Download the mole - empirical formula

THE MOLE - EMPIRICAL FORMULA

The ____________________________________ of elements in a compound is called the
_________________________________.

In contrast, the ________________________________ of a compound is the
_______________ ____________________________ of each element in the compound.

It can be calculated from the mole ratio or from percent composition data.
Mole Ratio Example:
What is the empirical formula for a compound if a 2.50 g sample contains 0.900 g of calcium and
1.60 g of chlorine?
Step One: Determine the number of __________ of Ca and Cl. (You must first determine the
atomic mass of each.)
Ca
0.900g Ca
1 mole Ca = 0.0225 mol
40.08g Ca
Cl
1.60g Cl
1 mole Cl = 0.0451 mol
35.45g Cl
Step Two: Obtain the ________________________________ for the set of moles. Divide both
numbers of moles by the smaller number of moles. The smallest is ___________ mol.
Ca 0.0225 mol = 1.00
0.0225 mol
Cl 0.0451 mol = 2.004 = 2.00
0.0225 mol
Step Three: Look at the ___________ of the two numbers. Round off to whole numbers (there
are exceptions to this).
Ca
Cl
1.00
2.01
becomes
becomes
Ca
Cl
1.00
2.00
The empirical formula is ____________.
PRACTICE:
Find the empirical formula of the compound with 63.0g Rb and 5.90g O.
Percent Composition Example:
 Sometimes you are given percentages, not masses, and asked to determine the empirical
formula.
 If this is the case, you can change the percentages directly to _______________ because you
assume a 100 g sample (100%)
Step One:
C
H
O
40.0 g C 1 mole C = __________ mol C
12.01g C
6.71 g H 1 mole H = __________ mol H
1.01g H
53.3 g O 1 mole O = __________ mol O
16.00g O
Step Two: To obtain the smallest ratio, divide both numbers of moles by the smaller number of
moles (3.33 mol).
C
3.33 mol =________ H 6.64 mol = _________ O 3.33 mol = _________
3.33 mol
3.33 mol
3.33 mol
Step Three: Look at the ratio of the two numbers. Round off to whole numbers.
C
1.00
becomes
C
____
H
1.99
becomes
H
____
O
1.00
becomes
O
____
The empirical formula is
PRACTICE:
A. What is the empirical formula for a compound that has 32.8% Cr and 67.2% Cl?
What happens when the mole ratios are not whole numbers?
B. What is the empirical formula of a compound that contains 53.73% Fe and 46.27% S?
RULES TO FOLLOW:
- If the ratio is: X.1 or X.9, round up or down.
- If the ratio is X.2 to X.8, determine what to multiply it by to get a whole number.
- Examples:
 X.3 x 3 = X.9 which can be rounded
 X.5 x 2 = X.0 which is a whole number
 X.4 x 5 = X.0 which is a whole number