Atomic and Molecular Masses, Stoichiometry, and Mole Concept - In Depth Notes

ISOTOPES

  • Definition: Variants of a chemical element characterized by:
    • Same number of protons
    • Different numbers of neutrons
    • Same atomic number but different mass numbers.

EXISTENCE AND STABILITY OF ISOTOPES

  • Stable Isotopes: Do not undergo radioactive decay. Examples include:
    • Carbon-12
    • Carbon-13
  • Unstable Isotopes (Radioactive):
    • Have an unstable neutron to proton ratio leading to decay.
    • Examples: Carbon-14 (radioactive), Uranium-238.
  • Stability Factors:
    • Strong nuclear force vs. electrostatic repulsion.
    • Balance between protons and neutrons determines stability.

TYPES OF ISOTOPES

  • Stable Isotopes: Not radioactive; examples are Carbon-12 (^12C) and Oxygen-16 (^16O).
  • Unstable Isotopes: Undergo decay and emit radiation; examples:
    • Carbon-14 (^14C)
    • Iodine-131 (^131I)
  • Artificial Isotopes: Created in labs; examples include:
    • Promethium-146
    • Technetium-95

RADIOACTIVE DECAY

  • Definition: Process by which an unstable nucleus loses energy.
  • Characteristics:
    • Occurs only in certain isotopes.
    • Creates new elements through emission of radiation (alpha, beta, gamma).
  • Decay Rate: Measured by half-life time; varies significantly across isotopes.

APPLICATIONS OF RADIOACTIVE DECAY

  • Important in fields like:
    • Nuclear physics, chemistry, geology, archaeology.
  • Applications include:
    • Radiometric dating,
    • Medical imaging,
    • Nuclear power generation.

STOICHIOMETRY

  • Definition: Study of the quantitative relationships in chemical reactions.
    • Combines "stoicheion" (element) and "metron" (measure).
  • Key concepts include:
    • Mass calculations.
    • Energy relationships in chemical reactions.

LAW OF CONSERVATION OF MASS

  • States mass cannot be created or destroyed in a chemical reaction.

LAW OF DEFINITE PROPORTIONS

  • Proust's principle that elements in compounds always combine in definite mass proportions.
    • Example: Water (H2O) is consistently in the ratio 1:8 (hydrogen to oxygen).

ATOMIC MASS

  • Definition: Mass of an atom relative to carbon-12.
    • Reported as a weighted average based on isotope abundance.
  • Example:
    • Carbon isotopes: Carbon-12 (12 amu), Carbon-13 (13 amu).
    • Average atomic mass of carbon is 12.01 amu.

CONCEPT OF RELATIVE ATOMIC MASS

  • Non-integer mass due to isotopes:
    • Relative abundance and isotopic masses contribute to average atomic mass.
  • Example:
    • Chlorine's weighted atomic mass calculated from its isotopes.

MOLECULAR MASS

  • Defined as the sum of atomic masses in a molecule.
  • Represented in amu or g/mol.
  • Example calculation for water (H2O):
    • Molecular mass = (2 x atomic mass of H) + (1 x atomic mass of O) = (2 x 1) + 16 = 18 amu.

RELATIVE MOLECULAR MASS

  • The mass of one molecule compared to carbon-12.
  • Example values:
    • Water = 18 g/mol
    • CO2 = 44 g/mol

CALCULATION OF RELATIVE MOLECULAR MASS

  • Examples based on chemical formulas:
    • extRelativeMolecularMassofH2SO4=(2imes1)+(1imes32)+(4imes16)=98ext{Relative Molecular Mass of } H2SO4 = (2 imes 1) + (1 imes 32) + (4 imes 16) = 98

MOLECULAR AND EMPIRICAL FORMULAE

  • Molecular Formula: Represents the actual composition (e.g. glucose = C6H12O6).
  • Empirical Formula: Simplest ratio of atoms (e.g. glucose = CH2O).

CONCEPT OF MOLE

  • Mole Definition: Quantity of substance with a mass equivalent to its atomic/molecular weight in grams.
    • Avogadro's number, N0=6.022imes1023N_0 = 6.022 imes 10^{23}, is the number of particles in a mole.

NUMERICALS ON MOLES

  • Methods for calculating moles based on mass or number of particles.

APPLICATIONS OF MOLE CONCEPT

  • Includes yield calculations:
    • Percentage yield = (Actual yield / Theoretical yield) x 100

PERCENTAGE PURITY

  • Definition: Measure of purity in a sample, reflecting actual vs possible mass of the desired compound.
    • Example: If an impure sample weighs 15 g and contains 13.5 g of pure substance, purity = (13.5 / 15) x 100 = 90%.

LIMITING REAGENT

  • Concept: Reactant that limits the amount of product formed due to insufficient quantity.
  • Calculation methods involve comparing relative amounts based on stoichiometry.

ATOM ECONOMY

  • Concept: Efficiency of a reaction based on how well reactants are converted to useful products.

CONCENTRATION

  • Expressed in g/dm³ or mol/dm³ (M).

TITRATION PROBLEMS

  • Process of determining concentration through neutralization.
  • Requires known concentration solution (standard solution) reacted with unknown concentration solution.