Compounds constant composition

Pure pyridine may be prepared from technical coal-tar pyridine in the following manner. The technical pyridine is first dried over solid sodium hydroxide, distilled through an efficient fractionating column, and the fraction, b.p. 114 116° collected. Four hundred ml. of the redistilled p)rridine are added to a reagent prepared by dissolving 340 g. of anhydrous zinc chloride in a mixture of 210 ml. of concentrated hydrochloric acid and 1 litre of absolute ethyl alcohol. A crystalline precipitate of an addition compound (probable composition 2C5H5N,ZnCl2,HCl ) separates and some heat is evolved. When cold, this is collected by suction filtration and washed with a little absolute ethyl alcohol. The yield is about 680 g. It is recrystaUised from absolute ethyl alcohol to a constant m.p. (151-8°). The base is liberated by the addition of excess of concentrated... 

The membrane consists of a binary compound AVaBVb of constant composition. Ion activities in the solid state are 1. At the membrane surface the activities of A and B in the aqueous phase are given by the solubility product ... 

The law of constant composition This tells us that a compound always contains the same elements in the same proportions by mass. If the atom ratio of the elements in a compound is fixed (postulate 3), their proportions by mass must also be fixed. 

Mercury(II) oxide, a red powder, can be decomposed by heating to produce liquid mercury and oxygen gas. When a sample of this compound is decomposed, 3.87 g of oxygen and 48.43 g of mercury are produced. In a second experiment, 15.68 g of mercury is allowed to react with an excess of oxygen and 16.93 g of red mercury(II) oxide is produced. Show that these results are consistent with the law of constant composition. 

Scientists are always on the lookout for patterns. When a pattern is observed in the data, it can be stated as a scientific law, a succinct summary of a wide range of observations. For example, water was found to have eight times the mass of oxygen as it has of hydrogen, regardless of the source of the water or the size of the sample. One of the earliest laws of chemistry summarized those types of observations as the law of constant composition, which states that a compound has the same composition regardless of the source of the sample. 

A separation involving a mobile phase of constant composition (irrespective of the number of components it contains) is termed isocratic elution, while that in which the composition of the mobile phase is changed is termed gradient elution. In the latter, a mobile phase is chosen which provides adequate separation of the early eluting analytes and a solvent which is known to elute the longer-retained compounds is added over a period of time. The rate at which the composition is changed may be determined by trial and error , or more formal optimization techniques may be used [5-7]. 

The stream leaving a reactor not only contains the product but may also include a series of by-products, unreacted raw material, solvents and various catalysts, surfactants, initiators, and so on. The unwanted material is removed from the product in a series of separations. Since each separation step is intended to remove certain compounds, the composition of the leaving stream must be monitored and controlled if the product quality is to be maintained constant. In the production of ethylene, c3 compounds, c4 compounds, and Cs compounds are all removed separately and are sold as by-products. Each of these by-product streams must also meet certain specifications and therefore must also be monitored and controlled (see Table 4-1). 

Dalton argued that these laws are entirely reasonable if the elements are composed of atoms. For example, the reason that mass is neither gained nor lost in a chemical reaction is that the atoms merely change partners with each other they do not appear or disappear. The constant composition of compounds stems from the fact that the compounds consist of a definite ratio of atoms, each with a definite mass. The law of multiple proportions is due to the fact that different numbers of atoms of... 

The above sections have focused upon homogeneous systems with a constant composition in which tracer diffusion coefficients give a close approximation to selfdiffusion coefficients. However, a diffusion coefficient can be defined for any transport of material across a solid, whether or not such limitations hold. For example, the diffusion processes taking place when a metal A is in contact with a metal B is usually characterized by the interdiffusion coefficient, which provides a measure of the total mixing that has taken place. The mixing that occurs when two chemical compounds, say oxide AO is in contact with oxide BO, is characterized by the chemical diffusion coefficient (see the Further Reading section for more information). 

Thus, the system comprising membrane, solution 2 of constant composition (internal filling solution), and electrode 2 (internal reference electrode) constitutes an ion selective electrode. The electrically neutral carrier antibiotics of the valinomycin group and related lipid-soluble compounds can serve as the active components of highly selective liquid... 

Constant composition that is easy to synthesise, preferably a single compound... 

A compound is an electrically neutral substance that consists of two or more different elements with their atoms present in a definite ratio. Water, for instance, is a compound of hydrogen and oxygen, with two hydrogen atoms for each oxygen atom. Whatever the source of the water, it has exactly the same composition indeed, a substance with a different ratio of atoms would not be water Chemists took a big step forward when they first noticed this invariance of composition, for it suggested an underlying order in nature. They summarized the observation as the law of constant composition. The law was important historically, because it suggested to chemists that compounds consisted of specific combinations of atoms. 

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. 

Law of constant composition Compounds always have the same elements joined together in the same proportions. 

The law of definite proportions, sometimes called the law of constant composition, was established in 1799 by Joseph Proust. He said a given compound always contained the same elements in the same proportion by mass. For example, water is always 88.9% oxygen by mass and 11.1% hydrogen by mass. 

Hereafter, attention will only be paid to the growth of a compound layer which is ideal both in the chemical (constant composition) and the physical (ordered structure, no macrodefects) sense. Influence of secondary factors such as stress, strain, specimen geometry, etc., will be neglected. 

In the case of chemical compounds of constant composition, application of these equations is, on the one hand, facilitated by the obvious fact that it is not necessary to take into account the concentration dependence of the diffusion coefficient. On the other hand, however, there arise serious, if not insurmountable, difficulties with the direct use of those equations because no concentration gradient can evidently exist in any growing layer, if a compound has no homogeneity range. It is therefore not surprising that... 

Equation (1.64) expresses the condition of the quasistationary concentration distribution when the concentrations of components A and B in the growing layer AB or ApBq are functions of only coordinate x and do not depend on time t. Simply speaking, on a time scale of the diffusion of atoms across the layer, the redistribution of concentration of the components along the layer takes place at a relatively high rate. For compounds of constant composition, this condition is clearly always satisfied. The distribution of concentration of the components in each particular case can readily be established, for example, by means of electron probe microanalysis. 

As in any binary system at most two phases can coexist under equilibrium, these are the compound layer of constant composition and one of initial substances (see Fig. 1.22b) or the compound layer itself, if the amounts of initial substances correspond to its composition. This is one of two modes of attaining equilibrium, namely, by consumption of an initial phase. It is typical of systems with no solid-state solubility. Another is a smooth homogenisation, as shown in Fig. 1.22c. Figure 1.22a represents a combination of the two extremes. First, phase B is completely consumed and then the AB layer attains the concentration cA] (the upper limit of the range of homogeneity in regard to component A). 

The formation of a compound from pure components is independent of the source of the material or of the method of preparation. If elements chemically react to form a compound, they always combine in definite proportions by weight. This concept is known as the Law of Constant Composition. 

Experiment 6 The empirical formula of a compound the Law of Constant Composition... 

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