You have learned that the
subscript numbers in a chemical formula represent the number of atoms in one molecule or
in one formula unit of an ionic compound. Now you will learn about the other
numbers, called coefficients, that you often see to the left of a chemical formula.
While a subscript number acts as a multiplier for a single element (unless there are
parenthesis), a coefficient number acts as a multiplier for all of the atoms in the entire
compound. As with subscripts, when no number is present then "1" is
understood. Look at the example below:
CO2
Here we have one molecule of carbon dioxide. The subscript 2 in the formula above only
pertains to the oxygen in the compound. The total number of atoms in the compound
is 3.
Now let us put a
coefficient in front of the molecule and see how that changes things.
5 CO2
The coefficient 5 refers to the entire molecule. It
shows that there are 5 molecules of carbon dioxide. Since each molecule is made up
of 3 atoms, the total number of atoms is now 15. There are 5 carbon atoms and 10
oxygen atoms.
Now, for an example with parenthesis;
Ba(NO3)2
Here we have one formula unit of the ionic compound, barium nitrate. We say
"formula unit" instead of "molecule" because ionic compounds don't
form molecules. The subscript 3 pertains to the oxygen, showing 3 oxygen
atoms for each polyatomic ion of nitrate. The subscript
2 is a
multiplier for everything in the parenthesis, because it is showing that there are two
nitrate ions for every barium ion. The total number of atoms for each formula unit
of barium nitrate is 9. There are; 1 barium atom, 2 nitrogen atoms, and 6 oxygen
atoms.
Now let's put a coefficient
in front of the formula unit and see how it changes the tally:
3 Ba(NO3)2
Now we have 3 formula units of barium nitrate. The 3 coefficient
acts as a multiplier for the entire compound. If there are 9 atoms in one formula
unit of barium nitrate, then there must be 27 atoms in three formula units. There
are: 3 barium atoms, 6 nitrogen atoms and 18 oxygen atoms.
Table 5-7a will show you some more examples of
coefficients and formulas. Study the table to make sure that you understand each
atomic tally.
Table 5-7a
Atomic Tallies for Specific Quantities of Molecules and Ionic Compounds |
Given |
2 NaNO3 |
6 C2H6 |
3 (NH4)2S |
Atomic Tally |
2 atoms Na 2 atoms N
6 atoms O
Total Atoms 10 |
12 atoms C 36 atoms H
Total Atoms 48 |
6 atoms N 24 atoms H
3 atoms S
Total atoms 33 |
Use the worksheets below to
practice this concept.
Using Coefficients
with Formulas Quizzes |
| |
Please forward all questions, comments and criticisms to Gregory L. Curran.
© Copyright 2004 Fordham Preparatory School, All Rights Reserved.
Last Modified February 07, 2008 |
