Basic Concepts of Chemistry Notes - Class 11 Chemistry - CBSE JEE NEET - ChemContent

Matter around us

Chemistry is defined as the branch of science which deals with the study of composition, structure and properties of matter.

Chemistry is related with all the fields of our life, e.g., food, fuels, textiles, dyes, drugs, disinfectants, perfumes, building materials, paints, inks, fertilizers, insecticide, pesticide, soap, detergents, etc.

Matter: Matter is anything that has mass and occupies space. For example, book, pen, pencil, water, air, all living beings etc. are composed of matter.

Units and Measurements

Accuracy: Freedom from mistake or error : The quality or state of being accurate or the ability to work or perform without making mistakes.

Accuracy = Mean value – True Value

Precision: The quality, condition, or fact of being exact and accurate or the closeness of the set of values obtained from identical measurements of quantity.

Precision = Individual Value – Arithmetic Mean Value

Significant Figures: The digits in a properly recorded measurement or the total number of figures in a number including the last digit whose value is uncertain are called significant figures, e.g., 180.00 has five significant figures, .018 has significant figures.

Laws of Chemical Combination

Elements combine to form compounds in accordance with the following five basic laws, called the laws of chemical combinations:

1. Law of Conservation of Mass: Mass can neither be created nor destroyed.
2. Law of Definite Proportions Compositions: A given compound always contains exactly the same proportion of elements by weight.
3. Law of Multiple Proportions: If two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers.
4. Lussac's Law of Gaseous Volumes: When gases combine or are produced in a chemical reaction they do so in a simple ratio by volume provided all gases are at same temperature and pressure.
5. Avogadro's Law: At the same temperature and pressure, equal volumes of gases should contain the equal number of molecules.

Dalton Atomic Theory

Dalton gave a theory about atoms which contain following postulates:

• All matter consists of tiny indivisible particles called atoms.
• Atoms are indestructible and unchangeable.
• All the atoms of a given element have identical properties including identical mass. Atoms of different elements differ in mass.
• When different elements react to form compounds their atoms combine in simple fixed, whole number ratio.
• In chemical reactions, atoms are neither created nor destroyed. They are reorganized in a chemical reaction.

Atomic Mass: Atomic mass of an element is defined as a mass exactly equal to one-twelfth the mass of C-12 atom.

Average Atomic Mass: The average atomic mass of an element refers to the atomic masses of the isotopes of the element, taking into account the abundances of the element's isotopes.

Average atomic mass = f₁M₁ + f₂M₂ + f₃M₃ + … + f_nM_n

where, f is the fraction representing the natural abundance of the isotope and M is the mass number (weight) of the isotope.

Molecular mass of a compound is the sum of atomic masses of all the constituent atoms. It is obtained by multiplying the atomic mass of each element by the number of its atoms and adding them together. 

Mole Concept and Percentage Composition

Mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly 12 g (or 0.012 kg) of the 12C isotope.

One mole of any element has same mass as the atomic weight of the element.

Avogadro constant is the number of entities present in 1 mole. It is denoted by ''NA'' and is equal to 6.0221367 × 10²³ atoms/mol.

Number of moles of a substance= Mass of substance (g)/Mass of 1 mole of substance (g )

= Number of particles/6.022 x 10²³

Number of molecules of a substance = (6.022 x 10²³) Volume of gas at NTP (in litres)/22.4

The mass of one mole of a substance in grams is called its molar mass.

Mass percent = (Mass of that element in ×100 the compound)/Molar mass of the compound

Empirical and Molecular Formulae

Empirical Formula: A formula that gives the simplest whole number ratio of various atoms present in a compound.

Molecular formula: It is a formula that shows the exact number of atoms of different elements present in a molecule of a compound.

Steps for calculation of empirical and molecular formula:

1. Conversion of mass percent to grams.
2. Convert into number of moles of each element.
3. Divide the mole value obtained above by the smallest number.
4. Write empirical formula by mentioning the numbers after writing the symbols of respective elements.
5. Writing molecular formula:

• Determine empirical formula mass by adding the atomic masses of various atoms present in the empirical formula.
• Divide molar mass by empirical formula mass.
• Multiply empirical formula by n obtained above to get the molecular formula.

How to measure the Concentration of a solution

1. Mass percent =  (Mass of solute/Mass of solution) ×100 

2. Mole fraction: In a binary solution, if a substance 'A' dissolves in substance 'B' and their number of moles are nA and nB, respectively, then

• Mole fraction of A=  nA/(nA+nB)
• Mole fraction of B=  nB/(nA+nB)
• Mole fraction of A + Mole fraction of B =1

3. Molarity (M) = Number of moles of solute/Volume of solution (litres)

4. Molality (m) = Number of moles of solute/Mass of solvent in kg

5. Normality (N)=Number of gram equivalent of solute/Volume of solution (l)

(Gram equivalent of solute = w₂/equivalent weight)

• Molality and mole fraction do not change with temperature.

6. Parts per million (ppm)= Number of parts of the component / Total no. of parts of all the components of the solution. 

7. Volume percent = (Volume of the component / Total volume of the solution) × 100

Chemical Equations, Stoichiometry and Limiting Reagent

Chemical equation: A chemical equation is the symbolic representation of a chemical reaction, e.g.,

AgNO₃ + NaCl → AgCl + NaNO₂

Reactants → Products

Stoichiometry and Stoichiometric Calculations:

Stoichiometry is the calculation of relative masses of reactants and products involved in chemical reactions.

Stoichiometric problems can be solved in just four simple steps :

1. Balance the equation.
2. Convert units of a given substances to moles.
3. Using the mole ratio, calculate the moles of substance yielded in the reaction.
4. Convert moles of wanted substance to desired units.

Limiting Reagent: In a chemical reaction, the reagent which is completely consumed and limits the amount of product formed is called as limiting reagent.

Molarity Equation 

M₁V₁ = M₂V₂

where, M₁ is initial molarity, V₁ is initial volume, M₂ is final molarity and V₂ is final volume.