Some Basic Concepts of Chemistry Test - 6

Precision means closeness of all the data values measured while accuracy means closeness of particular value with the real value.

For example ,

when the reported values are  2.01, and 1.99 g as the result  and  its truevalue is 2.00 g  .  

These values are both precise and accurate.

 Significant  digits are certain digits that have significance or meaning to certain digits in a number . Such digits help scientists to use them with more precise information about measurement and other numeric data. These digits also help them in rounding off very large or very small number

 Gay lussac’s law states that 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.

   Mathematically , the law is expressed as ,

 [ V / n ]   =  k 

where V represents volume of the gas  ,  n  denotes number of moles of the gas &  k is proportionality constant .

Thus, volume ratio must be related to the relative  number of molecules.

This relationship was crucial in establishing  the formula of simple molecules  at a time when a distinction between atoms and molecules was not clearly understood.

Avogadro's law  states that " equal volumes of all gases at same temperature  and pressure contain equal number of molecules. "

Molecular mass is the sum of atomic masses of the elements present in a molecule.

For example ;  Molecular mass of CO₂  =   Σ atomic mass of one carbon atom  &  atomic mass of  two oxygen atoms 

=[ 12+2(16) ]  u

= 44u.

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 12C12C 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 

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 "

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 

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. 

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 ,

Mollecular formula  =  ( Empirical formula ) _n 

where n represents a positive integer .