Stoichiometry deals with the numerical relationships of elements and compounds and the mathematical proportions of reactants and products in chemical transformations

1. STOICHIOMETRY
By Prof. Liwayway Memije-Cruz

3. STOICHIOMETRY - DEALS WITH THE NUMERICAL
RELATIONSHIPS OF ELEMENTS AND COMPOUNDS AND
THE MATHEMATICAL PROPORTIONS OF REACTANTS
AND PRODUCTSIN CHEMICAL TRANSFORMATIONS

4. AN UNDERSTANDING OF THE MOLE CONCEPT
TOGETHER WITH SOME SKILLS IN WRITING AND
BALANCING CHEMICAL EQUATIONS ENABLE US TO
SOLVE STOICHIOMETRIC PROBLEMS INVOLVING
MASS RELATIONS OF REACTANTS AND PRODUCTS IN
CHEMICAL REACTIONS.

5. In chemical calculations it is necessary to consider
quantities of substances in terms of the number of
atoms, ions or molecules present. The unit devised by
chemists in expressing numbers of atoms, ions, or
moleculesiscalled themole.
A moleisdefined as that quantity of asubstancethat
contains the same number of ultimate particles (atom,
ions, or units of ions) as are present in 12g of Carbon
-12.
MOLE

6. 1 MOLE OF ANY ELEMENT ISAN AMOUNT
EQUAL TO ITSATOMIC MASSIN GRAMS, ITS
MOLAR MASS.

7. MOLECULAR MASS
Molecular mass equals the sum of the atomic masses of all atoms
in themolecule.
Theunit of molecular massistheamu.
Example:
What isthemolecular weight of sugar cane: C12 H22 O11 ?
12 C atoms = 12(12.011) amu = 144.132 amu
22 H atoms = 22(1.0079) amu = 22.174 amu
11 O atoms = 11(15.9994) amu = 175.993 amu
394.299 amu

9. MOLAR MASS
Themolar massisthesum of themassesof theatoms
present in onemoleof asubstance, which can bean
element or acompound. It can beexpressed asaunit of
massper molelikegrams/mol, kg/mol.
Molar Massisan amount of thecompound equal to its
molecular massin grams. Theunit of molar massisthe
gram.

12. EXAMPLES:
1. Find themolar massof:
a. Al = 27 g/mol
b. Ca(OH)2­ = 74 g/mol
Ca= 1 at x 40 g/mol = 40 g
O = 2 at x 16 g/mol = 32 g
H = 2 at x 1 g/mol = 2 g
74 g/mol

13. 2. How many molesarepresent in:
a. 5.4 g of Al
5.4 g x 1 mol = 0.2 mol
27 g
b. 180 g of H2O
180 g x 1 mol = 10 mol
18 g
3. What isthemassof 5 mol of Ca(OH)2
5 mol x 74 g = 370 g
mol
2 mol x 27 g = 26 g
mol

14. EXERCISES:
1. Calculate the formula mass of ammonium
sulfate(NH4)2 SO4.
2. Find themolar massof Ca3(PO4)2.
3. Calculate the number of moles of Oxygen (O2 )
in 24.0 g of O2.
4. Calculate the molecular mass of C7 H5 NO3 S
(Saccharin).

15. AVOGADRO’SNUMBER
The number of molecules in a mole of any molecular substance the same as the
number of atoms in a gram-atom of any number. This number of atom is
called theAvogadro’snumber, theaccepted valueof which is6.02483 x 1023
atoms/gram-atom of any element.
A mole is the amount of substance that contains as Avogadro’s number of
particlesequal to 6.02 x 1023
. Theparticlescan beatoms, molecules, or ions.
Examples:
Thereare6.02 x 1023
atomsin 1 moleof carbon
Thereare6.02 x 1023
moleculesin 1 moleof H2­O
Thereare6.02 x 1023
Na
6.02 x 10 Cl in 1 moleof NaCl

18. EXAMPLES:
4. How many atomsarepresent in:
a. 5 mol of copper
5 mol x 6.02 x 1023
atoms = 30.10 or 3.01x 1023
atoms
mol
b. 48 g of carbon
48 g x 1 mol x 6.02 x 1023
atoms = 24.08 x 1023
or
12 g mol 24.08 x 1023
atoms
5. How many g of H2O will contain 3.01 x 1024
molecules?
3.01 x 1024
moleculesx 1 mol x 18 g = 90g
6.02 x 1023
molecules mol

20. PERCENTAGE COMPOSITION

21. PERCENTAGE COMPOSITION
the percentage of each element present in a compound.
In chemistry, this composition is always on a weight
basis unless specifically stated otherwise. Sometimes
the composition of the mixture of gases is given on a
volumetric basis.
based upon the meaning of the symbols and formulas.
Each symbol stands for one atomic weight’s worth of
the element it represents, and each formula stands for
one molecular weight’s worth of the compound it
represents.

22. EXAMPLES:
1. What isthepercentagecomposition of water?
% H = 2g x 100% = 11.11%
18g
% O = 18g x 100% = 88.89%
18g 100%
2. What isthepercentagecomposition of H2SO4?
% H = 2 g x 100% = 2.0%
98.1
% S = 32.1g x 100% = 32.7%
98.1g
% O = 64.0 g x 100% = 65.3%
98.1g 100%

23. EXERCISES:
1. Calculatethepercentage
composition of Vitamin E
(C29H50O2).
2. Calculatethepercentage
composition of Aluminum
hydroxide: Al(OH)3