Weight, atomic Molecular mass

The reactive nature of compound 22 is illustrated by the series of transformations shown in Scheme 7.12, in which its Zr—C bond reacts selectively with electrophilic reagents to produce a-haloboronates 36—38. Compound 22 also catalyzes the polymerization of styrene. The polymers thus obtained had weight-average molecular masses in the range 75000—100000 with polydispersities of 1.8—2.1. An X-ray analysis of 22 confirmed it to be a four-coordinate Zr complex with two cyclopentadienyl rings, chlorine, and the aliphatic C-l carbon atom as the ligands (Fig. 7.4). 

An unknown substance, X, was isolated from rabbit muscle. Its structure was determined from the following observations and experiments. Qualitative analysis showed that X was composed entirely of C, H, and 0. A weighed sample of X was completely oxidized, and the H20 and C02 produced were measured this quantitative analysis revealed that X contained 40.00% C, 6.71% H, and 53.29% O by weight. The molecular mass of X, determined by mass spectrometry, was 90.00 u (atomic mass units see Box 1-1). Infrared spectroscopy showed that X contained one double bond. X dissolved readily in water to give an acidic solution the solution demonstrated optical activity when tested in a polarimeter. 

The average mass of all molecules of a compound is its molecular mass (formerly called molecular weight). The molecular mass of a compound is calculated by multiplying the atomic mass of each element by the relative number of atoms of the element, then adding all the values obtained for each element in the compound. For example, the molecular mass of NH3 is 14.0 + 3 x 1.0 = 17.0. For another example, consider the following calculation of the molecular mass of ethylene, C2H4 ... 

If we know the molecular formula of a compound it tells us about the size of the molecule, and a molecular weight (or molecular mass) can be calculated from the weights (masses) of the individual component atoms. 

In all cases except perhaps the second you need a quick and reliable answer. Suppose you are trying to identify the heart drug propranolol, one of the famous beta blockers used to reduce high blood pressure and prevent heart attacks. You would first want to know the molecular weight and atomic composition and this would come from a mass spectrum propranolol has a molecular weight (relative molecular mass) of 259 and the composition C16H21NO2. Next you would need the carbon skeleton-—this would come from NMR, which would reveal the three fragments shown. 

The three types of formula masses correspond to the three types of formula units (1) atomic masses (also called atomic weights), (2) molecular masses (also called molecular weights), and (3) formula masses for ionic compounds (also called formula weights). The term atomic mass may be used whether an atom is combined or not, but it always refers to the mass of one atom of an element. 

Orthopedic UHMWPE has a weight average molecular mass of 2 X 10 amu. The polymer has a high viscosity, even in the molten state. Thus, macroradicals have very low mobility, either in the molten or in the solid state, whereas the H radical, which has a diameter smaller than 1 A, can diffuse in the polymer mass, even in the crystalline phase, where distances between C atoms are in the order of 4 A. 

The weighted-average molecular mass of H2O is 18.015 u, compared with a mass of exactly 12 u for a carbon-12 atom. If we compare samples of water molecules and carbon atoms by using Avogadro s number of each, we get a mass of 18.015 g H2O, compared with exactly 12 g for carbon-12. The molar mass of H2O is 18.015 g H20/mol H2O. If we know the formula of a compound, we can equate the following terms, as illustrated for H2O, MgCl2, and Mg(N03)2-... 

Clearly, there is one molecular property that can be exactly calculated fi om the contributions of its constituent atoms the molecular weight, or, more correctly, the molecular mass, which is exactly the sum of the masses of its constituent atoms. 

The molecular weight (mean relative molecular mass) was obtained by determination of density but, in order to determine that the gas was monatomic and its atomic and molecular weights identical, it was necessary to measure the velocity of sound in the gas and to derive from this the ratio of its specific heats kinetic theory predicts that Cp/C = 1.67 for a monatomic and 1.40 for a diatomic gas. 

Because MS identifies components on the basis of their atomic or molecular mass and, in the case of organic compounds, fragmentation pattern, it is a particularly useful and powerful detector. The molecular mass, which is commonly referred to as the molecular weight, is a particularly useful piece of information when trying to identify a compound. For elements, atomic mass determination can also determine the isotope present, which in turn can be used to identify the source and movement of an element through the environment. 

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