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## Wednesday, August 31, 2011

### 9th Atoms and Molecules - Mole Concept and Problems based on mole concept

Mole Concept
we can express the quantity of a substance is expressed in terms of mole.
Mole is a fundamental unit in the System International d' Unités, the SI system, and it is used to measure the amount of substance In Latin mole means a "massive heap" of material

Mole is defined as the amount of substance that contains as many specified elementary particles as the number of atoms in 12g of carbon-12 isotope.

One mole is also defined as the amount of substance which contains Avogadro number (6.023 x 1023) of particles.
Avogadro number: Number of atoms or molecules or ions present in one mole of a substance is called Avogadro number. Its value is 6.023 x 1023

Therefore, one mole of any substance contains Avogadro number of particles. The particles may be atoms, molecules, ions etc

E.g. 1 mole of oxygen atoms = 6.023 x 1023 atoms, molecules, ions etc  of oxygen
= 32g of O2
Problems (based on mole concept)
1. When the mass of the substance is given:

a. Calculate the number of moles in

i) 81g of aluminium

Sol: 1. Atomic mass of Al= 27gm
27g  of  aluminium = 1 mole of aluminium
81g  of aluminium   = 1/27 x 81=3 moles of aluminium

OR, Use formula , Number of moles = given mass/atomic mass

Self: ii) 4.6g sodium   iii) 5.1g of Ammonia  iv) 90g of water v) 2g of NaOH

b. Calculate the mass of 0.5 mole of iron

Atomic mass of iron = 55.9 g
Mass of the 1 mole of iron   = 55.9 g
Mass of the 0.5 mole of iron   = 0.5 x 55.9 g = 27.95 g

Or, Using formula: mass = atomic mass x number of moles

FOLLOW UP: Find the mass of 2.5 mole of oxygen atoms

2. Calculation of number of particles when the mass of the substance is given:

Number of particles =   Avogadro number x given mass / gram molecular mass

Calculate the number of molecules in 11g of CO2
Solution: gram molecular mass of CO2 = 44g
44g of CO= 6.023 x 1023  molecules
1 g of CO= ( 6.023 x 1023 ÷ 44 g ) molecules
11g of CO2 = ( 6.023 x 1023 ÷ 44 g ) x 11 =1.51 x 1023 molecules

FOLLOW UP: Calculate the number of molecules in 360g of glucose

3. Calculation of mass when number of particles of a substance is given:

Mass of a substance =  gram molecular mass x number of particles/ 6.023 x 1023

Eg. Calculate the mass of 18.069 x 1023  molecules of SO2
Sol: Gram molecular mass SO2 = 64g
6.023 x 1023 molecules of SO2  = 64 gm
1             molecules of SO2          = 64/(6.023 x 1023 )  gm
18.069 x 1023  molecules of SO2  = [ 64/(6.023 x 1023 )  x  18.069 x 1023  ]gm = 192 g

Calculate the mass of glucose in  2 x 1024 molecules
Gram molecular mass of glucose = 180g
Mass of glucose  [180 x 2 x 1024 ] / [6.023 x 1023  ] = 597.7g

FOLLOW UP: Calculate the mass of 12.046 x 1023 molecules in CaO.

4. Calculation of number of moles when you are given number of molecules:
Number of moles = Number of molecules/Avogadro Number
= [3.0115 x 1023 ] / [6.023 x 1023  ]      = 0.5 moles

FOLLOW UP:Calculate number of moles in 12.046x 1022 atoms of copper