Some Basic Concepts of Chemistry Test - 5

In chemistry the empirical formula of a compound  is the simplest positive integer ratio of atoms present in a compound.

However ,this differs from the chemical formula  which  expresses the number of atoms in each molecule of a compound .

An empirical formula makes no mention of the arrangement of atoms in  a compound.

It  is standard for many ionic compounds like CaCl2  or for macro - molecules such as silicates etc.

Stoichiometry is a method to express quantitative aspects of a chemical reaction.

Usually, the masses of reactants, as well as those of products in a chemical reaction, are calculated using corresponding  balanced chemical equation .

It is convenient and hence desirable to calculate volumes of gaseous reactants and products.  

The observation was first made by French chemist Joseph Proust , based on certain experiments conducted between 1798 and 1804.

Proust made the above statement  known as " Proust's law " or  "  Law of definite composition " or " Law of constant composition "

Since , M denotes molarity of a solution  & Molarity
= number of moles of a solute (ie given as NaOH )   / 
Volume of solution in Litres

∴.   0 . 2M solution means 0.2moles of solute(NaOH) present in 1L of solution.

One mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly 12 g of the 12C isotope.

A mole of a substance always contains the same number of entities , no  matter what the substance may be. In order to determine this   number precisely , the mass of carbon -12 atom was determined by mass spectrometer  and found to be equal to 1.992648 x 10^-23 g. Knowing that one mole of carbon weighs 12 g . the number of atoms in it is equal to :6.0221367 x 10²³ 

Avagadro's constant represents the number of entities ( viz, atoms, molecules or ions ) present in 1 mole of a substance.

The  constant ( denoted by  N )  =6.022 x 10²³ 

Molarity is the most widely used method of expressing the strength or concentration of a solution.

It is denoted by  ' M ' 

 Molarity is defined as number of . of moles of solute present per litre of solution.

Mathematically,
Molarity ( M )
=    Number of moles of solute  /   Volume of solution in Litres 

Molecular formula is the exact no. of atoms present in a molecule of a compound.
Molecular formula of a compound is related with its empirical formula  as,
Molecular formula  =  ( Empirical formula ) n
where n represents a positive integer.

According to law of conservation of mass
 total mass of reactants
= total mass of product
∴  the number . of atoms of each element in reactants
 =  number . of atoms of that element in products

Molality is defined as no. of moles of solute present per kg of solvent. It is denoted as " m". Mathematically,

Molality (m)
=   [ number of moles of the solute ] /  kg of solvent

Since , this mode of expressing the strength of a solution involves  ( weight / weight ) relationship of solute and solvent , the molality of the solution is not affected by variation in temperatures of the solution.