Download Using Coefficients with Formulas

Using Coefficients with Formulas
You have learned that the subscript numbers in a chemical formula represent the
number of atoms in one molecule or 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 of the ionic compound, barium nitrate. 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 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 molecules of barium nitrate. The 3 coefficient acts as a multiplier
for the entire compound.. 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
2 atoms
12 atoms 6 atoms N
Na
C
24 atoms
2 atoms N 36 atoms H
H
Atomic
6 atoms O
3 atoms S
Tally
Total
Total
Total
Atoms 48
Atoms 10
atoms 33