Revision: Some Basic Concepts of Chemistry Chemistry Science (English Medium) Class 11 CBSE

Mass is the amount of matter present in the object. The SI unit of mass is kg.

The weight of an object is defined as the force with which the earth attracts the object.

Avogadro s law states that "equal volumes of all gases under similar conditions of temperature and pressure contain the same number of molecules."

An atom is the smallest particle of an element that can take part in a chemical reaction; however, it may or may not exist independently. 

A molecule is the smallest particle of an element or a compound that can exist by itself; it never breaks up except for taking part in a chemical reaction.

It is the sum of total number of protons and neutrons present in a nucleus.

1. atomic mass number = number of protons + number of neutrons.
2. Atomic number = number of protons = number of electrons. It is denoted by Z.
3. Mass number = number of protons + number of neutrons. It is denoted by A.
4. For example: Carbon atom its,
   1. Number of proton = 6
   2. Number of neutrons = 6
   3. Number of electron = 6
5. Atomic number (Z) = number of protons = number of electrons = 6.
6. Mass number (A) = number of proton + number of neutrons = 6 + 6 =12.

A mass unit equal to exactly one-twelfth `(1/12^(th))` the mass of one atom of carbon-12 is called one atomic mass unit. It is written as ‘u’.

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

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.

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.

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

"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.

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

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.

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.

Percentage composition of a compound, is the percentage by weight of each element present in it.

• Unsaturated solution: If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution. (till it is dissolving).

• Saturated solution: When no more solute can be dissolved in a solution at a given temperature, it is called a saturated solution.

• Solubility: The amount of the solute present in the saturated solution at this temperature is called its solubility.

Percentage of an element in a compound \[=\frac{\text{Total wt. of the element in one molecule}}{\text{Gram molecular weight of the compound}}\times100\]