Chemistry Reference
In-Depth Information
Then
(8.15 g)
x
the mass of each nickel atom, and
(2.22 g)
x
the mass of each oxygen atom
Thus the ratio of masses is
(8.15 g)/
x
(2.22 g)/
x
3.67
Since the atomic mass of oxygen is 16.0 amu, the atomic mass of nickel is
amu.
After it has been determined as shown, the atomic mass of nickel can be
used to determine the atomic masses of other elements.
3.67(16.0 amu)
58.7
Practice Problem 3.7
Oxygen and selenium form a compound con-
taining twice as many oxygen atoms as selenium atoms. In a sample of the com-
pound, the ratio of masses of selenium to oxygen is 2.47 to 1.00. Calculate the
atomic mass of selenium.
How could the early chemists be sure that their samples of two elements
had equal numbers of atoms? They made a compound of the elements in which
the atomic ratio was 1 : 1. They did not need to know the exact number of atoms
of each element, only that the atoms were present in a 1 : 1 ratio.
Modern Determination of Atomic Mass
12
C
The modern method of determining atomic mass uses the isotope as the
standard, with a mass defined as exactly 12 amu. (The atomic mass of carbon
on this scale is 12.011 amu, and that of oxygen is 15.9994 amu.) The atomic
mass of an element is defined as the
weighted average
of the actual masses of
its naturally occurring isotopes (not the mass numbers of the isotopes). A
weighted average is the average taking into account the relative numbers of
atoms of each type of isotope. The concept of weighted average may be under-
stood using an analogy to a situation in everyday life, presented in Examples 3.8
and 3.9.
EXAMPLE 3.8
A supermarket is having a special, selling a package of seven 2.00-L bottles of
soda at regular price and including three 1.00-L bottles free. (a) What is the
average volume of two bottles—one bottle of each size? (b) What is the aver-
age volume of all 10 bottles together? (c) Explain the meaning of the term
weighted average
.
