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Some Basic Concepts in Chemistry
Definition of mole:
One mole is a collection of that many entities as there are number of atoms exactly in 12 gm of C -12 isotopes.
Or 1 mole = Collection of 6.02×1023species
1 mole of atoms is also termed as 1 gm-atom, 1 mole of ions is termed as 1 gm-ion and 1 mole of molecule termed as 1 gm-molecule.
Methods of Calculations of mole
a) If no. of some species is given, then no, of moles
b) If weight of a given species is given, then no of moles
c) If volume of a gas is given along with its temperature
Where R=0.0821 lit-atm/mol-K (When P is in atmosphere an V is in litre.)
1 mole of any gas at STP (0℃ &1 bar) occupies 22.7 litres.
1 mole of any gas at STP (0ᵒ C & 1 atm) occupies 22.4 litres.
Atom: Atom is smallest particle which cannot be divided into its constituents.
Atomic weight: It is the weight of an atom relative to one twelvth of weight of 1 atom of C-12
Laws of chemical combination
(a) Law of conservation of mass [Lavoisier]
In chemical change total mass remains conserved i.e. mass before the reaction is always equal to mass after the reaction.
1 mole ½ mole 1 mole
Mass before the reaction=1×2+1/2×32=18gm
Mass after the reaction=1×18=18 gm
(b) Law of constant composition [Proust]
All chemical compounds are found to have constant composition irrespective of their method of preparation or sources.
⇒ In H2O, Hydrogen & oxygen combine in 2: 1 molar ratio, this ratio remains constant
Whether it is Tap water, river water or seawater or produced by any chemical reaction.
(c) Law of multiple proportional [Dalton]
When one element combines with the other element to form two or more different compounds, the mass of one element, which combines with a constant mass of the other bear a simple ratio to one another.
(d) Law of reciprocal proportions[Richter]
When two elements combine separately with definite mass of a third element, then the ratio of their masses in which they do so is either the same or some whole number multiple of the ratio in which they combine with each other. This law can be understood easily with the help of the following examples.
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Q) What will be the maximum numbers of electrons having same spin, present in an atom for n+l=4?
The number of neutrons in a drop of water (20 drops =1mL) at 4ºC
Q)A long rectangular box is filled with Cl2 (atomic mass=35.45) which is known to contain only Cl35 and Cl37. If the box can be divided into a partition and the two types of chlorine molecule are put into two compartments respectively. Calculate where should partition to be made if pressure on both sides is to be same? The ratio of pressure to original pressure is 1/x.
So, the value of x is?