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Mole Concept and Stoichiometry 145
Let us say it Again
Normal temperature and pressure. The accepted values of the normal temperature and pressure are,
Normal temperature = 0°C = 273 K
Normal pressure = 1 atm = 76 cm Hg
= 760 mm Hg = 760 torr
Standard temperature and pressure. The accepted values of the standard temperature and pressure are,
Standard temperature = 0°C = 273 K
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Standard pressure = 1 bar = 1 × 10 Pa (1 atm = 1.01325 bar)
Gay-Lussac’s law of combining volumes. When gases react, they do so in volumes which bear a simple ratio to one
another and to the volumes to the gaseous products, provided all volumes are measured at the same temperature and
pressure.
Avogadro’s law. Under similar conditions of temperature and pressure, equal volumes of all gases contain the same
number of molecules.
Atomicity. The number of atoms of all the elements present in a molecule and of any substance is called its atomicity.
Noble gases are monatomic; oxygen, nitrogen, hydrogen are diatomic; and ozone, carbon dioxide are triatomic gases.
Atom. Atom is the smallest particle which takes part in a chemical reaction.
Atomic mass unit. The mass equal to 1/12th of the mass of a C atom is called one atomic mass unit. The atomic mass
12
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unit is abbreviated as amu, and denoted by the symbol u.
12
An atom of carbon-12 ( C) has been assigned the mass of 12 atomic mass units.
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Atomic mass. The average mass of an atom of an element is termed as its atomic mass. Atomic masses can be expressed
in gram, kilogram or atomic mass units (u). The accepted unit of atomic and molecular masses is atomic mass unit (u).
Gram-atomic mass. The number of grams of an element numerically equal to its atomic mass is termed as gram-atomic
mass or gram-atom.
Molecule. The smallest unit of a substance which can exist independently is called a molecule.
Molecular mass. The average mass of a molecule of a substance in atomic mass units is termed as its molecular mass.
Formula mass. The sum of the atomic masses of all atoms in a formula unit of a substance is called its formula mass.
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Molar mass. Mass (expressed in gram units) of 6.02 × 10 chemical units of a substance is called the molar mass of that
substance. Molar mass has the units of g/mol.
Molar volume. All gases, under NTP conditions occupy 22.4 litre of volume. The volume occupied by one mole of a
gaseous substance is called molar volume (V ).
m
Mole. Mole is the amount of a substance which contains the same number of chemical units (atoms, molecules, ions) as
there are atoms in exactly 12 g of pure carbon-12.
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One mole of any substance has 6.02 × 10 chemical units (atoms, molecules, ions).
Mass of the substance
No. of moles of a substance =
Molar mass of the substance
The mole concept summarized:
