Stiochiometry Lab: Empirical Formula and Hydration Number
Part A:
Empirical Formula of Magnesium Oxide
Introduction
Metal oxides are formed when a metal is reacted with oxygen. In this lab, elemental magnesium metal, Mg, is to be oxidized
to form magnesium oxide. Magnesium is an alkaline earth metal, which reacts vigorously when heated in the presence of air
to produce magnesium oxide and magnesium nitride. We only want magnesium oxide, however, so water is added to react
with the magnesium nitride and convert it to magnesium hydroxide. This is then further oxidized by burning to convert all of
the Mg to magnesium oxide. Based on the masses of the reactants and the product, the percent composition of the product
can be found. Using this information, the empirical formula of the oxide, MgxOy can be determined.
Purpose
To determine the empirical formula of magnesium oxide by oxidizing magnesium.
Materials
Magnesium ribbon
Ceramic crucible & cover
Crucible tongs
Distilled water
Bunsen Burner
Ring stand, clay triangle, wire gauze
Electronic Balance (200.00g ± 0.01g)
Safety
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Burning magnesium produces a very bright white flame. DO NOT look directly at the flame, it can damage your eyes.
Crucibles can become very hot but still look cold. Be careful around any crucibles because they may be extremely hot.
Always use crucible tongs to carry crucibles carefully, this is also so that oils and impurities from your body do not get onto them.
Procedure
1.
Place a clean, dry crucible and crucible cover on a clay triangle on a ring stand over a Bunsen burner. Adjust the height of the ring so the
bottom of the crucible will be in the hot part of the flame. Place the cover so it is slightly ajar, allowing air to leave the crucible.
2. Heat the empty covered crucible for about 10 minutes. The bottom of the crucible should be red hot. Turn off the burner and allow the
crucible to cool for 5 minutes.
3. When the crucible is cooled to room temperature, transfer it to the balance with crucible tongs and record the mass.
4. Prepare to heat the magnesium inside the crucible. Record whatever measurements you need. Never touch the crucible with your hands
or place on the lab bench, this may result in contamination.
5. Obtain about 0.15-0.20g of magnesium ribbon. If the metal is not shiny, rub it with steel wool to remove oxidized metal.
6. Fold the magnesium ribbon and then place it inside the crucible. Place the crucible on the clay triangle over the Bunsen burner and start
heating.
7. As soon as the magnesium glows, place the cover on the crucible, so that no smoke escapes. Periodically lift the cover to check the
progress of the reaction and to allow oxygen into the crucible. When the reaction is complete, remove from heat and place on wire gauze.
8. Add 10 drops of distilled water and observe, including smell.
9. Heat the crucible slightly covered until the water has completely evaporated. Allow the crucible to cool down for 5 minutes, then record
readings.
10. Repeat the procedure for 2 additional samples of magnesium.
Calculations
1.
2.
3.
4.
5.
Determine the mass percent composition of your product.
Determine the empirical formula of magnesium oxide.
Find out the correct empirical formula of magnesium oxide and compare your results. Give some possible reasons for any discrepancies.
Why is it important to heat the empty crucible at the beginning of the lab?
Write a balanced chemical equation for the conversion of magnesium to magnesium oxide, using the experimental empirical formula found
for MgxOy.
Conclusion and Evaluation
• State the empirical formula that you found from your data.
• Identify the empirical formula that you expected and compare this with your experimental empirical formula.
• Comment on the random and systematic errors in this investigation
• Evaluate the accuracy and effectiveness of the experiment
• Suggest practical ways to improve the experiment other than performing the experiment yourself
Part B:
Determination of the Hydration Number an Ionic Compound
Introduction
Several ionic compounds form solid salts that contain water molecules integrated into the crystal lattice in definite whole
number ratios. The number of water molecules per formula unit of the salt is called its hydration number.
In this lab, the hydration number of an ionic compound was determined by dehydrating the hydrated salt and then calculating
the mass change from the hydrate to its anhydrous (no H2O) form. The mass change corresponds to the mass of water
removed. Using molar masses the relative amounts of the water and the anhydrous salt can be determined. From this
information, the hydration number of the salt can be determined.
Purpose
To determine the hydration number of _______________________________.
Materials
hydrated salt (specify)
Ceramic crucible & cover
Crucible tongs
Distilled water
Bunsen Burner
Ring stand, clay triangle, wire gauze
Electronic Balance (200.00g ± 0.01g)
Safety
•
•
•
Rapidly heating hydrates may cause the water to be released quickly causing pieces to pop. Heat the hydrate gently.
Crucibles can become very hot but still look cold. Be careful around any crucibles because they may be extremely hot.
Always use crucible tongs to carry crucibles carefully, this is also so that oils and impurities from your body do not get onto them.
Procedure
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•
•
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The mass of a clean, dry crucible was determined using a digital scale (±0.01g).
Exactly 5.00 g of the hydrated crystals was placed in the crucible.
The hydrated crystals were observed and their appearance recorded.
The crucible containing the hydrate was heated on a hot plate to remove the water and form an anhydrous product.
Its behaviour and appearance were observed and recorded.
After about 10 minutes, the crucible and hydrate was removed from the hot plate, allowed to cool, and its mass determined
This was repeated it irregular intervals until no significant change in mass was observed between successive weighings
The appearance of the final product (anhydrous) was observed and recorded.
The hydration number (lowest whole number ratio of moles of water to moles of anhydrous salt) was determined.
Data Collection, Processing and Presentation
• Show ALL calculations needed to determine the hydration number of the hydrated salt from your data.
Conclusion and Evaluation
• State the formula of the hydrated salt that you found from your data.
• Research the expected hydration number for this salt.
• Evaluate the accuracy and effectiveness of the experiment
• Suggest practical ways to improve the experiment other than performing the experiment yourself