Name:_____________
Section:____________
Answer the following questions after reading the background information at the beginning of the lab. This should be completed before coming to lab.
1.
Name the following compounds from today’s experiment: a) MgO b) Mg
3
N
2
c) Mg(OH)
2
d) H
2
O (not water)
2.
Balance the following Equations: a)
2
3
2 b)
3
2
2
2
3
3.
Draw the electron dot formula and give the molecular geometry for the following compounds: a) NH
3 b) SO
2
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Objectives:

Calculate the molecular formula of Magnesium Oxide

Determine percent Composition
Background Information :
Determination of the percent composition of a compound and subsequent calculation of the molecular formula is readily accomplished for metal oxides, a class of binary compounds formed from the reaction of a metal and oxygen. Many metals form the oxide merely by heating the metal in air as shown in Equation 1. The percent composition is determined using the analysis outlined below. metal + oxygen → metal oxide Equation 1.
Magnesium may be converted to magnesium oxide at temperatures safe in an undergraduate lab. The reaction is shown in Equation 2. Theis conversion is complicated by the simultaneous reaction of magnesium with atmospheric nitrogen when heated as shown in Equation 3: magnesium + oxygen → magnesium oxide Equation 2. magnesium + nitrogen → magnesium nitride Equation 3.
This contamination issue (magnesium nitride) can be fixed by converting the magnesium nitride to the desired magnesium oxide with the addition of water and further heating. The reactions are shown in Equations 4 and 5: magnesium nitride + water → magnesium hydroxide + ammonia Equation 4. magnesium hydroxide →magnesium oxide + water Equation 5.
Once the magnesium nitride is converted completely into magnesium oxide, the mass of the product can be determined. With the mass of the magnesium oxide and the initial mass of the magnesium metal we can calculate the masses of the reacted oxygen, the compound’s mass and the % of oxygen and % of magnesium in the magnesium oxide.
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This portion of the experiment will convert the metallic magnesium ribbon into magnesium oxide. This is done by heating the ribbon to extremely high temperatures in air. Caution : The crucible, its lid and the wire mesh will be extremely hot so care when working around these needs to be taken.
Equipment List :

Large Ring Clamp

Medium Ring Clamp

Ceramic Crucible w/ Lid

Crucible Tongs

Disposable Pipette
 Steel Wool

Pipette Bulb

Magnesium Ribbon

Wire Mesh

Propane Torch

Ring Stand
Procedure:
1.
Using the Ring Stand, large and medium ring clamps, the torch and wire mesh, set up the heating station for oxidizing the magnesium ribbon.
2.
Get a crucible and lid. Clean it. Light the Torch.
3.
Heat the crucible for 5 minutes at full heat (the bottom of the crucible should sit just above the flame tip.
4.
Let the crucible cool until it can be held.
5.
Get the mass of the crucible with the lid on it. Record this value on the chart on the following page.
6.
Get a Piece of Magnesium Ribbon (1-1.5 inches in length).
Clean the ribbon with steel wool. Then cut ribbon into several pieces (~0.3 inches in length and put a bend in the middle).
7.
Place the pieces inside the crucible and get the mass of the crucible, metal, and the lid. Record this value in the Chart on the next page.
8.
Light the Torch. Using the crucible tongs place the crucible with the lid on, on the wire mesh above the flame.
9.
Heat the crucible, lid and magnesium for 10 minutes at full heat. It is important that the lid is ajar to ensure access to oxygen.
10.
Cool the crucible, lid and contents in the manner previously described. While cooling, heat approximately 30 mL of water in a beaker to the point of steaming but not boiling.
11.
When the crucible is cooled, add 20 drops of the hot water to the crucible. Note if there is odor being given off by the addition of the water.
12.
Place the crucible with the lid slightly ajar upon the wire mesh support and heat gently for 10 minutes. Then allow the crucible to cool.
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13.
Once the crucible is cool to touch, record the mass of the crucible, lid and magnesium oxide in the data table.
14.
Dispose of the magnesium oxide in the trash and clean the crucible and lid.
15.
Repeat steps 3-14 for a second trial.
Chart of Recorded Masses
Trial 1 Trial 2
Mass of Crucible with Lid
Mass of Crucible, Lid and
Magnesium
Mass of Magnesium
Mass of Crucible, Lid and
Magnesium Oxide
Mass of Magnesium Oxide
Mass of Oxygen
Procedure:
1.
Using the recorded masses, calculate the amount of magnesium that was initially put in the crucible, the amount of magnesium oxide after heating and the amount of absorbed oxygen. Record these values on the above chart.
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2.
Calculate the % composition for magnesium and Oxygen for each trial. Enter Values in the Chart below.
Element
% Oxygen
Trial 1 Trial 2 Average
% Magnesium
3.
In order to calculate the Molecular formula, we must convert from % composition to molecular formula following these steps (use the average values):
A.
Assume 100g, make the percentages of each element into grams by dividing the %s by 100.
B.
Convert this to moles, by dividing by the atomic mass of the element.
C.
Divide both by the smaller of the two molar units.
D.
Convert to integers to get the formula units.
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Name:_________________
Partner:________________
Section:________________
Determination of Magnesium Oxide: Fill in the Chart with Your Results
1. Average % Composition of Oxygen
2. Average % Composition of
Magnesium
3. Molecular formula of Magnesium
Oxide
Additional Questions :
1. Why is the water added to the crucible after the initial heating of the magnesium?
2. Mg(OH)
2
exists briefly in this experiment. If there are 1.5 moles of Mg(OH)
2
: a) How many molecules of Mg(OH)
2
are there? b) How many atoms of Mg are there? c) How many atoms of O are there?
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