ABSTRACT
At the beginning of the nineteenth century, the English chemist John Dalton discovered important laws that are the basis of modern chemistry. The ‚rst is the law of de‚nite proportions. This law states that a chemical compound always contains exactly the same proportion of elements by mass. For example, oxygen makes up 8/9 of the mass of any sample of pure water, while hydrogen makes up the remaining 1/9 of the mass. In the case other than this ratio between oxygen and hydrogen, the elements do not make water, but whichever excess element remains. The second is the law of multiple proportions. The law states that when chemical elements combine, they do so in a ratio of small whole numbers. For example, a reaction of carbon of 12 g with oxygen of 32 g makes carbon dioxide (CO2). Carbon monoxide (CO) is a combination of carbon of 12 g with oxygen of 16 g. In comparison with these two compounds, oxygen jumps by twice the amount, from 16 g to 32 g. Dalton’s atomic model constructed the modern view for materials that compounds are produced by combination of atoms with the mass. This idea was supported by contemporary scientists. Gay-Lussac’s law of combining volumes states that a simple integer ratio holds between the volumes of individual gases before reaction and the volumes of produced gases under the condition of constant pressure and temperature. This law means the volume of the gas is in proportion to the number of molecules under the above condition. Avogadro proposed the hypothesis that all gases of the same volume are the group of molecules with the same number. The mass of a material, A gram, consisting of an element with the atomic mass A is 1 g atom of the material. The mass of a compound, M gram, consisting of the molecular mass M is 1 g molecule or 1 mole. The number of atoms (molecules) included in the material of 1 g atom (molecule) is the common constant for all materials, called the Avogadro constant, NA. The value of NA is 6.022 × 1023 mol-1.
