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The law of conservation of mass states that matter cannot be created or destroyed during a chemical reaction. In other words, the total mass of the reactants in a chemical reaction will always be equal to the total mass of the products. This principle was first established by French chemist Antoine Lavoisier in 1789, and it is a fundamental concept in chemistry. Example: Consider the chemical reaction between hydrogen gas and oxygen gas to form water: \[2H_2(g) + O_2(g) \rightarrow 2H_2O(l)\] In this reaction, two molecules of hydrogen gas (each with a mass of 2) combine with one molecule of oxygen gas (with a mass of 32) to produce two molecules of water (each with a mass of 18). The total mass of reactants (2 x 2 + 32) equals the total mass of products (2 x 18), illustrating the law of conservation of mass.

The law of definite proportions, also known as the law of constant composition, states that any pure chemical compound will always consist of the same elements combined in the same proportion by mass. This was first proposed by French chemist Joseph Proust in the early 19th century, and it is a foundational concept of modern chemistry. Example: Consider the chemical compound water (H2O). In every sample of pure water, the proportion of hydrogen to oxygen by mass is always the same. In this case, the mass ratio is 2:16, meaning that for every 2 grams of hydrogen present, there are 16 grams of oxygen. This consistent mass ratio demonstrates the law of definite proportions.

The law of multiple proportions was developed by English chemist John Dalton in the early 19th century. This law states that when two elements can form more than one compound, the masses of one of the elements that combine with a fixed mass of the other element will be in a simple whole number ratio. Example: Consider the two compounds of carbon and oxygen, carbon monoxide (CO) and carbon dioxide (CO2). In carbon monoxide, one atom of carbon (with a mass of 12) is combined with one atom of oxygen (with a mass of 16). In carbon dioxide, one atom of carbon (with a mass of 12) is combined with two atoms of oxygen (with a total mass of 32). The mass ratio of oxygen in these two compounds is 1:2, which is a simple whole number ratio, thus demonstrating the law of multiple proportions.