| Stoichiometry
: Empirical Formulas |
|
Empirical Formula - A formula that gives the simplest
whole-number ratio of atoms in a compound.
Steps for Determining an Empirical
Formula
-
Start with the number of grams of each element, given in the problem.
-
If percentages are given, assume that the total mass is 100 grams so that
the mass of each element = the percent given.
-
Convert the mass of each element to moles using the molar mass from the
periodic table.
-
Divide each mole value by the smallest number of moles calculated.
-
Round to the nearest whole number. This is the mole ratio of the elements
and is
represented by subscripts in the empirical formula.
-
If the number is too far to round (x.1 ~ x.9), then multiply each solution
by the same
factor to get the lowest whole number multiple.
-
e.g. If one solution is 1.5, then multiply each solution in the problem
by 2 to get 3.
-
e.g. If one solution is 1.25, then multiply each solution in the problem
by 4 to get 5.
|
-
Once the empirical formula is found, the molecular formula for a compound
can be determined if the molar mass of the compound is known. Simply
calculate the mass of the empirical formula and divide the molar mass of
the compound by the mass of the empirical formula to find the ratio between
the molecular formula and the empirical formula. Multiply all the atoms
(subscripts) by this ratio to find the molecular formula. (See Example
#2)
Example Problem #1
A compound was analyzed and found to contain 13.5 g Ca, 10.8 g O, and
0.675 g H. What is the empirical formula of the compound?
Start with the number of grams of each element, given in the problem.
Convert the mass of each element to moles using the molar mass from the periodic
table.
Divide each mole value by the smallest number of moles calculated. Round
to the nearest whole number.
This is the mole ratio of the elements and is represented by subscripts in
the empirical formula.
Example Problem #2
NutraSweet is 57.14% C, 6.16% H, 9.52% N, and 27.18% O. Calculate
the empirical formula of NutraSweet and find the molecular formula. (The
molar mass of NutraSweet is 294.30 g/mol)
Start with the number of grams of each element, given in the problem.
-
If percentages are given, assume that the total mass is 100 grams so that
the mass of each element = the percent given.
Convert the mass of each element to moles using the molar mass from the periodic
table.
Divide each mole value by the smallest number of moles calculated. Round
to the nearest whole number.
This is the mole ratio of the elements and is represented by subscripts in
the empirical formula.
-
If the number is too far to round (x.1 ~ x.9), then multiply each solution
by the same
factor to get the lowest whole number multiple.
Now, we can find the molecular formula by finding the mass of the empirical
formula and setting up a ratio:
Next: "Limiting Reactant"