Chapter 1 – Some Basic Concepts of Chemistry
1. Importance of Chemistry
Chemistry is the branch of science that deals with the study of composition,
structure, properties, and changes of matter.
It forms the foundation for medicine, engineering, material science, and biology.
2. Laws of Chemical Combination
(a) Law of Conservation of Mass
Mass can neither be created nor destroyed during a chemical reaction.
Total mass of reactants = Total mass of products
(b) Law of Definite Proportions
A chemical compound always contains the same elements combined in the same fixed proportion by mass.
(c) Law of Multiple Proportions
When two elements combine to form more than one compound,
the masses of one element that combine with a fixed mass of the other are in simple whole-number ratios.
3. Dalton’s Atomic Theory
- Matter is made up of atoms
- Atoms are indivisible
- Atoms of same element are identical
- Chemical reactions involve rearrangement of atoms
Modern chemistry has modified Dalton’s theory due to discovery of subatomic particles.
4. Atomic Mass
Atomic mass of an element is the average mass of its naturally occurring isotopes
relative to $^{12}\text{C}$ taken as 12 units.
Atomic mass = $\sum (\text{fractional abundance} \times \text{isotopic mass})$
5. Molecular Mass
Molecular mass is the sum of atomic masses of all atoms present in a molecule.
Molecular mass of $H_2O = 2(1) + 16 = 18\,\text{u}$
6. Formula Mass
Formula mass is used for ionic compounds where discrete molecules do not exist.
Formula mass of NaCl = 23 + 35.5 = 58.5 u
7. Mole Concept
A mole is the amount of substance that contains
$6.022 \times 10^{23}$ particles (Avogadro number).
$1\;\text{mol} = 6.022 \times 10^{23}\;\text{entities}$
8. Molar Mass
Molar mass is the mass of one mole of a substance expressed in g/mol.
Number of moles $n = \frac{\text{given mass}}{\text{molar mass}}$
9. Relationship Between Mass, Moles, and Particles
$n = \frac{m}{M}$
Number of particles = $n \times N_A$
10. Percentage Composition
Percentage composition tells how much each element contributes to the mass of a compound.
$\%\text{ of element} = \frac{\text{mass of element}}{\text{molar mass of compound}} \times 100$
11. Empirical Formula
Empirical formula shows the simplest whole-number ratio of atoms in a compound.
Steps:
- Convert % to mass
- Convert mass to moles
- Divide by smallest mole value
12. Molecular Formula
Molecular formula shows actual number of atoms in a molecule.
Molecular formula = $(\text{Empirical formula})_n$
$n = \frac{\text{Molar mass}}{\text{Empirical formula mass}}$
13. Stoichiometry
Stoichiometry deals with quantitative relationships between reactants and products in a chemical reaction.
14. Limiting Reagent
The reactant which gets completely consumed first and limits the amount of product formed
is called the limiting reagent.
Always calculate moles of reactants to identify the limiting reagent.
15. Concentration of Solutions
(a) Mass Percentage
$\%\text{ by mass} = \frac{\text{mass of solute}}{\text{mass of solution}} \times 100$
(b) Molarity
$M = \frac{\text{moles of solute}}{\text{volume of solution (in litres)}}$
(c) Molality
$m = \frac{\text{moles of solute}}{\text{mass of solvent (in kg)}}$
16. Final Exam Checklist
You have mastered this chapter if you can:
- Apply mole concept confidently
- Calculate empirical and molecular formula
- Identify limiting reagent
- Solve stoichiometric problems
- Convert between mass, moles, and particles