EMPIRICAL FORMULA
•The empirical formula of a compound gives the simplest whole number ratio of the atoms or ions present in the compound and can be found only by experiment.
•The ratio of elements by mass must be converted to ratio of elements by numbers (amount of substance).
Steps in calculating an empirical formula:
1.Write down elements symbols.
2.Calculate moles (n) for all elements.
3.Divide all of the moles by the smallest number of moles.
4.If necessary, multiply by a factor to convert all numbers to whole numbers.
Example:
A compound of sulfur contains 2.4 % hydrogen, 39.0 % sulfur and 58.6 % oxygen.
Find the empirical formula of the compound.
Solution:
n (H) : n (S) : n (0)
2.4 : 39.0 : 58.6
1 32 16
2.4 : 1.2 : 3.7
(Divide by smallest number – 1.2)
2.4 : 1.2 : 3.7
1.2 1.2 1.2
2 : 1 : 3
Empirical Formula is H2SO3
Empirical formula of a hydrated salt
•A hydrated compound is an ionic compound that produces water when heated.
•An example of a hydrated salt is copper sulfate pentahydrate. CuSO4.5H20
•When calculating the empirical formula of a hydrated compound you are finding the number of water molecules in the formula.
Example:
5.00 g of sodium carbonate were dried and 1.85 g remained.
Find the empirical formula of the hydrated compound.
Solution:
n (Na2CO3) : n (H2O)
1.85 : 3.15
106 18
0.0175 : 0.175
(Divide by smallest number – 0.0175)
0.0175 : 0.175
0.0175 0.0175
1 : 10
Empirical Formula is Na2CO3 . 10H2O
MOLECULAR FORMULA
•The molecular formula of a compound represents the actual composition of a compound that is made up of molecules.
•A molecular formula is either the same as the empirical formula, or is a whole number multiple of it.
Example:
The Empirical formula of hydrazine is NH2. The compound has a Molar Mass of 32g mol^-1. What is its molecular formula?
M(NH2) = 16 g mol^-1- this is twice the molar mass of the molecule- 16x2 = 32 g mol^-1.
So the molecular formula is double the empirical formula- N2H4.
PERCENTAGE COMPOSITION
•The percentage composition of a compound expresses in terms of percentage what element contributes to its overall mass.
·To calculate percentage composition do the following:
% of X in compound containing X = mass of X in compound x 100
Molar mass of compound 1
Example:
Find the % composition by mass of each element in stannous fluoride, SnF2.
% Sn
= (mass (Sn) / M (SnF2) )x 100
= (1 x 118.7 / 156.7) x 100
= 75.7 %
% F
= (mass (F) / M (SnF2) )x 100
= (2 x 19 / 156.7) x 100
= 24.3 %
•The empirical formula of a compound gives the simplest whole number ratio of the atoms or ions present in the compound and can be found only by experiment.
•The ratio of elements by mass must be converted to ratio of elements by numbers (amount of substance).
Steps in calculating an empirical formula:
1.Write down elements symbols.
2.Calculate moles (n) for all elements.
3.Divide all of the moles by the smallest number of moles.
4.If necessary, multiply by a factor to convert all numbers to whole numbers.
Example:
A compound of sulfur contains 2.4 % hydrogen, 39.0 % sulfur and 58.6 % oxygen.
Find the empirical formula of the compound.
Solution:
n (H) : n (S) : n (0)
2.4 : 39.0 : 58.6
1 32 16
2.4 : 1.2 : 3.7
(Divide by smallest number – 1.2)
2.4 : 1.2 : 3.7
1.2 1.2 1.2
2 : 1 : 3
Empirical Formula is H2SO3
Empirical formula of a hydrated salt
•A hydrated compound is an ionic compound that produces water when heated.
•An example of a hydrated salt is copper sulfate pentahydrate. CuSO4.5H20
•When calculating the empirical formula of a hydrated compound you are finding the number of water molecules in the formula.
Example:
5.00 g of sodium carbonate were dried and 1.85 g remained.
Find the empirical formula of the hydrated compound.
Solution:
n (Na2CO3) : n (H2O)
1.85 : 3.15
106 18
0.0175 : 0.175
(Divide by smallest number – 0.0175)
0.0175 : 0.175
0.0175 0.0175
1 : 10
Empirical Formula is Na2CO3 . 10H2O
MOLECULAR FORMULA
•The molecular formula of a compound represents the actual composition of a compound that is made up of molecules.
•A molecular formula is either the same as the empirical formula, or is a whole number multiple of it.
Example:
The Empirical formula of hydrazine is NH2. The compound has a Molar Mass of 32g mol^-1. What is its molecular formula?
M(NH2) = 16 g mol^-1- this is twice the molar mass of the molecule- 16x2 = 32 g mol^-1.
So the molecular formula is double the empirical formula- N2H4.
PERCENTAGE COMPOSITION
•The percentage composition of a compound expresses in terms of percentage what element contributes to its overall mass.
·To calculate percentage composition do the following:
% of X in compound containing X = mass of X in compound x 100
Molar mass of compound 1
Example:
Find the % composition by mass of each element in stannous fluoride, SnF2.
% Sn
= (mass (Sn) / M (SnF2) )x 100
= (1 x 118.7 / 156.7) x 100
= 75.7 %
% F
= (mass (F) / M (SnF2) )x 100
= (2 x 19 / 156.7) x 100
= 24.3 %