Thursday, 7 March 2013

Molecular Weight- Calculate and find what is Molecular weight


Calculation of  Molecular Weight and find out Molecular weight - Sometimes it is also called as Molecular Mass . In order to find out exact M.W. We need two things first molecular formula to find out weight of any compound or  chemical and second is a periodic table.

The molecular formula are used to know total atoms present, If we know the molecular formula. Periodic table can be used to  find out atomic weight of each element and add up all the atomic weight and do not forget to multiple how many times atoms are repeated. 

Unit of Molecular weight is gram per mol (g/mol). Sometimes the term Formula weight and the Molecular Weight are used for finding weight , So Let us check it out  the difference between formula weight and molecular weight. 

Molecular weight is a term used for molecules. While Formula wight is a term used for compounds which are ionic in nature. But  the calculation of both are same overhere. Example water (H2O) is a molecule hence it's weight can be considered as Molecular weight while  Na2SO4 or NaCl  are not molecules and they are ionic compounds hence their weight are called Formula weight.


  1. You must know the molecular formula.
  2. Second step is to use periodic table to find atomic weight.
  3. Multiply the atomic weight. Example Oxygen (O2), Here oxygen is present two times hence atomic weight of oxygen given in the periodic table is  16 so now it is multiplied twice , Hence by multiplying we get  16*2=32
  4. Add up all the atomic weight.
  5. Molecular mass or molecular weight or the formula weight of a compound will be the final result obtained after summation.

  1. H2O- Water, We know that atomic weight of H is 1 hence 2*1=2, Now atomic weight of Oxygen is  given in periodic table is 16 Just add up atomic weights i.e., 2+16=18 Hence Molecular weight of Water is 18g/Mol
  2. In the same way For H2SO4 . The weight is found 2+32+4*16=98 g/mol. Here atomic weight of hydrogen is 2 for sulphur is 32 and oxygen having 16*4 as there are 4 times oxygen is present. Hence by adding all the atomic weight we get 98g/mol

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