CHM 3773 Lecture Notes - Magnesium Oxide, Antoine Lavoisier, Joseph Proust

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8 Feb 2013
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Law of Conservation of Mass
In 1774, Antoine Lavoisier (1774-1794) performed an experiment in which he
heated a sealed glass vessel containing a sample of tin and some air. He found that
the glass before heating and the glass after heating were the same. Through further
investigation, he proved that the product of the reaction, tin calx, consisted of the
original tin together with a portion of the air. This proved that oxygen from air is
essential to combustion and it led to the formulation of the Law of Conservation
of Mass: the total mass of substance present after a chemical reaction is the same
as the total mass of substances before the reaction.
Problem: A 0.455g sample of Mg is allowed to burn in 2.315 g of O2 gas. The sole
produce is MgO. After the reaction, no magnesium remains and the mass of
unreacted oxygen is 2.015g. What mass of magnesium oxide is produced?
Solution
Mass before reaction: 0.455g magnesium + 2.315g oxygen = 2.770 g
2.770g mass after reaction =? g magnesium oxide after reaction + 2.015g oxygen
after reaction
? g magnesium oxide after reaction = 2.770 g mass after reaction – 2.015 g oxygen
after reaction = 0.755 g magnesium oxide after reaction.
Law of Constant Composition
In 1799, Joseph Proust: Law of constant composition or law of definite
proportions: All samples of a compound have the same composition – the same
proportions by mass of the constituent elements.
Consider H2O at 10.000g: 1.119 g H = 11.19 % Consider H2O at 27.000g: 3.021
g H = 11.19% 8.881 g O = 88.81%
23.979 g O= 88.81%
Problem: 0.455g of Mg reacted with an excess of oxygen to produce 0.755g of
magnesium oxide. According to the law of constant composition, the mast ratio:
0.455 g magnesium/0.755 g magnesium oxide should exist in all samples of
magnesium oxide. Thus, in 0.500 g of magnesium oxide, the mass of magnesium is:
Solution:
? g magnesium = 0.500 g magnesium oxide x (0.455 g magnesium/0.755 g
magnesium oxide) = 0.301 g magnesium
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