Revision: Basic Concepts of Chemistry and Chemical Calculations Chemistry HSC Science Class 11 Tamil Nadu Board of Secondary Education

Relative atomic mass is defined as the ratio of the average atomic mass to the unified atomic mass unit.

Relative atomic mass (A_r) = `"Average mass of the atom"/"Unified atomic mass"`

The sum of the atomic masses of the atoms present in the formula is called the formula mass.

The mass of a single atom of an element is called the atomic mass.

The sum of the atomic masses of all atoms in a molecule is called the molecular mass.

The weighted average of the masses of all its isotopes in a sample of that element is called the average molecular mass.

The relative molecular mass of a compound is the number that represents how many times one molecule of the substance is heavier than `1/12` of the mass of an atom of carbon ₆C¹².

The quantity of the element which weighs equal to its gram atomic mass is called one gram atom of that element.

Avogadro's number is defined as the number of atoms present in 12 g (gram atomic mass) of C-12 isotope, i.e., 6·022 x 1023 atoms.

OR

Avogadro's number is the number of elementary units, i.e., atoms, ions or molecules present in one mole of a substance. It is denoted by N_A.

Avogadro’s number is defined as the number of atoms present in 12g of ₆C¹² isotope i.e. 6.023 × 10²³ atoms.

"The relative atomic mass or atomic weight of an element is the number of times one atom of the element is heavier than `1/12` times of the mass of an atom of carbon - 12".
Relative atomic mass = Mass of 1 atom of the element `1/12` of the mass of one C¹² atom.

Mole is the amount of a substance containing elementary particles like atoms, molecules or ions in 12 g of carbon - 12.

One mole of any gaseous molecules occupies 22.4 dm³ (litre) or 22400 cm³ (ml) at standard temperature and pressure (STP). This volume is known as the molar volume.

A mole is the amount of pure substance containing the same number of chemical units as there are atoms in exactly 12 grams of carbon -12.

Gram equivalent mass of an element, compound or ion is the mass that combines or displaces 1.008 g hydrogen or 8 g oxygen or 35.5 g chlorine. The equivalent mass has no unit but the gram equivalent mass has the unit g eq^-1.

Gram equivalent mass = `("Molar mass" ("gmol"^(−1)))/("Equivalence factor" ("eqmol"^(-1)))`

Any reaction that involves both oxidation and reduction occurring simultaneously is called an oxidation-reduction reaction or simply a redox reaction.

\[M_{\mathrm{avg.}}=\frac{M_{1}\times r_{1}+M_{2}\times r_{2}+M_{3}\times r_{3}}{r_{1}+r_{2}+r_{3}}\]

where M₁, M₂, M₃ are atomic masses of isotopes and r₁, r₂, r₃ are their relative abundances.

Matter is categorised based on its chemical composition into two broad groups:

1. Pure Substances have a definite, fixed chemical composition. They are further divided into:

• Elements — the simplest form of matter; cannot be broken down further by ordinary chemical means. Example: pure silver.

• Compounds — formed when two or more elements chemically combine in a fixed ratio. Example: common salt (NaCl).

2. Mixtures have no fixed composition and therefore no definite properties. They are divided into:

• Homogeneous Mixtures — constituents are uniformly distributed throughout the sample. Example: vinegar.

• Heterogeneous Mixtures — constituents are not uniformly distributed. Example: tomato sauce.

Quick memory trick:

Pure → Fixed composition. Mixture → Variable composition.

Redox Reactions:

• A substance that oxidises another substance (and is itself reduced) is called an oxidising agent.
• A substance that reduces another substance (and is itself oxidised) is called a reducing agent.

What is Oxidation and Reduction?