Moles of an element

For purposes of chemical bookkeeping, it is unnecessary to know the isotopic molar masses and isotopic distributions of the elements. All we need to know is the mass of one mole of an element containing its natural composition of isotopes. These molar masses usually are included in the periodic table, and they appear on the inside front and back covers of this textbook. 

Note Samples with the same number of atoms or moles of an element have the same mass. 

The mass of one atom of any element is infinitessimal and is impossible to measure on any existing balance. A more convenient mass unit was needed for laboratory work, and the concept of the mole emerged, where one mole of an element is a quantity equal to the atomic weight in grams. One mole of carbon, for example, is 12.01 grams, and one mole of iron is 55.85 grams. 

The actual number of atoms in one mole of an element has been determined by several elegant experimental procedures to be 6.02 X 10 This quantity is known as Avogadro s number, in honor of one of the pioneers of the atomic theory. One can then see that one mole of carbon atoms (12.01 grams) will contain exactly the same number of atoms as one mole (55.85 grams) of iron. Using the mole concept, the chemist can now go into the laboratory and weigh out equal quantities of atoms of the various elements. 

The mass balance states that the moles of an element in all species in a mixture equal the total moles of that element delivered to the solution. 

Students will determine the mass of one mole of an element and the mass of one mole of a compound. 

How can you use this relationship to relate mass and moles The periodic table tells us the average mass of a single atom in atomic mass units (u). For example, zinc has an average atomic mass of 65.39 u. One mole of an element has a mass expressed in grams numerically equivalent to the element s average atomic mass expressed in atomic mass units. One mole of zinc atoms has a mass of 65.39 g. This relationship allows chemists to use a balance to count atoms. You can use the periodic table to determine the mass of one mole of an element. 

The number of moles of an element in a mole of componnd can also be used to calculate the number of moles of the compound involved in a reaction. The ratio of the number of moles of an element within a compound to the number of moles of the compound is determined by the compound s chemical formula (Section 7.3). Thns, the snbscripts of the formula may be used to form conversion factors. 

The mass of 1 mole of an element is equal to its atomic mass in grams. 

The mole is the SI unit for amount. The molar mass, or mass in grams of one mole of an element or compound, is numerically equal to the atomic mass of monatomic elements and the formula mass of compounds and diatomic elements. To find a monatomic element s molar mass, use the atomic mass, but instead of having units of amu, the molar mass will have units of g/mol. So, the molar mass of carbon is 12.01 g/mol, and the molar mass of iron is 55.85 g/mol. How to find the molar mass of compounds and diatomic elements is shown in the next section. 

Calculate the number of moles of an element when given the number of atoms of the element. 

Molar mass is used to convert from moles of an element to mass, and the inverse of molar mass is used to convert from mass of an element to moles. 

What helped make Mendeleev s system seem more plausible was the coincidental publication of Lothar Meyer s periodic table. Meyer had attended the 1860 Karlsruhe conference just as Mendeleev had, and, like Mendeleev, he had gone away convinced that there was some pattern that united the elements through atomic weights. He had sketched out an incomplete table of elements in his book Die modernen Theorien der Chemie (1864), and in 1868 he worked out a much more complete table. He did not publish this until 1870, by which time he was fully aware of Mendeleev s work. In addition to providing independent confirmation of Mendeleev s system, Meyer also added a supporting observation, showing that there was a periodic relationship between atomic number and atomic volume. Atomic volume represents the volume occupied by one mole of an element in its solid state. Meyer showed that atomic volume equaled atomic weight divided by the density of the solid. 

Faraday s law of electrolysis Relates to the number of electrons required to discharge one mole of an element. One equivalent weight of a substance is produced at each electrode during the passage of 96,487 coulombs of charge through an electrolytic cell. 

To complete these steps, we need one additional kind of conversion factor that converts between moles of an element and moles of a compound containing that element. We obtain this conversion factor from the compound s chemical formula. For example, the formula for hexane, C6H14, tells us that each hexane molecule contains six... 

The standard enthalpy of atomization of an element is the energy required to convert one mole of an element in its most stable form at 25°C to one mole of monatomic gas. Given that the standard enthalpy of atomization for sodium is 108.4 kJ/mol, calculate the energy in kilojoules required to convert one mole of sodium metal at 25°C to one mole of gaseous Na ions. 

Any quantity that can be converted to moles of reactant or product may be presented in the statement of a problem. Examples are number of formula units of reactant or product, or number of moles of an element in one of the reactants or products, as well as data on solutions or gases that will be presented later (in Chapters 6 and 7). In any case, merely convert the quantity given to moles, use the balanced chemical equation as presented in Section 5.1 to determine the number of moles of reactant or product that was asked about, and finish the problem as required. 

Calculating the Mass and Number of Atoms in a Given Number of Moles of an Element... 

One mole of an element has a mass equal to the element s atomic mass expressed in grams. The molar mass of any compound is the mass (in grams) of 1 mole of the compound and is the sum of the masses of the component atoms. 

Figure 3.2 The relationships between mass (m in grams) of an element and number of moles of an element (n) and between number of moles of an element and number of atoms (N) of an element M is the molar mass (g/mol) of the element and Na is...

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