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Law of Definite Proportions

The Law of Definite Proportions states that a chemical compound always contains exactly the same proportion of elements by mass. This law stems from the chemical nature of compounds, where there is always a fixed ratio of atoms, each bonded in a well-defined manner in three dimensional space.

For example, carbon dioxide with the chemical formula CO2 has the molecular weight of 44 g/mole. The carbon atom weighs 12 g/mole, while the two oxygen atoms weigh 32 g/mole. Thus, according to the Law of Definite Proportions, carbon makes up about 3/11 of the mass of any sample of CO2, while oxygen makes up the remaining 8/11 of the mass.

These proportions can be determined by dividing the atomic weight of each element, by the total molecular weight.

So for the element carbon in CO2:

(12 g/mole) / (44 g/mole) = 3/11

For the element oxygen in CO2:

(2x 16 g/mole) / (44 g/mole) = 8/11

Thus, understanding the Law of Definite Proportions is fundamental for understanding stoichiometry, as well as determining the relative quantities of reactants and products in a chemical reaction.

Law of Definite Proportions

1. Calculate and record the following for the experiment performed with 20g KClO3: (a) mass of the crucible with manganese(IV) dioxide (in grams): 53.300g (b) mass of the crucible after potassium chlorate has been added (in grams): 73.300g (c) mass of the crucible after heating (in grams): 21.00 degrees C