Experiment Report: Determining the Empirical Formula of Magnesium Oxide

Introduction

In chemistry, compounds can be distinguished by using the empirical formula, which provides the simplest positive integer ratio of elements in a compound. The empirical formula is particularly useful for identifying ionic compounds with a non-directional nature of bonding, where ions can be surrounded by various oppositely charged ions. This results in a repeating lattice structure of ions, making the empirical formula a valuable tool for defining ionic-bonded substances.

Calculating the Empirical Formula

To calculate the empirical formula of a compound formed during a chemical reaction, follow these steps:

1. Record the masses of all elements present in the compound.
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2. Convert the masses into moles (by dividing by the atomic weight in grams).
3. Divide the moles by the smallest number of moles to obtain a ratio.
4. If the numbers are not whole, multiply by a suitable small factor to get whole numbers.
5. Round off the numbers if necessary to determine the empirical formula.

For example, the molecular formula of butane is C₄H₁₀, but the ratio of carbon to hydrogen atoms can be reduced to 2:5 to represent the empirical formula as C₂H₅.

The Mole and its Formula

A mole is the quantity of a substance that contains as many elementary units (atoms, ions, molecules) as there are atoms in exactly 12 grams of carbon-12 isotope. A mole of an element is numerically equal to the element's atomic weight in grams.

For compounds, a mole is numerically equal to the molecular weight in grams. In simpler terms, a mole of a substance is its atomic/molecular weight in grams. For example, a mole of copper (atomic weight 63.6) is 63.6 grams.

Experiment Setup

The experiment involves the combustion of magnesium metal to determine the empirical formula of magnesium oxide. The setup includes the use of a crucible and Bunsen burner to heat the magnesium metal for burning.

Equipment

• Bunsen burner
• Crucible and lid
• Pipe clay triangle
• Tongs
• Steel wool

Aim

The purpose of this experiment is to determine the empirical formula of magnesium oxide by performing the combustion of magnesium and collecting precise mass data.

Method

1. Set up the Bunsen burner with a tripod and place the pipe clay triangle securely over it.
2. Position the crucible containing magnesium in the pipe clay triangle with the lid on.
3. Turn on the gas and ignite the Bunsen burner to a blue flame.
4. Heat the crucible strongly for 5 minutes until the bottom of the crucible glows red over the blue flame to remove contaminants.
5. Remove the flame and allow the crucible to cool with the lid on.
6. Cut a piece of magnesium ribbon (about 5 cm long).
7. Thoroughly clean the surface of the magnesium ribbon with steel wool and record its appearance.
8. Weigh the cooled crucible and lid (1st mass to 2 decimal places).
9. Coil the cleaned magnesium ribbon to fit inside the crucible and cover it with the lid.
10. Weigh the crucible containing the magnesium and lid (2nd mass to 2 decimal places).
11. Position the crucible without the lid onto the pipe triangle setup.
12. Turn on the gas again and ignite the Bunsen burner to a red flame (air hole fully open).
13. As the magnesium begins to glow, carefully cover the crucible with its lid using tongs.
14. Heat strongly for about 10 minutes, occasionally lifting the lid carefully to admit oxygen.
15. Keep heating and lifting the lid until all the magnesium turns into gray-white powder or no further reaction is witnessed (for around 5 minutes).
16. Turn off the gas and allow the apparatus to cool.
17. Weigh the completely cooled crucible containing magnesium oxide with the lid carefully (3rd mass to 2 decimal places).

Risk Assessment

Ensure safety during the experiment by wearing safety glasses to protect eyes from the intense UV light produced during the combustion of magnesium. Avoid inhaling the smoke produced when magnesium burns, as it can cause irritation. Use tongs to handle hot objects to prevent burns. Do not cool the crucible or lid under cold water immediately after heating, as it may cause cracking and potential injury.

Results

Masses

Total mass of Mg oxide in crucible + lid:

n/a (To be calculated)

Total mass of crucible + lid + magnesium:

24.39g (To be calculated)

Percent composition of Magnesium in compound:

Mass of Mg in 1 mole / Mass of MgO in 1 mole (To be calculated)

Percent composition of Oxygen in compound:

Mass of O in 1 mole / Mass of MgO in 1 mole (To be calculated)

Observations

Upon observation, the 5cm magnesium ribbon had a slightly greasy texture and a brittle and coarse surface. After cleaning its surface thoroughly with steel wool, it became shiny and glossy, no longer feeling waxy. The purpose of this cleaning was to remove the oxide layer on the magnesium ribbon's surface, which can hinder combustion. The crucible and lid were also cleaned by firing them under a blue Bunsen flame to remove any contaminants and moisture.

Data Analysis

In this experiment, the combustion of magnesium with oxygen produced magnesium oxide. The balanced chemical equation is: 2Mg(s) + O₂(g) → 2MgO(s)

To calculate the mass of each substance, we use the following formulas:

Mass of Magnesium = Mass 2 - Mass 1 = 24.39g - 24.31g = 0.08g

Mass of Oxygen = Mass 3 - Mass 2 = 24.44g - 24.39g = 0.05g

Mass of Magnesium Oxide = Mass 3 - Mass 1 = 24.44g - 24.31g = 0.13g

Now, we can calculate the number of moles using the formula: Number of Moles = Mass / Molar Mass

Number of Moles of Magnesium: N = 0.08g / 24.31g/mol = 0.0033 moles (rounded to 4 decimal places)

Number of Moles of Oxygen: N = 0.05g / (16g/mol * 2) = 0.0016 moles (rounded to 4 decimal places)

Divide by the smallest mole value (Oxygen):

Magnesium Mole ratio = 0.0033 / 0.0016 ≈ 2.06

Oxygen Mole ratio = 0.0016 / 0.0016 = 1

Rounding to the nearest whole number:

Magnesium Mole ratio = 2

Oxygen Mole ratio = 1

So, the empirical formula for magnesium oxide is MgO, which matches the theoretical formula.

Discussion

The experiment successfully demonstrated the ability to determine the empirical formula of magnesium oxide. It also showcased the quantitative stoichiometric relationships between moles, mass, and molar mass in a chemical reaction. The balanced chemical equation for the combustion of magnesium was confirmed through the law of conservation of mass, where the total mass of reactants equaled the total mass of products.

The experiment highlighted the formation of an ionic bond between a metal (magnesium) and a non-metal (oxygen) to produce magnesium oxide. The empirical formula indicated the simplest ratio of elements in the compound, and it was found to be MgO, which aligns with theoretical expectations.

Experimental Errors and Improvements

Three experimental errors that could affect the results include the potential escape of magnesium oxide fumes, incomplete combustion of magnesium, and inadequate firing of the crucible and lid. To improve results, consider the following:

1. Monitor the reaction carefully and keep the crucible lid in place to prevent the escape of magnesium oxide fumes.
2. Extend the heating time for magnesium and use a stirring rod to ensure complete combustion.
3. Fire the crucible and lid twice to thoroughly remove any contaminants and moisture.

Conclusion

In conclusion, this experiment successfully determined the empirical formula of magnesium oxide to be MgO through careful measurements, calculations, and the application of stoichiometry principles. The results align with theoretical expectations, and the experiment provided valuable insights into chemical reactions, stoichiometry, and the formation of ionic compounds.