An empirical
formula for a compound is the formula written in its most reduced
form. For example, the empirical formula for C6H12O6
is CH2O. (Note:
The 6, 12 & 6 in the original formula are all divisible by 6 so it reduces
to 1, 2 & 1.) The amount of each element in a compound must be divisible
in order to reduce. For example calcium oxalate, CaC2O4,
cannot be reduced because there is 1 calcium, 2 carbon and 4 oxygen. Thus,
CaC2O4,
is already an empirical formula.
Sometimes you
will have to calculate a substance's empirical formula from its percent
composition or mass composition. In either case (mass composition or percent
composition), the steps are the same. Look at the example below.
A compound consists of 72.2% magnesium and 27.8% nitrogen by mass. What
is the empirical formula?
Description
of Action |
Action |
| 1. Divide each
element’s percent composition or mass composition by its
atomic weight. Round your answer to at least three places
after the decimal. |
Mg:
72.2 ÷
24.3 = 2.971
N: 27.8 ÷ 14.0 = 1.986 |
| 2. Divide each
result by the smallest result. Round your answer to at least two
places after the decimal. |
Mg:
2.971 ÷
1.986 = 1.50
N: 1.986 ÷ 1.986 = 1.00 |
3. Multiply
each result by the same whole number until both equal a whole
number (or at least within a couple hundredths). If your result
ends in one or the following, multiply all results by same factor.
If your number ends in...
.25 --- multiply all by 4
.33 --- multiply all by 3
.50 --- multiply all by 2
.66 --- multiply all by 3
.75 --- multiply all by 4 |
Mg: 1.50 x
2 = 3
N: 1.00 x 2 = 2 |
| 4. Write the
formula with the each element’s result as its subscript. (Remember
to write the cation first in ionic compounds.) |
Mg3N2
|
Simplified,
the steps above are:
| Given:
Percentage or mass of each element or compound. |
1. Divide
each percentage or mass by either the element’s atomic weight.
|
2. Divide each result by the smallest result. |
3.
Multiply each result by the SAME whole number to get a whole number
result. The number you use depends on what your results are. The
chart below outlines what to multiply by.
Result
ends in |
Multiply
all by |
.25 |
multiply
all by 4 |
.33 |
multiply
all by 3 |
.50 |
multiply
all by 2 |
.66 |
multiply
all by 3 |
.75 |
multiply
all by 4 |
If your results after completing step 2 do not end in one of the
numbers in the table above you should round your result to the
nearest value from the table. (For example a result that ends
in .35 should be rounded to .33.)
|
|
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