Law of Fixed Proportions

In addition to his research in chemistry and physics, Dalton investigated colorblindness, an ailment he suffered from. However, he is best known for his work in the early 1800s in which he noticed that substances always combined in fixed proportions. A fixed weight of oxygen always combines with a fixed weight of hydrogen to produce a predictable amount of water. The Law of Fixed Proportions led Dalton to propose the first scientific theory of the atom ... 

More significant still, in 1799 Proust had put forward the Law of Fixed Proportions, according to which When combination between elements takes place, it is In definite proportions by weight, so that the composition of a pure cheonical compound is independent of the way in w hich it is prepared. This law, also known as the Law of Constant Composition, may be restated in the words The same compound always contains the same elements combined together in the same proportions by weight. 

It has been found that a number of compounds exist which do not rigidly obey the law of fixed proportions. In the spinel group of compounds, of which MgAlgO is the type, certain constituents of the lattice may be lost but the lattice still retains its structure. Spinel itself may undergo the continuous transition MgAl204M 2A1q/304 (y-alumina). The excess of one component is due either to the existence of gaps in the lattice where that component should be, or to an interstitial excess-of another component. The behaviour is shown by certain oxides, sulphides and halides. 

The law of multiple proportions This law, formulated by Dalton himself, was crucial to establishing atomic theory. It applies to situations in which two elements form more than one compound. The law states that in these compounds. the masses of one element that combine with a fixed mass of the second element are in a rath of small whole numbers. 

Lattice Points positions in a unit cell occupied by atom, molecules, or ions Law of Definite Proportion law that states that different samples of the same compound always contain elemental mass percentages that are constant Law of Mass Action mathematical expression based on the ratio between products and reactants at equilibrium, an equation to determine the equilibrium rate constant Law of Multiple Proportions law that states when two elements combine to form more than one compound that the mass of one element compared to the fixed mass of... 

But, if atoms were little balls that always united in the same simple ratios to make compound particles , this explained why chemical reactions between elements always took place in constant and simple proportions. It was why, for example, a certain mass of mercury always combined, during calcination, with another fixed mass of oxygen. French chemist Louis Joseph Proust enshrined this principle in his Law of Definite Proportions in 1788. Not that... 

The trend was definitely toward the principle of fixed composition, but the empirical evidence in its support was still unreliable and allowed room for the doubts of the honest sceptic. Credit is usually given to Joseph-Louis Proust for bringing the law of definite proportions into the continuing consciousness of the chemical community. Proust thought his data justified the assumption of fixed composition and took it as a firm operating principle, very much as Lavoisier had assumed the conservation principle as an axiom. For example, Proust claimed that the quantity of copper oxide prepared from copper carbonate was always the same whatever process used, and that every chemical entity was characterized by a fixed composition. 

The ratio of atoms within a chemical compound is usually constant. Compounds are made up of fixed proportions of elements they have a fixed composition. Chemists call this the Law of constant composition. 

Not all chemical species obey the law of definite proportions. Systems which obey the law, i.e., those which have fixed ratios of... 

Another law of historical importance, the Law of Multiple Proportions, states that the relative amounts of an element combining with a fixed amount of a second element in a series of compounds are the ratios of small whole numbers. For example, three oxides of nitrogen contain 63.65% N (A), 46.68% N (B), and 30.45% N (C). Table 2-4 shows the mass of N and the mass of O contained in 100 g of each compound, and the ratios,... 

According to the law of multiple proportions, the relative amounts of an element combining with some fixed amount of a second element in a series of compounds are in ratios of small whole numbers. 

Daltons law of multiple proportions states that when two elements combine in a series of compounds, the ratios of the weights of one element that combine with a fixed weight of the second are in a ratio of small whole numbers (see Chapter 7). 

There were other, less theoretical but no less persuasive objections. Some substances, such as ammonium chloride, dissociate in the vapor phase. That is, a single particle of vapor turns into two or more particles. Two or more particles occupy two or more times the volume that one particle does. That wreaks havoc with measurements of gas volumes and provides empirical evidence that fails to obey Gay-Lussac s law, making apparent nonsense of Avogadro s hypothesis. It was not until the phenomenon of dissociation was understood, and interpreted in terms of reaction kinetics, that this objection could be countered. Similar objections were raised against Dalton s laws of combining proportions, which work only for compounds of fixed composition. Metallic alloys and salt solutions, to take two of the most obvious exceptions, do seem to share some of the characteristics of chemical compounds, but they do not fit Daltons laws. The simplest way to avoid that objection was to say that only those substances that did fit Dalton s laws were true chemical compounds, but that is a circular argument that did not convince critics. 

Law of definite proportion-. Elements always combine to form compounds in fixed proportions by mass. 

Chemical formulas such as CO and C02 reflect an important law called the law of multiple proportions. This law applies when two elements (such as carbon and oxygen) combine to form two or more different compounds. In these cases, the masses of the element (such as 02 in CO and C02) that combine with a fixed amount of the second element are in ratios of small whole numbers. For example, two moles of carbon can combine with one mole of oxygen to form carbon monoxide, or with two moles of oxygen to form carbon dioxide. The ratio of the two different amounts of oxygen that combine with the fixed amount of carbon is 1 2. 

When determining the percentage composition by mass of a homogeneous sample, the size of the sample does not matter. According to the law of definite proportions, there is a fixed proportion of each element in the compound, no matter how much of the compound you have. This means that you can choose a convenient sample size when calculating percentage composition from a formula. 

From this table it is clear that the proportions of oxygen in combination with a fixed weight of nitrogen are as I 2 3 4 5. This law together with the law of definite proportions has profoundly influenced the development of the atomic theory of Dalton. 

As the law of multiple proportions states, the different masses of copper that combine with a fixed mass of chlorine in the two different copper compounds, shown in Figure 3-22, can be expressed as a small whole-number ratio, in this case 2 1. 

As Chapter 1 emphasizes, the law of definite proportions was one of the principal pieces of evidence that led to the acceptance of Dalton s atomic theory. It is now recognized that a great many solid-state binary compounds do not have fixed and unvarying compositions but exist over a range of compositions in a single phase. Thus, FeO (wiistite) has the composition range Feo.ssOi.oo to Feo.ssOi.oo and is never found with its nominal 1 1 composition. The compounds NiO and CuiS also deviate considerably from their nominal stoichiometries. 

Furthermore, we may say that a compound is a pure substance consisting of two or more different elements in a fixed ratio. Water is 11.1% hydrogen and 88.9% oxygen by mass. Similarly, carbon dioxide is 27.3% carbon and 72.7% oxygen by mass, and calcium oxide (the white solid A in the previous discussion) is 71.5% calcium and 28.5% oxygen by mass. We could also combine the numbers in the previous paragraph to show that calcium carbonate is 40.1% calcium, 12.0% carbon, and 47.9% oxygen by mass. Observations such as these on innumerable pure compounds led to the statement of the Law of Definite Proportions (also known as the Law of Constant Composition) ... 

The chemical Law of Definite Proportions, which expresses the notion that a pure compound always has a fixed and consistent composition in terms of the elements it contains, is an exact law for many, perhaps most compounds. There is nothing approximate in the assertion that pure sodium chloride contains equal numbers of sodium and chlorine atoms and, therefore, 61.72% chlorine, or that pure water contains just twice as many hydrogen atoms as oxygen and, therefore, 11.19% hydrogen. The whole of analytical chemistry is based on the reliability of such ratios. But there are a few classes of material that chemists would prefer to regard as pure compounds that do not show this consistency of composition. The Law of Definite Proportions is an exact law, but one that has clear exceptions. 

Unfortunately, Berthollet got a bit carried away by the idea that a reaction could be influenced by the amounts of substances present, and maintained that the same should be true for the compositions of individual compounds. This brought him into conflict with the recently accepted Law of Definite Proportions (that a compound is made up of fixed number, s of its constituent atoms), so his ideas (the good along with the bad) were promptly discredited and remained largely forgotten for 50 years. (Ironically, it is now known that certain classes of compounds do in fact exhibit variable composition of the kind that Berthollet envisioned.)... 

The Law of Multiple Proportions If two elements can combine to form two or more different compounds, the mass of one element compared to a fixed mass of the other will always be in a ratio of small, whole numbers. This really requires an explanation to make sense. 

The second hypothesis suggests that, in order to form a certain compound, we need not only atoms of the right kinds of elements, but specific numbers of these atoms as well. This idea is an extension of a law published in 1799 by Joseph Proust, a French chemist. Proust s law of definite proportions states that different samples of the same compound always contain its constituent elements in the same proportion by mass. Thus, if we were to analyze samples of carbon dioxide gas obtained from different sources, we would find in each sample the same ratio by mass of carbon to oxygen. It stands to reason, then, that if the ratio of the masses of different elements in a given compound is fixed, the ratio of the atoms of these elements in the compound also must be constant. 

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