STOICHIOMETRY

At the beginning of chemistry, the quantitative aspects of chemical change, ie, stoichiometry of chemical reactions, do not get much attention. Even when attention has been given, experimental techniques and tools do not produce the correct result.One example involves the theory of phlogiston. Flogistonis tried to explain the phenomenon of combustion with the term "flammable substances". According to the flogitonis, combustion is the release of substances can etrbakar (from the burning substance). This substance is then called "phlogiston". Based on this theory, they define arson as the release of phlogiston from burning substances. Mass changes when burning wood fits well with this theory. However, the mass change of the metal when calcined incompatible with this theory. Nevertheless flogistonis accept that both processes are essentially identical. Increasing the mass of metal calcined is a fact. Flogistonis tried to explain this anomaly by arguing that the negative-mass phlogiston.Philosopher of Flanders in January Baptista van Helmont (1579-1644) conducted experiments "willow" famous. He is growing willow seedlings after measuring the mass of flower pots and soil. Since there is no change in flower pots and the soil mass during the seed grows, it assumes that the masses are obtained only due to water entering the ore.He concludes that "the root of all matter is water". Based on current outlook, hypothesis and experiment is far from perfect, but his theory is a good example of the attitude of the quantitative aspects of chemistry that is growing. Helmont recognize the importance of stoichiometry, and clearly ahead of his time.In the late 18th century, German chemist Jeremias Benjamin Richter (1762-1807) invented the concept of equivalent (in terms of modern chemistry chemical equivalent) with careful observation of the reaction of acid / base, the quantitative relationship between acid and base in a neutralization reaction. Richter equivalent, or which is now called the chemical equivalent, indicating a certain amount of material in the reaction. The neutralization equivalent in regard to the relationship between the number of acid and base number for mentralkannya. Appropriate knowledge about the equivalent is essential to produce soap and gunpowder good. Thus, such knowledge is very important in practice.At the same time Lavoisier established the law of conservation of mass, and provide a basis equivalent to the concept of an accurate and creative experiments. Thus, the stoichiometry of chemical reactions deal with the quantitative aspects of the basic methodology of chemistry. All the fundamental laws of chemistry, from the mass conservation law, comparative law remains the law until all based gas reaction stoichiometry. These fundamental laws are the basis of atomic theory, and is consistently explained by atomic theory. However, it is interesting to note that, the concept of equivalent used before the atomic theory was introduced.
b. The relative atomic mass and atomic massDalton recognized that it is important to determine the mass of each atom due to its mass varies for each type of atom. Atom is very small so it is not possible to determine the mass of one atom. So he focuses on the relative value of the mass and making atomic mass table (figure 1.3) for the first time in human history. In the table, the mass of the lightest elements, hydrogen adoption as a standard one (H = 1). Atomic masses are relative values, it means a dimensionless ratio. Although several different atomic masses with modern values, most of the proposed values ​​in the range of compatibility with the current value. This suggests that the right ideas and experiment.
 Then the Swedish chemist Jons Jakob Baron Berzelius (1779-1848) to determine the mass of atoms with oxygen as a standard (O = 100). Since Berzelius get this value based on the analysis of oxides, it has a clear reason to choose oxygen as the standard.However, the standard hydrogen is clearly superior in terms of its simplicity. Now, after much discussion and modification, used carbon standards. In this method, the mass of 12C carbon with 6 protons and 6 neutrons is defined as 12.0000. Atomic mass is the mass of an atom relative to this standard. Although carbon has been declared as a standard, the actual way this can be considered as a modified standard hydrogen.equal to the mass of the proton, which in turn is almost equal to the mass of the neutron, and electron mass is very small to negligible. However, most of the naturally occurring element that is a mixture of several isotopes, and the mass of the atom depends on the distribution of isotopes. For example, the mass of hydrogen and oxygen atoms are 1.00704 and 15.9994. The mass of the oxygen atoms are very close to the value of 16 is a bit smaller.
Molecular mass and formula massEach compound is defined enumerated by the chemical formula that indicates the type and number of atoms that menyususn compound. Mass formula (or the chemical formula mass) is defined as the sum of atomic mass based on the type and number of atoms are defined in the chemical formula. The chemical formula of molecules called molecular formula, chemical formula and the mass is called the mass molekul.5 example, the molecular formula of carbon dioxide is CO2, and the molecular mass is 12 + (2x 6) = 44.As in the atomic mass, both mass and molecular mass formula should not be integers. For example, the mass of a hydrogen chloride molecule HCl is 36.5. Even if the type and number of atoms that make up the molecule are identical, the two molecules may have different molecular masses when there are different isostop involved.It is impossible to define the molecular compounds such as sodium chloride. Mass formula for sodium chloride is used instead of molecular mass.Molecular mass difference of H2O and D2O is very substantial, and this difference in physical and chemical properties anatara both types of compounds can not be ignored.H2O is more easily electrolyzed than D2O. Thus, the remaining water after electrolysis tend to contain more D2O than in natural water.d. The quantity of matter and moleQuantitative methods are best suited to express the amount of matter is the number of particles such as atoms, molecules that make up the material being discussed. However, to calculate the atomic or molecular particles are very small and can not be very difficult.Instead of counting the number of particles is directly the number of particles, we can use the mass of a certain number of particles. Then, how a certain number of numbers selected? Tolong story short, the number of particles in a 22.4 L of gas at STP (0 ℃, 1atm) was selected as the standard amount. This number is called Avogadro's number. Loschmidt number name is also proposed to honor the Austrian chemist Joseph Loschmidt (1821-1895) who first with the experiment (1865).Since 1962, according to the SI (Systeme Internationale) decided bahwam in the world of chemistry, moles are used as the unit amount of material. Avogadro's number defined number of carbon atoms in 12 g of 126C and renamed Avogadro constant.There are several definitions of "mole":(I) The amount of material that contains a number of particles contained in 12 g of 12C. (Ii) one mole of material that contains Avogadro's number of particles constant.(Iii) A material that contains 6.02 x 1023 particles in a mole.
e. Atomic mass units (sma)Because the standard atomic mass in Dalton is the mass of hydrogen systems, the standard of mass in the right SI 1 / 12 the mass of 12C. This value is called the atomic mass unit (sma) and is equal to 1.6605402 x 10-27 kg and D (Dalton) was used as its symbol. Atomic mass is defined as the ratio of the average sma elements with natural isotopic distribution with 1 / 12 sma 12C.