Combining Magnesium and Oxygen
Combining Magnesium and Oxygen
Purpose:
The purpose of this experiment is to show that magnesium and oxygen combine in a definite whole- number ratio by mass, by using mass relationships.
Data and Observations:
a. Mass of empty crucible 11.8g
b. Mass of crucible + Mg 12.35g
c. Mass of crucible + oxide 13.11g
d. Odor of vapor in step 6: The odor was similar to that of burning plastic.
Calculations:
. Mass of magnesium used: 0.55g
2. Mass of oxygen that reacted: 0.76g
3. Number of moles of Mg used: 0.023 mol
4. Number of moles of O that reacted: 0.048 mol
5. Ratio of moles of Mg to moles of O: MgO2 (1:2)
Questions:
. The empirical formula of magnesium oxide is MgO2.
2. The ratio of the mass in grams of magnesium used to the mass in grams of
oxygen that reacted is 3 to 4 (3:4). The law of definite proportions states that
specific substances always contain elements in the same ratio by mass. Therefore
magnesium oxide will always contain a ratio of 3:4 by mass in grams.
3. The ratio found in calculation 5 is different from the ratio found in question 2
because the ratio in calculation 5 is the ratio of moles of magnesium to the moles
of oxygen, whereas in question 2 the ratio is of the mass in grams of magnesium
used to the mass in grams of oxygen that reacted. They are different because
they are in different units, one is in moles and the other is in grams.
4. ...
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magnesium oxide will always contain a ratio of 3:4 by mass in grams.
3. The ratio found in calculation 5 is different from the ratio found in question 2
because the ratio in calculation 5 is the ratio of moles of magnesium to the moles
of oxygen, whereas in question 2 the ratio is of the mass in grams of magnesium
used to the mass in grams of oxygen that reacted. They are different because
they are in different units, one is in moles and the other is in grams.
4. In a chemical formula such as H2O, the "2" or subscript means that there are two hydrogens for every oxygen. To convert H2O from grams to moles you divide by the molar mass. To find the molar mass of H2O, add the mass of hydrogen multiplied by two (1.0079 x 2) plus the mass of oxygen (15.9994). Then divide this total into the amount of grams. Once the H2O is converted into moles, it can be converted into atoms or molecules by multiplying the amount of moles by Avogadro's number (6.023x1023). Therefore subscripts are needed or "significant" in finding the amount of moles of a substance, but they are not needed when converting to atoms or molecules, only Avogadro's number and the amount of moles is needed.
5. The empirical formula of hydrogen peroxide is HO.
6. The chemical composition of carbon monoxide, CO, is similar to that of carbon dioxide, CO2, because both compositions are made up of the same elements: carbon and oxygen. Although the compositions are different because their empirical formulas are different; carbon monoxide has one carbon and one oxygen, whereas as carbon dioxide has one carbon and two oxygens.
7. The empirical formula of a compound having a 3.20g mass, with 1.28g of sulfur and 1.92g of oxygen, is SO3 ( sulfite).
Conclusion:
. Procedure:
First clean a crucible and a lid. Dry them by heating them in the hottest part of the burner's flame for three minutes. Then allow the crucible and lid to cool. Now measure the mass of just the crucible and record this measurement as (a) under "Observations and Data".
Next place 0.5g of magnesium turnings in the crucible and measure the mass of the crucible and its contents, record this measurement as (b). Then cover the crucible and place it in a clay triangle. Heat the crucible gently for two minutes. Then, using crucible tongs, carefully tilt the cover (lid) to provide an opening for air to enter the crucible. Next heat the partially covered crucible strongly for ten minutes.
Then turn off the burner, cover the crucible, and allow the contents to cool. When the crucible is cool enough to touch, remove the cover and examine the contents. If any unreacted magnesium remains, replace the cover at a slight tilt, and reheat the crucible strongly for several minutes.
Then put the cover all the way on and allow to cool. After making sure that all the magnesium has reacted, use a dropper pipet to add enough water to the crucible to just cover the contents. Next wash any material that may have spattered onto the inside of the cover into the crucible.
Next, holding the burner in your hand, gently heat the contents of the uncovered crucible by moving the burner back and forth, slowly. Avoid spattering. Observe the odor of the vapor given off by wafting it toward your nose. Record your observations as (d).
When all the liquid has boiled off, repeat steps 5 and 6. Finally allow to cool and then remass.
2. Results:
The results showed that 0.55g or 1mole of magnesium was used, 0.76g or 2 moles of oxygen reacted, and that the empirical formula of magnesium oxide is MgO2. The heated crucible resulted in a vapor that had an odor of burning plastic.
3.
I learned from this lab how to set up mass ratios and how they relate to the Law of Definite Proportions. I also learned how to convert grams into moles and moles into molecules, atoms, and ions. I also learned how to determine the empirical formula of a compound and how to obtain gram atomic masses.
Discussion:
. Sources of Error:
One source of error may have been when we reheated the magnesium and
let it heat for too long. The mass of the magnesium may have differed if it had not
been heated as long as it was. Another source of error may have been when we
remassed the magnesium before letting it cool completely. If it had more time to
cool, the mass could have been less.
2. Lab Skills:
In this lab I learned how to properly hold a crucible and a crucible lid with
crucible tongs. I also learned how to use a clay triangle when heating a crucible.
I also became more familiar with the Bunsen burner and how to use it when
heating a substance "gently" compared to heating a substance "strongly."
