Formula units defined

Molarity (M) A concentration unit defined to be the number of moles of solute per liter of solution, 95q, 259 concentration unit conversion, 261-262 potassium chromate, 263 Mole A collection of6.0122 X 1023 items. The mass in grams of one mole of a substance is numerically equal to its formula mass, 55. See also Amount Mole fraction (X) A concentration unit defined as the number of moles of a component divided by the total number of moles, 116-117,261 Mole-gram conversions, 55-56,68-68q... 

FIGURE 7.27 The standard Gibbs free energy of formation of a compound is defined as the standard reaction Gibbs free energy per mole of formula units of the compound when the compound is formed from its elements. It represents a "thermodynamic altitude" with respect to the elements at "sea level." The numerical values are in kilojoules per mole. 

There arc two NH4 groups, each containing four H atoms, for a total of eight H atoms per formula unit. There is only one S atom the 3" defines the number of O atoms. 

The relation between orbitals and electrons is effected by the average parameters A, Ai, <5j, dt, and yn as defined per formula unit through... 

A more complex example may be represented by TaSe2 its modification called 2H-TaSe2 is hexagonal (space group P6 mmc, with two formula units in the unit cell). This layered compound shows a displacive 2D modulation (defined by two vectors) its symmetry may be therefore described in terms of a supergroup in a 5D superspace (Janner and Janssen 1980). A general point is therefore denoted by the 5 parameters x,y,z, t, u, and a position vector by the five components xa + yb + zc + td + ue of the superspace, with a, b, c basis in the position space and d and e in the internal subspace . 

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.)... 

The parameter Z is used to denote the number of molecules or formula units in the asymmetric unit of a crystal structure, i.e. the number of molecules that cannot be related to one another by the symmetry operations defined by the crystal space group. Of course, Z is therefore crucially dependent on the somewhat subjective definition of what constitutes the formula unit . Strictly Z is defined as the number of formula units in the unit cell divided by the number of independent general positions. The... 

In molecular crystals, the asymmetric unit of the unit cell may be composed of a part of the chemical unit, of the chemical unit itself, or even of more than one chemical unit. The chemical unit is understood as the single molecule or the formula unit of the substance. For the free molecule (or formula unit) we can define "chemically equivalent"sites within the molecule. 

The mole is defined as the amount of substance that contains as many elementary entities (atoms, molecules, or formula units) as exactly 12 g of carbon-12. 

The mole is defined as the number of atoms in exactly 12 g of which is 6.02 X 10 —Avogadro s number. Equal numbers of moles of two (or more) different substances have the same number of formula units but not the same mass. The molar mass is the mass in grams of one mole of a substance. The number of grams per mole—the molar mass— is a frequently used conversion factor, used for converting between grams and moles. (Section 7.3)... 

Define or identify each of the following molecule, ion, formula unit, formula mass, mole, molecular mass, Avogadro s number, percent, empirical formula, molecular formula, molar mass, empirical formula mass, molecular weight. 

The mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kg of carbon-12. Although it is defined in terms of the number of entities, in practice, 1 mol of atoms, molecules, or specific formula units of a substance is measured by weighing M x (1 mol) of the substance, where M is the molar mass, the mass per unit amount of substance. Molar mass is synonymous with the terms atomic weight, for atoms, and molecular weight, for molecules or formula units, respectively, and is reported in grams per mole (g mol ). 

In the case of alkali feldspars the unit cell is quite complicated. It contains four formula units, and 53 atoms in the asymmetric unit. As a result, the simulated sample of Tsatskis and Salje (1996) had the form of a very thin slab (or film) the computational unit cell defined for the whole slab contained slightly more than four formula units. In the simulation the slab had 101 orientation, which allowed the observation of only the... 

Let us define the polymerization degree of silicate melts as the fraction of bridging oxygen atoms in a single formula unit. The assumptions, on the basis of which activities of components in the mixture are calculated, are as follows ... 

If one looks up the term component in practically any text on physical chemistry or thermodynamics, one finds it is defined as the minimum number of chemical formula units needed to describe the composition of all parts of the system. We say formulas rather than substances because the chemical formulas need not correspond to any actual compounds. For example, a solution of salt in water has two components, NaCl and H2O, even if there is a vapor phase and/or a solid phase (ice or halite), because some combination of those two formulas can describe the composition of every phase. Similarly, a mixture of nitrogen and hydrogen needs only two components, such as N2 and H2, despite the fact that much of the gas may exist as species NH3. Note that although there is always a wide choice of components for a given system (we could equally well choose N and H as our components, or N10 and H10), the number of components for a given system is fixed. The components are simply building blocks , or mathematical entities, with which we are able to describe the bulk composition of any phase in the system. The list of components chosen to represent a system is, in mathematical terms, a basis vector, or simply the basis . 

The subscripts in the formula of a compound give the ratio of the number of atoms of each element to the number of atoms of each other element in the formula. The collection of atoms written to represent the compound is defined as one formula imit. That is, the formula unit of ammonium sulfide, (NH4)2S, contains two atoms of nitrogen, eight atoms of hydrogen, and one atom of sulfur. The term formula mass (sometimes called formula weight) refers to the sum of the atomic masses of every atom (not merely every element) in a formula unit. There are several names for formula masses corresponding to different kinds of formulas. For uncombined atoms, the formula mass is the atomic mass. For covalent compounds, which consist of molecules, the formula mass can be called the molecular mass. For ionic compounds, there is no special name for formula mass. These terms are summarized in Table 4-1. 

Atoms, most molecules, and formula units of ionic compounds are extremely tiny. Their formula masses are measured in atomic mass units, which are useful for comparison purposes only, in order to get weighable quantities of matter, a huge collection of formula units is required. The mole is defined as the number of atoms in exactly 12 grams of A millimole is 0.001 mol, and is useful for calculations with small quantities of substances. The mole is abbreviated mol, not m or M, which are used for related quantities, and millimole is abbreviated mmol. 

---