Molecular mass from chemical formulas

It is remarkable that no empirical mixture parameters and no experimental data are required to use the equation. The only parameters in the Flory-Huggins equation are the hard core volumes V, which are a pme-component property, and the atomic or group contribution values are found in standard compilations. Since the v/s are significant in the FH equation only in terms of their ratios, pure-liquid molar volumes are often used for V in place of hard core volumes. For solutions of polymers of the same chemical formula, molecular masses are legitimate substitutes for V , for the same reason. Thus the volume fractions ( ) can be substituted by mass fractions W . Either volume fraction or mass fraction is directly related to laboratory data. To avoid mole fractions, the activity tti from Equations (4.368) and (4.369) can be used to calculate by / = aj. ... 

Appendix A lists molecular and fragment formulas in order of the unit masses. Under each unit mass, the formulas are listed in the standard Chemical Abstract system. The formula mass (FM) to four decimal places is given for each formula. Appendix A is designed for browsing, on the assumption that the student has a unit molecular mass from a unit-resolution mass spectrometer and clues from other spectra. Note that the table includes only C, H, N, and O. 

Add Names from Anion Names 57 Binary Covalent Compounds 58 The Simplest Organic Compounds Straight-Chain Alkanes 58 Molecular Masses from Chemical Formulas 59... 

P.C. Jurs, B.R. Kowalski and T.L. Isenhour, Computerized learning machines applied to chemical problems. Molecular formula determination from low resolution mass spectrometry. Anal. Chem., 41 (1969)21-27. 

In the problem above, we determined the percentage data from the chemical formula. We can determine the empirical formula if we know the percent compositions of the various elements. The empirical formula tells us what elements are present in the compound and the simplest whole-number ratio of elements. The data may be in terms of percentage, or mass or even moles. However, the procedure is still the same—convert each element to moles, divide each by the smallest, and then use an appropriate multiplier if necessary. We can then determine the empirical formula mass. If we know the actual molecular mass, dividing the molecular formula mass by the empirical formula mass, gives an integer (rounded if needed) that we can multiply each of the subscripts in the empirical formula. This gives the molecular (actual) formula, which tells what elements are in the compound and the actual number of each. 

The most fundamental properties of a chemical substance are those of the substance in pure form, in most cases as a solid or liquid. Molecular mass can be deduced readily from the chemical formula or structure, although a range of values may exist for commercial mixtures. In some cases, the substance may adopt different structural (e.g., cis-trans) or enantiomeric forms, usually with relatively small physical property differences but with potentially substantial differences in ability to induce toxicity or other biological responses. The hexachlorocyclohexane isomers and enantiomers are examples, the insecticide lindane or y HCH being the most active form. 

Each element has a specific symbol that is different from the symbol for any other element. In a chemical formula, the symbol stands for an atom of an element. Molecular substances are composed of two or more atoms that are tightly bound together. The formula for a molecular substance consists of the symbols for the atoms that are found in that molecule. For instance, the formula for carbon dioxide is CO2. Note the use of the subscript to show that each molecule contains two oxygen atoms in addition to the one carbon atom. Also note that the 1 for the one carbon atom is not written. The molecular mass of CO2 is the sum of the atomic mass of carbon plus twice the atomic mass of oxygen and is expressed in u. As was discussed directly above, the molar mass of CO2 is the mass in grams equal to the molecular mass in u. A mole of carbon dioxide is 12.0 u + 2(16.0 u) = 44 u. This result can be expressed as 44 g to indicate one Avogadro s number, Na, of CO2 molecules. Recall that Na is 6.0221 x 1023 things—molecules in this case. 

As seen from the structural formulas in Figure 16.4, the organochlorine insecticides are of intermediate molecular mass and contain at least one aromatic or nonaromatic ring. They can be placed in four major chemical classes. The first of these consists of the chloroethylene derivatives, of which DDT and methoxychlor are the prime examples. The second major class is composed of chlorinated cyclodiene compounds, including aldrin, dieldrin, and heptachlor. The most highly chlorinated members of this class, such as chloredecone, are manufactured from hexachlorocyclo-pentadiene (see Section 16.3). The benzene hexachloride stereoisomers make up a third class of organochlorine insecticides, and the third group, known collectively as toxaphene, constitutes a fourth. 

Here we compare the thermodynamic parameters of trehalose, maltose and sucrose because they have the same chemical formula (C12H22O11) and mass (molecular weight 342.3), but different structures which could be responsible for their different hydration properties. The anomaly of hydration of trehalose is understood from the following observation [10]. Namely, the amount of water used for the preparation of 1.5 M trehalose solution is smaller than the amount used for the preparation of other sugar solutions. In a 1.5 M solution, trehalose itself occupies 37.5% of the volume of the solution. However, in a 1.5 M solution, sucrose occupies 13% and maltose occupies 14%. These data suggest that trehalose has a larger hydrated volume than the other sugars. This hypothesis can be demonstrated from various thermodynamic parameters as shown in Table 12.1. 

The determination of the empirical formula of a compound can be made experimentally, by determining the percentage amounts of elements present in the substance using the methods of quantitative chemical analysis. At the same time the relative molecular mass of the compound has to be measured as well. From these data the empirical formula can be determined by a simple calculation. If, for some reason, it is impossible to determine the relative molecular mass the simplest (assumed) formula only can be calculated from the results of chemical analysis the true formula might contain multiples of the atoms given in the assumed formula. 

Microscopic foam films have been used to study the steric interaction between two liquid/gas interfaces [130]. Two ABA triblock copolymers of the Synperonic PE series were employed P85 and F108. These commercial non-ionic surfactant were used as obtained from ICI Surfactants, Witton, UK. Blocks A are hydrophilic polyethylene oxide (PEO) chains, while block B is a hydrophobic polypropylene oxide (PPO) chain. The molecular masses and average EO contents are known from the manufacturer and yield approximate chemical formulae (Table 3.3). Data about the surface tension of electrolyte-free aqueous copolymer solutions can be seen in Fig. 3.31 [130]. It was additionally checked that NaCl (up to 510 2 mol dm 3) had no influence on these values. 

The meaning of a chemical formula was discussed in Chapter 5, and we learned how to interpret formulas in terms of the numbers of atoms of each element per formula unit. In this chapter, we will learn how to calculate the number of grams of each element in any given quantity of a compound from its formula and to do other calculations involving formulas. Formula masses are presented in Section 7.1, and percent composition is considered in Section 7.2. Section 7.3 discusses the mole—the basic chemical quantity of any substance. Moles can be used to count atoms, molecules, or ions and to calculate the mass of any known number of formula units of a substance. Section 7.4 shows how to use relative mass data to determine empirical formulas, and the method is extended to molecular formulas in Section 7.5. 

An average molecular mass (weight) of the polymer also needs to be defined, since components of various molecular masses (weights) M, are present in the polymer. The number of moles of species i in the polymer can be obtained from the typical formula ni = W] / Mi where W is the weight fraction of the component i", and M, is the molecular mass of species i . The masses of molecules or groups can be calculated using two different conventions. One convention considers the natural isotopic abundance of elements and takes their sum based on the compound chemical formula. For the masses of polymers, the first convention is typically used. The other convention considers only the masses of the most abundant isotope, which is useful for MS... 

DENDRAL uses a set of rule-based methods to deduce the molecular structure of organic chemical compounds from chemical analysis and mass-spectrometry data. This process includes detecting structural fragments, generating structural formula from these fragments, and verifying it by determining the correlation between... 

In 1989 a controversy arose concerning the chemical daminozide, or Alar , which was sprayed on apple trees to yield redder, firmer, and more shapely apples. Concerns about Alar s safety stemmed from the suspicion that one of its breakdown products, unsymmetrical dimethylhydrazine (UDMH), was carcinogenic. Alar is no longer sold for food uses. UDMH has the empirical formula of CNH4 and has a molecular mass of 60.099. What is the molecular formula for UDMH ... 

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