Some Basic Concepts of
Chemistry
Sub-Topics
Matter and its nature, Dalton's atomic theory: Concept of atom,
molecule, element, and compound; Physical quantities and
their measurements in Chemistry, precision, and accuracy,
significant figures, S.I. Units, dimensional analysis: Laws of
chemical combination; Atomic and molecular masses, mole
concept, molar mass, percentage composition, empirical and
molecular formulae; Chemical equations and stoichiometry.
LAWS OF CHEMICAL COMBINATION
There are five laws of chemical combination.
(1) Law of Conservation of Mass (Lavoisier 1774). It
deals with the mass of reactants and products and states
that, in a chemical change, the total mass of the products
is equal to the total mass of the reactants.
e.g., C + O2 → CO2
12g + 32g = 44g
(2) Law of Constant Composition (Proust 1799). A
chemical compound always contains the same elements
combined together in the same proportion by mass e.g.,
H2O prepared from any source contains H and O in the
ratio of 1 ꓽ 8 by mass.
(3) Law of Multiple Proportion (Dalton 1804). When
two elements combine to form two or more compounds
then the masses of one of which combine with a fixed
mass of the other element bear a simple whole-number
ratio to one another.
(4) Gay Lussac's Law of Combining Volumes. It
states that at a given temperature and pressure when the
gases combine they do so in volumes that bear a simple
ratio to each other and also to the volume of gaseous
product
, e.g., H2(g) + Cl2 (g) → 2HCI(g). The ratio of their volumes is
1ꓽ1ꓽ2
(1L) (1L) (2L)
MOLE CONCEPT
A mole is nothing but a collection of 6.022 x 10 23 particles
(atoms or molecules or ions) as well as it is equal to the atomic
weight (in g), molecular weight (in g), and ionic weight (in g) for
atoms, molecules, and ions respectively.
1 mole is represented in the form of atoms, molecules, and ions
as:
For atoms → 1 gm atom
For molecules → 1 gm molecule
Some For ions → 1 gm ion
Moles can be calculated in the following manner:
(a) Number of moles of molecules = Weight of substance
(in g) / Molecular weight
(b) Number of moles of atoms = Weight of substance (in
g) / Atomic weight
(c) Number of moles of gases = Volume of gas at NTP (in
litres) / 22.4
(1 mole of any gas occupies a volume of 22.4 litres at
N.T.P., N.T.P. Corresponds to 0°C and 1 atm pressure)
(d) Number of moles of atoms/molecules/ions = (Number
of atoms/molecules/ions) / Avogadro constant
(Avogadro constant is equal to 6.022 x 10 23)
MEASUREMENT OF CONCENTRATION
The concentration of a solution reflects the relative proportion
of solute and solvent present in the solution. The various
concentration terms are
(a) % w/W (weight percent or Mass percent)
x % w/W means that x g solute is present in 100 g of
solution.
Chemistry
Sub-Topics
Matter and its nature, Dalton's atomic theory: Concept of atom,
molecule, element, and compound; Physical quantities and
their measurements in Chemistry, precision, and accuracy,
significant figures, S.I. Units, dimensional analysis: Laws of
chemical combination; Atomic and molecular masses, mole
concept, molar mass, percentage composition, empirical and
molecular formulae; Chemical equations and stoichiometry.
LAWS OF CHEMICAL COMBINATION
There are five laws of chemical combination.
(1) Law of Conservation of Mass (Lavoisier 1774). It
deals with the mass of reactants and products and states
that, in a chemical change, the total mass of the products
is equal to the total mass of the reactants.
e.g., C + O2 → CO2
12g + 32g = 44g
(2) Law of Constant Composition (Proust 1799). A
chemical compound always contains the same elements
combined together in the same proportion by mass e.g.,
H2O prepared from any source contains H and O in the
ratio of 1 ꓽ 8 by mass.
(3) Law of Multiple Proportion (Dalton 1804). When
two elements combine to form two or more compounds
then the masses of one of which combine with a fixed
mass of the other element bear a simple whole-number
ratio to one another.
(4) Gay Lussac's Law of Combining Volumes. It
states that at a given temperature and pressure when the
gases combine they do so in volumes that bear a simple
ratio to each other and also to the volume of gaseous
product
, e.g., H2(g) + Cl2 (g) → 2HCI(g). The ratio of their volumes is
1ꓽ1ꓽ2
(1L) (1L) (2L)
MOLE CONCEPT
A mole is nothing but a collection of 6.022 x 10 23 particles
(atoms or molecules or ions) as well as it is equal to the atomic
weight (in g), molecular weight (in g), and ionic weight (in g) for
atoms, molecules, and ions respectively.
1 mole is represented in the form of atoms, molecules, and ions
as:
For atoms → 1 gm atom
For molecules → 1 gm molecule
Some For ions → 1 gm ion
Moles can be calculated in the following manner:
(a) Number of moles of molecules = Weight of substance
(in g) / Molecular weight
(b) Number of moles of atoms = Weight of substance (in
g) / Atomic weight
(c) Number of moles of gases = Volume of gas at NTP (in
litres) / 22.4
(1 mole of any gas occupies a volume of 22.4 litres at
N.T.P., N.T.P. Corresponds to 0°C and 1 atm pressure)
(d) Number of moles of atoms/molecules/ions = (Number
of atoms/molecules/ions) / Avogadro constant
(Avogadro constant is equal to 6.022 x 10 23)
MEASUREMENT OF CONCENTRATION
The concentration of a solution reflects the relative proportion
of solute and solvent present in the solution. The various
concentration terms are
(a) % w/W (weight percent or Mass percent)
x % w/W means that x g solute is present in 100 g of
solution.
