Mole chemistry

Introduction:
All substance is made up of smallest particle, called atom. In chemistry, the smallest particle, used for calculation, is said to be a mole. Any chemical equation or a chemical expression is given with moles of a substance under consideration.

Mole Definition:
Mole is the smallest unit used in all calculations in chemistry. Mole is a unit of measurement, which gives the same number of chemical entities (atoms, molecules, ions, electrons), as in number of atoms in 12 grams of carbon.

Mole concept is used in calculation of concentrations of solutions, in the calculation of molecular mass, etc.

Molar mass of a substance is defined as “mass per mole” of a substance.
Mole of a substance can also be defined as: “one mole of a substance contains Avogadro number of molecules or atoms”.
The value of Avogadro’s number is – 6.023 x 10²³
This is given by:

Mole =
Mole Calculation

Many calculations, regarding a chemical compound can be obtained from the mole concept.
Mole of a substance is used to calculate-
a.     Grams of a substance, if molar mass is known.
b.     Molar mass of a substance, if grams are known.
c.      Molarity, molality, mole fraction.
d.     Number of atoms present, with the help of Avogadro’s number.

Thus, moles play a very important role in chemical calculations.

Chemistry Mole Problems-
Example – 1:
A sample of magnesium hydroxide contains 12 grams of the substance. Calculate the number of moles of Mg(OH)₂ present.
Answer:
Molar mass of Magnesium hydroxide is: 58.32

Moles of Magnesium hydroxide = Mass in grams / Molar mass
                                                         = 12 grams / 58.32 grams/mole
                                                        = 0.206 moles.

Example – 2
13.65 moles of methane gas was obtained in a reaction. Find the mass in grams of methane.

Answer:
Molar mass of methane is 16.04 grams/mole.
Moles of methane = Grams of methane / Molar mass
Grams of methane = Moles of methane x Molar mass
                                    = 13.65 moles x 16.04 grams / mole
                                    =   218.94 grams of methane
Mole Problems Chemistry
Finding number of atoms, with Avogadro number
Example – 3
Calculate the number of atoms present in 4.2 moles of Sodium.
Answer:

1 Mole of a substance contains Avogadro number of atoms/molecules/ions.
Therefore, Avogadro’s constant =

We have – 4.2 moles of Sodium.

Number of atoms of Sodium present =


    4.2 moles of Na x   = 25.29 x 10²³ atoms of Na

Example – 4
 There are 3.01 x 10³² molecules of carbon dioxide present. Calculate:

i)                   Number of moles

ii)                Number of grams of CO₂.

Answer:

i)  To calculate the number of moles-

Moles of Carbon dioxide

= 3.01 x 10³² atoms of CO₂ x

= 4.99 x 10⁹ moles of Carbon dioxide

ii)                 To calculate the mass in grams of carbon dioxide –

Mass in grams = Moles of CO2 x Molar mass

                          = 4.99 x 10⁹moles x 44 grams / mole

                           = 219.56 x 10⁹ grams of Carbon dioxide