STOICHIOMETRY

By Prof. Liwayway Memije-Cruz

STOICHIOMETRY - DEALS WITH THE NUMERICAL RELATIONSHIPS OF ELEMENTS AND COMPOUNDS AND THE MATHEMATICAL PROPORTIONS OF REACTANTS AND PRODUCTS IN CHEMICAL TRANSFORMATIONS

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.

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 molecules is called the mole. A mole is defined as that quantity of a substance that contains the same number of ultimate particles (atom, ions, or units of ions) as are present in 12g of Carbon -12.

MOLE

1 MOLE OF ANY ELEMENT IS AN AMOUNT EQUAL TO ITS ATOMIC MASS IN GRAMS, ITS MOLAR MASS.

MOLECULAR MASSMolecular mass equals the sum of the atomic masses of all atoms

in the molecule.The unit of molecular mass is the amu.Example:What is the molecular weight of sugar cane: C12 H22 O11 ?

12 C atoms = 12(12.011) amu = 144.132 amu22 H atoms = 22(1.0079) amu = 22.174 amu11 O atoms = 11(15.9994) amu = 175.993 amu 394.299 amu

MOLAR MASSThe molar mass is the sum of the masses of the

atoms present in one mole of a substance, which can be an element or a compound. It can be expressed as a unit of mass per mole like grams/mol, kg/mol.

Molar Mass is an amount of the compound equal to its molecular mass in grams. The unit of molar mass is the gram.

EXAMPLES:

1. Find the molar mass of:a. Al = 27 g/molb. 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

2. How many moles are present 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 g3. What is the mass of 5 mol of Ca(OH)2

5 mol x 74 g = 370 g mol 2 mol x 27 g = 26 g mol

EXERCISES:1. Calculate the formula mass of ammonium

sulfate (NH4)2 SO4.2. Find the molar mass of 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).

AVOGADRO’S NUMBERThe 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 the Avogadro’s number, the accepted value of which is 6.02483 x 1023 atoms/gram-atom of any element.

A mole is the amount of substance that contains as Avogadro’s number of particles equal to 6.02 x 1023. The particles can be atoms, molecules, or ions.

Examples:There are 6.02 x 1023 atoms in 1 mole of carbonThere are 6.02 x 1023 molecules in 1 mole of H2­O

There are 6.02 x 1023 Na 6.02 x 10 Cl in 1 mole of NaCl

EXAMPLES:4. How many atoms are present in:

a. 5 mol of copper5 mol x 6.02 x 1023 atoms = 30.10 or 3.01x 1023 atoms

molb. 48 g of carbon48 g x 1 mol x 6.02 x 1023 atoms = 24.08 x 1023 or

12 g mol 24.08 x 1023 atoms5. How many g of H2O will contain 3.01 x 1024 molecules? 

3.01 x 1024 molecules x 1 mol x 18 g = 90g 6.02 x 1023 molecules mol

PERCENTAGE COMPOSITION

PERCENTAGE COMPOSITIONthe 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.

EXAMPLES:1. What is the percentage composition of water? % H = 2g x 100% = 11.11% 18g % O = 16g x 100% = 88.89% 18g 100% 2. What is the percentage composition 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%

EXERCISES:1. Calculate the percentage

composition of Vitamin E (C29 H50O2).2. Calculate the percentage

composition of Aluminum hydroxide: Al(OH)3