Answer:
Step-by-step explanation: Chemistry is the study of how atoms and molecules interact with each other which occurs on the atomic scale. Chemists need a way of simply determining how many molecules they have in a beaker. The mole concept, which we will introduce here, bridges that gap by relating the mass of a single atom or molecule in amu to the mass of a collection of a large number of such molecules in grams.
As you learned, the mass number is the sum of the numbers of protons and neutrons present in the nucleus of an atom. The mass number is an integer that is approximately equal to the numerical value of the atomic mass. Although the mass number is unitless, it is assigned units called atomic mass units (amu). Because a molecule or a polyatomic ion is an assembly of atoms whose identities are given in its molecular or ionic formula, we can calculate the average atomic mass of any molecule or polyatomic ion from its composition by adding together the masses of the constituent atoms. The average mass of a monatomic ion is the same as the average mass of an atom of the element because the mass of electrons is so small that it is insignificant in most calculations. This is not much help in the laboratory for the typical chemist who only has a balance to weight out chemicals but needs to know how the number of atoms or molecules. Clearly something clever is needed but first let us briefly review how to calculated molecular masses.
Molecular
The molecular mass of a substance is the sum of the average masses of the atoms in one molecule of a substance. It is calculated by adding together the atomic masses of the elements in the substance, each multiplied by its subscript (written or implied) in the molecular formula. Because the units of atomic mass are atomic mass units, the units of molecular mass are also atomic mass units. The procedure for calculating molecular masses is illustrated in Example 1.
Example 1.7.1
Calculate the molecular mass of ethanol, whose condensed structural formula is CH3CH2OH. Among its many uses, ethanol is a fuel for internal combustion engines.
Given: molecule
Asked for: molecular mass
Strategy:
A Determine the number of atoms of each element in the molecule.
B Obtain the atomic masses of each element from the periodic table and multiply the atomic mass of each element by the number of atoms of that element.
C Add together the masses to give the molecular mass.
Solution:
A The molecular formula of ethanol may be written in three different ways: CH3CH2OH (which illustrates the presence of an ethyl group, CH3CH2−, and an −OH group), C2H5OH, and C2H6O; all show that ethanol has two carbon atoms, six hydrogen atoms, and one oxygen atom.
B Taking the atomic masses from the periodic table, we obtain
2×atomic; massofcarbon=2atoms(12.011amuatom)=24.022amu
6×atomic; massofhydrogen=6atoms(1.0079amuatom)=6.0474amu
1×atomic; massofoxygen=1atoms(15.9994amuatom)=15.9994amu
C Adding together the masses gives the molecular mass:
24.022 amu + 6.0474 amu + 15.9994 amu = 46.069 amu
Alternatively, we could have used unit conversions to reach the result in one step.
The same calculation can also be done in a tabular format, which is especially helpful for more complex molecules:
2C (2 atoms)(12.011 amu/atom) = 24.022 amu
6H (6 atoms)(1.0079 amu/atom) = 6.0474 amu
1O (1 atoms)(15.9994 amu/atom) = 15.9994 amu
C2H6O molecular mass of ethanol = 46.069 amu
and boom, there you are, a fraction, nice and dandy