Introduction to fundamental concepts of chemistry

Empirical formula and Molecular Formula
Combustion Analysis
Concept of mole and Avogadro’s Number
Molar Volume
Stoichiometry
Limiting reactant
Yield

Atomic Structure

Gases

Liquids

Solids

Chemical Equilibrium

Reaction Kinetics

Thermo-chemistry and Energetics of chemical reactions

Electrochemistry

Chemical bonding

S and p block elements

Transition Elements

Fundamental principles of organic chemistry

Chemistry of Hydrocarbons

Alkyl halides

Alcohols & phenols

Aldehydes and Ketones

Carboxylic acid

Macromolecules

Relative Atomic mass

Relative Atomic Mass

The mass of an atom of an element as compared to 1/12th mass of an atom of carbon taken as 12 is called relative atomic mass (atomic mass unit).

The mass of an atom is very small. It cannot be weighed by any known physical balance. That is why we use the relative atomic mass unit scale.

Examples:

H = 1.008 a.m.u, Cl = 35.5 a.m.u. These are the average of atomic masses of isotopes proportional to their relative abundances.

Unit:

The unit used to express atomic mass is called atomic mass unit (a.m.u).

  • 1 a.m.u = 1.661 × 10–27 kg
  • 1 a.m.u = 1.661 × 10–24 g

Relative Isotopic Mass

The mass of an isotope of an element as compared to 1/12th mass of an atom of carbon-12 is called relative isotopic mass.

Examples:

  • Relative atomic mass of Chlorine-35 = 35 a.m.u.
  • Relative atomic mass of Chlorine-37 = 37 a.m.u.

Relative Molecular Mass

The mass of a molecule as compared to 1/12th mass of an atom of carbon-12 is called relative molecular mass.

Examples:

  • Relative molecular mass of water (H2O) = 18 a.m.u
  • Relative molecular mass of carbon dioxide (CO2) = 44 a.m.u

Relative Formula Mass

The mass of a formula unit as compared to 1/12th mass of an atom of carbon-12 is called relative formula mass.

Examples:

  • Relative formula mass of sodium chloride (NaCl) = 58.5 a.m.u
  • Relative formula mass of sodium sulphate (Na2SO4) = 142 a.m.u

Note: Carbon-12 is taken as standard on the C-12 scale because:

  • It is a highly stable isotope.
  • Its mass is exactly a whole number (12.000).
  • It can be handled easily because it is a solid.

Average Atomic Masses

The average atomic mass depends on the number of possible isotopes and their natural abundance.

Formula:

Average atomic mass = Σ (isotopic mass × fractional abundance)

Example (Neon isotopes):

  • Neon-20: Abundance = 90.92%, Mass = 20 a.m.u
  • Neon-21: Abundance = 0.26%, Mass = 21 a.m.u
  • Neon-22: Abundance = 8.82%, Mass = 22 a.m.u

Relative atomic mass of Neon = (20 × 0.9092) + (21 × 0.0026) + (22 × 0.0882) = 20.18 a.m.u

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