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The mole concept and Avogadro’s constant


A mole is the amount of a substance which contains the same number of chemical species as there are atoms in exactly 12 g of the isotope carbon-12.

A chemical species may be an atom, a molecule or an ion.

The mass of one mole of a species is called the molar mass. It is the relative mass
expressed in g and has units of g mol-1.
The molar mass of an element which exists as atoms is the relative atomic mass expressed in g.
The relative molecular mass (Mr) is defined as the sum of the relative atomic masses of the atoms in the molecular formula. The molar mass (M) of a compound is the relative molecular mass expressed in g.


Amounts of substance

Number of moles (n) = (mass (m))/(molar mass(M))

n = m/M



The relative formula mass of a compound is the sum of the relative atomic masses of the atoms in the formula.
The molar mass of а compound is the relative formula mass expressed in g.

Avogadro’s constant (L) has the value 6.02 х 1023 mol/l.
It has units as it is the number of particles per mole.
Number of particles (N) = number of moles (n) x Avogadro’s constant (L)
N = n x L

Example
Calculate the amount of water H2O, that contains 1.80 x 1024 molecules.
Solution
Use the shorthand notation: N = n x L
n = N/L = (1.75 x 1024)/(6.02 x 1023) = 2.91 mol = 3.0

The molecular formula shows the actual number of atoms of each element present in a molecule.

States of matter

Matter can exist in three states: gas, liquid, and solid
State symbols indicate the state of a substance:
(s) is for solid,
(l) is for liquid,
(g) is for gas
(aq) is for aqueous – dissolved in water.

KCl(aq) + AgNO3(aq) = AgCl(s)↓ + KNO3(aq)

Cu(s) + 4HNO3(aq) = Cu(NO3)2 (aq) + NO2 (g)↑ + H2O (l)

2HCl(aq) + CaCO3(s) = CaCl2(aq) + CO2 (g)↑ + H2O (l)

The difference in physical properties is explained by kinetic theory.

The basic ideas of kinetic theory are:

- All matter consists of particles (atoms or molecules) in motion.
- As the temperature increases, the movement of the particles increases.


The three states can be characterized in terms of the arrangement and movement of the particles and the forces between them. Most substances can exist in all three states. The state at a given temperature and pressure is determined by the strength of the interparticle forces.

Solid: the particles are closely packed in fixed positions. The interparticle forces restrict the movement to vibration about a fixed position. Solid has a definite shape and volume. Solid is essentially incompressible

Liquid: the particles are still relatively close together. The interparticle forces are sufficiently weak to allow the particles to change places with each other, but their movement is constrained to a fixed volume. Liquids can change shape but not volume. Liquids are slightly compressible.

Gas: the interparticle forces between the particles are negligible; they are zero for an ideal gas. The particles move freely occupying all the space available to them. Gases have no definite shape or volume. Gases are highly compressible.


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