Molecular/atomic formulae

This distinction looks forward to the question of the status of molecular/atomic formulas and the equations that use them. They can be read as shorthand for material transformations in chemical terms, or they can be read as shorthand for descriptions of the formation of ions, electron transfers such as oxidation and reduction and so on. 

If the only constituents of petroleum were the hydrocarbons, the complexity is further illustrated by the number of potential isomers (i.e., molecules having the same atomic formula) that can exist for a given number of paraffinic carbon atoms and that increases rapidly as molecular weight increases ... 

When referring to the enormous numbers of molecules or ions that take part in a visible chemical reaction, it s convenient to use a special unit called a mole, abbreviated mol. One mole of any substance is the amount whose mass—its molar mass—is equal to the molecular or formula mass of the substance in grams. One mole of ethylene has a mass of 28.0 g, one mole of HC1 has a mass of 36.5 g, one mole of NaCl has a mass of 58.5 g, and so on. (To be more precise, one mole is formally defined as the amount of a substance that contains the same number of molecules or formula units as there are atoms in exactly 12 g of carbon-12.)... 

Substance Atomic/ Formula Molecular Wt. Valency Gram Equiv. Wt 1 meq/l equals ppm... 

If molecular densities were determined on the basis of Eq. (3.38), atomic densities might be evaluated by contraction of those results. Equation (3.38) provides a derivation of the previously mentioned conditional density of Eq. (3.4). This point hints at a physical issue that we note. As we have emphasized, the potential distribution theorem doesn t require simplified models of the potential energy surface. A model that implies chemical formation of molecular structures can be a satisfactory description of such molecular systems. Then, an atomic formula such as Eq. (3.35) is fundamentally satisfactory. On the other hand, if it is clear that atoms combine to form molecules, then a molecular description with Eq. (3.38) may be more convenient. These issues will be relevant again in the discussion of quasi-chemical theories in Chapter 7 of this book. This issue comes up in just the same way in the next section. 

Le Bel published his stereochemical ideas two months later, in November 1874, under the title, The relations that exist between the atomic formulas of organic compounds and the rotatory power of their solutions" (14). An English translation is presented in Le Bel (15). Le Bel approached the problem from a different direction from van t Hoff. His hypothesis was based on neither the tetrahedral model of the carbon atom nor the concept of fixed valences between the atoms. He proceeded purely from symmetry arguments he spoke of the asymmetry, not of individual atoms, but of the entire molecule, so that his views would nowadays be classed under the heading of molecular asymmetry. Only once does he mention the tetrahedral carbon atom, which he regarded as not a general principle but a special case. Today, substituted allenes, spiranes, and biphenyls are but a few examples of asymmetric molecules that do not contain any asymmetric carbons, thus confirming Le Bel s views on molecular asymmetry. The reason for the different approaches by van t Hoff and le Bel is easy to understand, van t Hoff came from the camp of structural chemists, and he... 

In practice, one mole is the relative atomic or molecular or formula mass of any substance expressed in grammes, and it contains 6 X 10 particles. We do not have to go around counting the particles instead we can use the relative masses of elements or compounds. These are easily measured by weighing quantities. 

Find the molar mass of the solute, CaCl2. Remember The process of finding the molar mass of a substance is the same as finding the molecular or formula mass of the substance. We look up the masses listed on the Periodic Table of Elements for each of the elements involved and multiply by the appropriate subscripts. The only difference is that you use the unit symbol g for grams, instead of u for atomic mass units. 

Formula The molecular (empirical or atomic) formula is regularly given. Structural formulas are used in special cases of unusual importance or interest. 

Whether you are dealing with elements or compounds, the molar mass of a species is the mass in grams of 1 mole (6.022 x 1023) of that species 1 mole of atoms, 1 mole of molecules, or 1 mole of formula units. With compounds, you re dealing with molecules and formula units, so it is necessary to calculate the molecular or formula mass of each compound to get its molar mass. It s easy to get confused by the language ... 

Atomic mass unit. The unit of atomic, molecular, and formula... 

Molecular Weight - The molecular weight (formula weight) is the sum of the atomic weights of all the atoms in a molecule (molecular formula). Also called MW, formula weight, average molecular weight. 

Chemical formulas describe the simplest atom ratio (empirical formula), actual atom number (molecular formula), and atom arrangement (structural formula) of one unit of a compound. An ionic compound is named with cation first and anion second. For metals that can form more than one ion, the charge is shown with a Roman numeral. Oxoanions have suffixes, and sometimes prefixes, attached to the element root name to indicate the number of oxygen atoms. Names of hydrates give the number of associated water molecules with a numerical prefix. Acid names are based on anion names. Covalent compounds have the first word of the name for the element that is leftmost or lower down in the periodic table, and prefixes show the number of each atom. The molecular (or formula) mass of a compound is the sum of the atomic masses in the formula. Molecules are depicted by various types of formulas and models. 

RHODIUM (metal fume and insoluble compounds, as Rh)(The atomic formula for rhodium metal is Rh. The atomic weight for rhodium metal is 102.91. Insoluble rhodium compounds have variable molecular formulas and variable formula weights, depending upon the specific compound.)... 

Thus, the molecular weight of a particular compound must be calculated from the atomic formula or read off the reagent bottle. 

Riboflavin or vitamin makes a prosthetic group of flavin enzymes where it can be reversibly reduced by hydrogen atoms (formula [38]). When exposed to light, riboflavin absorbs energy and reacts, via a triplet exited state, with other molecules such as protonated substrates or molecular oxygen, generating reactive species. In the type I photodynamic reaction, energy is transferred from a triplet sensi-tiser to O2 with the formation of O2. 

We can use atomic masses, not ionic masses, because electron loss equals electron gain, so electron mass is balanced. In the next two sample problems, the name or molecular depiction is used to find a compound s molecular or formula mass. 

Plan Each of the conqrounds contains only two elements, so to find the formula, we find the simplest whole-number ratio of one atom to the other. Erom the formula, we determine the name and the molecular (or formula) mass. 

The molecular (or formula) mass of a compound is the sum of the atomic masses. 

A molecular structure formula differs from a molecular formula in that it indicates the relative connectivity or arrangement of the atoms in a molecule. For example, the molecular structure of sarin with the molecular formula of C4H,oF02P is shown Figure 2.1 ... 

A chemical formula is a notation used to convey the relative proportions of the atoms of the different elements in a substance. If the substance is molecular, the formula gives the precise number of each kind of atom in the molecule. If the substance is ionic, the formula gives the relative number of different ions in the compound. 

Figure A molecular view of some nonmetals (not to scale). Many nonmetals naturally exist as polyatomic (many-atom) molecules, (a) The noble gases (Group 8A/18) occur as 1-atom formula units. An example is helium, He. (b) Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine occur as 2-atom formula units. For example, chlorine is CI2. (c) Phosphorus naturally occurs as 4-atom formula units, (d) Sulfur commonly occurs in 8-atom formula units, (e) One form of carbon consists of 60-atom formula units, commonly referred to as buckyballs. The official name of C q is buckminsterfullerene. 

Figure 7.1 Molecular (or formula) mass. Molecular mass is the sum of the atomic masses of each atom in the molecule. The molecular mass of a carbon dioxide molecule is the sum of the masses of one atom of carbon and two atoms of oxygen. 

If you use the periodic table on your shield as your source of atomic masses— and we encourage this—most of your molecular or formula masses will be rounded to two decimal places. But there are some exceptions. 

It is all very well to calculate the atomic, molecular, and formula masses of atoms, molecules, and other compounds, but since we cannot weigh an individual particle, these masses have a limited usefulness. To make measurements of mass useful, we must express chemical quantities at the macroscopic level. The bridge between the particulate and the macroscopic levels is molar mass, the mass in grams of one mole of a substance. The units of molar mass follow from its definition grams per mole (g/mol). Mathematically, the defining equation of molar mass is... 

The molar mass of any substance in grams per mole is numerically equal to the atomic, molecular, or formula mass of that substance in atomic mass units. 

Atomic mass, molecular mass, formula mass, molar mass... 

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