)}\), where \(\ce{Mg3N2}\) is a by-produced, and \(x\) and \(y\) in the oxide of magnesium are positive integers. Some of the oxide particles may be lost as smoke if the lid is not placed over the crucible.
Continue heating the crucible with the lid on for a minute or so, then remove the lid again to replenish the oxygen consumed. When the magnesium ribbon starts to burn again, replace the lid to extinguish the flame.
Repeat this heating process until the magnesium ribbon no longer catches fire.
With the lid on, heat the crucible and sample strongly for 5 minutes. Make sure that the bottom of the crucible turns ‘red’ hot.
Turn off the burner, and let the crucible, lid, and sample cool on the clay triangle.
When the crucible is cool to touch, remove the lid, then add about 10 drops of laboratory water into the burned magnesium ribbon. Ensure that the entire sample is wet, not just one spot. Water reacts with the by product (\(\ce{Mg3N2(s) + 6H2O(l) -> 3Mg(OH)3(s) + 2NH3(g)}\)
Light the Bunsen burner and heat the crucible (without a lid) with a low flame for about a minute. This evaporates the surplus water you just added. After the water has evaporated, increase the flame and heat it strongly for about 10 minutes. The bottom of the crucible does not need to turn ‘red’ hot during this heating. This heating converts \(\ce{Mg(OH)3}\) into magnesium oxide by the following unbalanced equation: \(\ce{Mg(OH)3(s) ->[heat] Mg_{x}O_{y}(s) + H2O(g)}\)
Turn off the burner, and let the crucible and the sample cool on the clay triangle.
When the crucible is cool to touch, weigh the crucible, product, and lid on the digital balance.	Mass of crucible, product, and lid: (c)________________ grams

Section 4: Calculations

From the masses you recorded in Parts 1 and 2 of this experiment, calculate the mass of the magnesium ribbon. (b) – (a)	(d) _________________ g Mg
From the masses you recorded in Parts 2 and 3 of this experiment, calculate the mass of magnesium oxide produced from the reaction. (c) – (a)	(e) _________________ g MgO
From the mass of magnesium oxide (e) and the mass of magnesium ribbon (d), calculate the mass of oxygen that combined with magnesium during the reaction. (e) – (d)	(f) ________________ g O
Refer to a periodic table to obtain the molar mass of magnesium. Then, using this information and the mass of magnesium ribbon (d), calculate the number of moles of magnesium that you started with. Molar mass of Mg: ___________g/mol (from a Periodic Table) Show your equation here: EXAMPLE: If a student used 0.4532 g of Mg, then set up your equation like this. Use your molar mass of Mg value. Set up the equation so that the units cancel.	(g) ____________ mol Mg
Obtain the molar mass of oxygen from a periodic table. Using this information and the mass of oxygen you calculated (f), calculate the number of moles of oxygen that combined with the magnesium. Molar mass of O: ____________ g/mol (from a periodic table) Show your equation here: EXAMPLE: If a student measured a mass gain of 0.3005 g due to the combustion of oxygen, then set up your equation like this. Use your molar mass of oxygen value. Set up the equation so that the units cancel. \(\frac{\text {0.3005 grams} \quad \text {O}}{1} \times \frac{\text {moles of O}}{\text {grams of O}}=\) mol of O	(h) _______________ mol O
Calculate the mole ratio between magnesium and oxygen by dividing the mol Mg (g) by the mol O (h). Show your equation here: EXAMPLE: If a student measured a moles of Mg and b moles of O, the equation should be set up as Set up the equation so that the units cancel. \(\frac{\text {a} \quad \text {moles of Mg}}{\text {b} \quad \text {moles of O}}=\) mole ratio of Mg to O in magnesium oxide	(i) ________________
Using your mole ratio determined in (i), write the empirical formula of magnesium oxide by replacing \(x\) and \(y\) in \(\ce{Mg_{x}O_{y}}\) with positive integers. Round off the ratio to the nearest whole number to obtain the \(x\) and \(y\) integers.	Empirical Formula:

Post Lab Questions:

1. Some experimental errors could result in a high or low molar ratio between Mg and O. In each case below, decide whether the situation described would lead to a calculated ratio of too much oxygen or too little oxygen, and explain your answer.

(a) You forgot to perform the initial drying step and proceeded directly to weighing the crucible and lid you obtained from the stockroom.

(b) Your magnesium ribbon is not shiny. But you did not polish it with steel wool before use, as indicated in the experiment.

(c) You added more laboratory water than is needed in Part 3 Step 7, and you did not dry it out completely.

(d) After strong heating of the crucible, you removed the lid but dropped it and broke it. You then obtained a new lid for the final weighing.

2. A similar experiment is performed to determine the empirical formula of an oxide of copper, and the following data were collected. Predict the empirical formula of copper oxide from these data.

Mass of crucible, cover, and copper sample: 21.53 g

Mass of empty crucible with cover: 19.66 g

Mass of crucible, cover, and sample (after heating): 21.76 g

How can the experiment for the determination of the empirical formula of an oxide of copper be improved?