Molecules empirical formulas

The earlier sections have only considered the way atoms are bonded to each other in a molecule (topology) and how this is translated into a computer-readable form. Chemists define this arrangement of the bonds as the constitution of a molecule. The example in Figure 2-39, Section 2.5.2.1, shows that molecules with a given empirical formula, e.g., C H O, can have several different structures, which are called isomers [lOOj. Isomeric structures can be divided into constitutional isomers and stereoisomers (see Figure 2-67). 

With copolymers, it is not sufficient merely to describe the empirical formula to characterize the molecule. Another question that can be asked concerns the distribution of the different kinds of repeat units in the molecule. Starting from monomers A and B, the following distribution patterns are obtained in linear polymers ... 

Human growth hormone is a single polypeptide chain of 191 amino acids (qv) having two disulfide bonds, one between Cys-53 and Cys-165, forming a large loop in the molecule, and the other between Cys-182 and Cys-189, forming a small loop near the C-terminus. The stmcture of hGH is shown in Figure 1 molecular mass is 22,125 the empirical formula is C qH 29N 262 300 7 ... 

Tellurium nitride was first obtained by the reaction of TeBt4 with liquid ammonia more than 100 years ago. The empirical formula TeN was assigned to this yellow, highly insoluble and explosive substance. However, subsequent analytical data indicated the composition is Tc3N4 which, in contrast to 5.6a and 5.6b, would involve tetravalent tellurium. This conclusion is supported by the recent preparation and structural determination of Te6N8(TeCl4)4 from tellurium tetrachloride and tris(trimethylsilyl)amine (Eq. 5.5). The TceNs molecule (5.12), which is a dimer of Tc3N4, forms a rhombic dodecahedron in which the... 

To represent the composition of such a solid, we use the formula NaCl. However, this formula does not indicate that molecules of sodium chloride are present—it is not a molecular formula. It shows the composition by giving the relative number of each kind of atom present and is called an empirical formula. 

The empirical formula tells only the relative number of atoms of each element in a molecule. For example, consider ethane. Analysis shows that this is a compound of carbon and hydrogen and that there are three hydrogen atoms for... 

The monomer unit in starch and that in cellulose each has the empirical formula C6Hi0Os. These units are about 5.0 A long. Approximately how many units occur and how long are the molecules of cellulose and of the soluble starch ... 

To find the empirical formula of vitamin C from the data in Example F.l we must express the ratios of numbers of atoms as the simplest whole numbers. First, we divide each number by the smallest value (3.41), which gives a ratio of 1.00 1.33 1.00. Molecules contain only whole numbers of atoms, and one of these numbers is still not a whole number. Hence, we must multiply each number by the correct factor so that all numbers can be rounded off to whole numbers. Because 1.33 is Vi (within experimental error), we multiply all three numbers by 3 to obtain 3.00 3.99 3.00, or approximately 3 4 3. Now we know that the empirical formula of vitamin C is C3H403. 

We have determined that the empirical formula of vitamin C is C3II403. However, the empirical formula tells us only that the C, H, and O atoms are present in the sample in the ratio 3 4 3, not the number of each type of atom in a molecule. The molecular formula could be C3H4O3, CbH806, C9H1209, or any other whole-numher multiple of the empirical formula. 

To find the molecular formula of a compound, we need one more piece of information—its molar mass. Then all we have to do is to calculate how many empirical formula units are needed to account for the molar mass. One of the best ways of determining the molar mass of an organic compound is by mass spectrometry. We saw this technique applied to atoms in Section B. It can be applied to molecules, too and, although there are important changes of detail, the technique is essentially the same. 

F.13 Osmium forms a number of molecular compounds with carbon monoxide. One light-vellow compound was analyzed to give the following elemental composition 15.89% C, 21.18% O, and 62.93% Os. (a) What is the empirical formula of this compound (b) From the mass spectrum of the compound, the molecule was determined to have a molar mass of 907 g-mol 1. What is its molecular formula ... 

The ratio of two chloride ions for each calcium ion results in the formula CaCl2. However, note that this is an empirical formula (Section E). There are no CaCl2 molecules. Crystals of CaCl2 contain many of these ions in three-dimensional arrays. 

A hydrocarbon of empirical formula C,H, takes 349 s to effuse through a porous plug under the same conditions of temperature and pressure, it took 210. s for the same number of molecules of argon to effuse. What is the molar mass and molecular formula of the hydrocarbon ... 

Most commercial polymers are substantially linear. They have a single chain of mers that forms the backbone of the molecule. Side-chains can occur and can have a major affect on physical properties. An elemental analysis of any polyolefin, (e.g., polyethylene, polypropylene, poly(l-butene), etc.) gives the same empirical formula, CH2, and it is only the nature of the side-chains that distinguishes between the polyolefins. Polypropylene has methyl side-chains on every other carbon atom along the backbone. Side-chains at random locations are called branches. Branching and other polymer structures can be deduced using analytical techniques such as NMR. 

Empirical formula — simplest ratio of atoms in a molecule Molecular formula — actual number of atoms in a molecule Structural formula — shows the order of atom linkage... 

The principal oxide of silicon is silica. The empirical formula of silica is Si02, but the compound consists of a continuous network of Si—O bonds rather than individual Si02 molecules. Figure 9-17 shows part of this network. Each silicon atom is at the center of a regular tetrahedron, bonded to four oxygen atoms. As in water... 

We can use Pauling s empirical formula, which defines the Pauling electronegativity scale, for dissociation energies of a diatomic gold molecule. 

For anabolic reactions, which result in the production of new cells, it is important to know the approximate chemical composition of the biomass. The bacterial protoplasm comprises 75 to 80% water. The solid material is composed of several complex organic molecules, such as proteins, carbohydrates, and DNA. The mean composition of these molecules can be approximated by a relatively simple empirical formula, C60H87O23N12P, or in an even more simple form as C5H7O2N10.Numerous other elements such as sulfur, sodium, potassium, calcium, magnesium,... 

A compound consists of 85.7% C and 14.3% H. Its molecular weight is 42.0amu. (a) Calculate its empirical formula. (b) Calculate its empirical formula weight, (c) Calculate the number of empirical formula units in one molecule, (d) Calculate its molecular formula. 

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