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Molar Mass

Since the particles are a group of atoms chemically joined together the final mass for these molecules known from the masses of their atoms i.e., we use relative mass to compare various molecules in terms of mass. Meaning that:-

 

Molar mass of matter =Total relativistic masses of atoms which make up matter

 

Molar mass is the mass of 1 mole of a compound in gram unit. (Molar mass used to be called as molecular mass formerly.)

 

For example, if we want to calculate the molar mass of methane gas (CH4) or if we ask what is the mass of 1 mole of methane gas, as each methane molecule has 1 C and 4 H atoms, 1 mole of methane molecule contains 1 mole of C atom and 4 moles of H atoms. According to this, we can calculate the mass of 1 mole of methane gas as follows:

 

·       Mass of 1 mole of C = 1 × 12 = 12 g/mol

·       Mass of 4 moles of H = 4 × 1= 4 g/mol

·       Mass of 1 mole of CH4 = 16 g/mol

 

In the same manner, we can calculate molar mass of H2SO4 acid.

·       Mass of 2 moles of H = 2 × 1= 2 g/mol

·       Mass of 1 mole of S = 1 × 32 = 32 g/mol

·       Mass of 4 moles of O = 4 × 16 = 64 g/mol

·       Mass of 1 mole of H2SO4= 98 g/mol

 

If 16g of methane gas makes up 1 mole of CH4 and 98 g of sulfuric acid makes up 1 mole of H2SO4, logically we can call those masses as molar mass. As we understand from previous calculations we have made, in chemistry, all atoms and molecules are bound with the following:

 

1.      Atomic mass and molar mass

2.     Mole

3.     Avogadro’s number

 

Avogadro’s number or Avogadro’s constant is the number of atoms in 1 mole of an element or number of molecules in 1 mole of a molecule. It is shown with NA. Previously, we have mentioned that atoms, molecules or ions as many as Avogadro’s number are called as a mole.

Molar Mass Calculation


 

Example 1

Find the molar mass for the following compounds:


a. Aqueous sodium sulfate Na2SO4.7H2O

b. Juglone C10H6O3

c. Sulfur dioxide SO2


Solution:

Find the molar mass for the following compounds:  a. Aqueous sodium sulfate Na2SO4.7H2O  b. Juglone C10H6O3  c. Sulfur dioxide SO2


 

Example 2

Find out the mole numbers of the following compounds.


a)9.6 g of SO2

b) 85 g of NH3


Solution:


a) Molar mass of SO2:

Find out the mole numbers of the following compounds.    a)9.6 g of SO2  b) 85 g of NH3


 

b) Molar mass of NH3:

Find out the mole numbers of the following compounds.    a)9.6 g of SO2  b) 85 g of NH3


  

Example 3

Calculate the mass of 0.7 mol of manganese dioxide (MnO2).


Solution:

Calculate the mass of 0.7 mol of manganese dioxide (MnO2).


 

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