Weight, atomic Formula mass

Thus, the mass observed for the molecular ion of CO, for example, is the sum of the exact formula masses of the most abundant isotope of carbon and of oxygen. This differs from a molecular weight of CO based on atomic weights that are the average of weights of all natural isotopes of an element (e.g., C = 12.01, O = 15.999). 

Mass ratios are determined by using the molecular masses (also called molecular weights) of the substances involved in a reaction. Just as the atomic mass of an element is the average mass of the element s atoms, the molecular mass of a substance is the average mass of the substance s molecules. Numerically, molecular mass (or more generally formula mass) equals the sum of the atomic masses of all atoms in the molecule. 

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. 

The relative formula mass (RFM) equals the atomic or molecular weight of the substance, so for a compoimd such as sodium chloride (NaCl) this is simply the sum of the atomic weights of Na (23) and Cl (35.5), taken from the Periodic Table of the elements (see Appendix 2). The terms relative formula mass and relative atomic mass are often taken as interchangeable. 

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. 

The formula mass (formula weight) of a substance is determined by adding the atomic masses (atomic weights) of each atom (not each element) in a formnla unit. Molecular mass is one type of formula mass (for substances that form molecnles) and is calculated in the same way as the formula mass for an ionic compound. For example, the formula mass of NH3 is 17.0 amu, the atomic mass of three hydrogen atoms plus that of one nitrogen atom. Three or more significant digits should be used to report formula masses. (Section 7.1)... 

The molecular masses of the complex compounds are conveniently determined by soft-ionization methods, such as fast atom bombardment mass spectrometry (FAB-MS), in the positive and/or negative mode. This method can easily be applied to the ionization of non-volatile, non-derivatized, polar compounds, and is particularly suited to the analysis of saponins. Extremely valuable is the use of high resolution FAB-MS (HRFAB-MS), which provides the exact mass of high molecular weight compounds, and consequently their molecular formulae. 

Formality, F The number of formula masses of solute contained in each liter of solution synonymous with analytical molarity. Formal potential, The electrode potential for a couple when the analytical concentrations of all participants are unity and the concentrations of other species in the solution are defined. Formula weight The summation of atomic masses in the chemical formula of a substance synonymous with gram formula weight and molar ma.ss. 

The formula weight (FW) of a substance is the sum of the atomic weights (AW) of the elements in the formula, each taken the number of times the element occurs. Hence a formula weight gives the mass of one formula unit in atomic mass units. 

As we saw with mass and moles of elements and molecular compounds, it is important to be able to convert between mass and moles of ionic substances. The development of the tools for this conversion starts with the determination of the formula mass, which is the weighted average of the masses of the naturally occurring formula units of the substance. (It is analogous to the atomic mass for an element and the molecular mass for a molecular substance.)... 

Formula mass The weighted average of the masses of the naturally occurring formula units of the substance. It is the sum of the atomic masses of the atoms in a formula unit. 

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. 

Just as a mole of atoms is based on the atomic mass, a mole of a compound is based on the formula mass or formula weight. The formula weight is calculated by addition of the masses of all the atoms or ions of which the unit is composed. To calculate the formula weight, the formula imit must be known. 

Name Formula Atomic or molecular weight Equivalence factor Mass in grams... 

We can extend this idea of the molar mass of an element to determine the molar mass of compounds. By analogy, the molar mass of a compound is simply the sum of the atomic weight of each element in grams multiplied by the subscript on the element in the formula of the compound. For instance, one mole of methane molecules (CH ) contains one mole of carbon atoms (molar mass = 12.0 g) and 4 moles of hydrogen atoms (4 X 1.0 g/mol =... 

Mass spectrometry is useful to organic chemists because of the information it provides about molecular structure. For example, if the molecular ion is present, that peak can be used to determine the molecular weight (MW) of the neutral molecule. With precise measurement of the m/z ratio (to 0.0001,for example), the elemental formula of the molecular ion can be determined. For example, N2O and CO2 both have a molecular weight of 44 however, measurement of their exact masses (44.0011 and 43.9898, respectively) can be used to assign their elemental formula. Mass spectrometry was used to originally determine the exact mass of each element (see Table 8.34), and these exact masses, not the atomic weights, are used to calculate mass. 

Molecular masses, molecular weights, formula masses, and formula weights are expres.sed in atomic mass units (amu). 

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