Class 9 Mole Concept

Mole Concept.

Mole is the number.

Earlier in banks, bankers have lots of coin to calculate the amount and it was very tedious job for them.

So they have found easier method to calculate the no. of coins.

They have taken the mass of all the coins = 2000 g.

Then they have calculated the mass of only one coin = 2g

In order to calculate the total no.s of coin = Mass of total coin / mass of one coin.

1000 coins = 2000g / 2g

In above method the mass of coins was used to calculate the no.s of coins.

In the same way Avogadro scientist has also calculated the no. of atoms and molecules present in the gram atomic and gram molecular mass of an element or compounds.

How the scientist have derived the Avogadro's No. 6.022×1023.

Let us calculate the no. of atoms present in 12g of carbon.

Relative atomic mass of carbon is 12 u.

1 u = 1.66×10 -24g

Absolute atomic mass of 1 atom of carbon = 12 × 1.66×10 -24g

1 atom of carbon = 12 × 1.66×10 -24g

1 / 12 × 1.66×10 -24g = 1g

12×1/ 12 × 1.66×10 -24 = 12g

1/12 × 1.66×10 -24 = 6.022×1023

Gram atomic mass: The atomic mass of an element expressed in gram is called its gram atomic mass.

For example: Atomic mass of oxygen = 16 u

Gram atomic mass of oxygen = 16 g.

The gram atomic mass of a substance represents the mass of one mole of atom of that substance.

Gram molecular mass: The molecular mass of a compound expressed in gram is called gram molecular mass.

For example: Molecular mass of oxygen = 32 u

Gram molecular mass of oxygen = 32 gram.

The gram molecular mass of a substance represent the mass of 1 mole of a molecules of that particular compound.

Molar mass: The molar mass of a substance is the mass of one mole of that substance.

unit of molar mass : g/mol

The molar mass of an element is the mass of one mole of its atom.

The molar mass of an element is equal to the atomic mass of the element expressed in the unit of gram per mole.

* 1 pair = 2 units

1dozen = 12 unit

1 mole = 6.022×1023 units

1 mole of the element = 6.022×1023 atoms

1 mole of an element = Gram atomic mass of the element

6.022×1023 units = Gram atomic mass of the element

1 mole of the element = 6.022×1023 atoms = Gram atomic mass of the element.

1 mole of the compound = 6.022×1023 molecules

1 mole of an compound = Gram molecular mass of the compound

6.022×1023 molecules = Gram molecular mass of the compound

https://us04web.zoom.us/j/76556978321?pwd=SmJyYmUrNHErZCtqQ3JIaXpObm5LZz09he compound = 6.022×1023 molecules = Gram molecular mass of the compound.

No. of moles = Given mass/molar mass

Numericals

Q1. What is the mass of 4 moles of aluminium atoms?

(Atomic mass of aluminium = 27 u)

Ans.

Given: Atomic mass of aluminium = 27 u

Calculate: Mass of 4 moles of aluminium atom.

1 mole of the atom = Gram atomic mass of atom.

Gram atomic mass of aluminium = 27 g

1 mole of the element = Gram atomic mass of the element.

1 mole = 27 g

4 mole = 27 × 4 = 100g.

Q2.If one mole of carbon atoms weighs 12 grams, what is the mass in grams of one atom of carbon?

Ans.

Given: Mass of 1 mole of carbon atoms = 12gms

Calculate: Mass of 1 atom of carbon

1 mole of the element = 6.022×1023 atoms = Gram atomic mass of the element.

1 mole of carbon atoms = 6.022×1023 carbon atoms

6.022×1023 carbon atoms = 12gms

1 carbon atom = 12 / 6.022×1023 = 1.99 ×10-23 g.

Q3. Which has more number of atoms, 100 g of sodium or 100 gram of iron?

(Atomic masses of Na = 23 u & Fe = 56 u)

Ans. Gram atomic mass of sodium = 23 g

Gram molecular mass of Fe = 56 g

Gram atomic mass of sodium = 6.022×1023 atoms

23 g = 6.022×1023 atoms

1 g = 6.022×1023 / 23

100 g = 6.022×1023 / 23 × 100

= 26.1×1023

Gram atomic mass of iron = 6.022×1023 atoms

56 g = 6.022×1023 atoms

1 g = 6.022×1023 / 56

100 g = 6.022×1023 / 56 × 100

= 10.7 ×1023

Sodium contains more no. of atoms than iron.

OR

Calculate the no. of moles for 100 g of sodium and 100 g of iron atom.

Moles of sodium = Mass of sodium / molar mass of sodium

= 100 / 23

= 4.34

Moles of iron = Mass of iron / molar mass of iron

= 100 / 56

= 1.78

100 g of sodium contains more moles of atoms as compared to iron.

Q4. Convert 22 g of carbon dioxide into moles? ( Atomic mass of C = 12 u and oxygen = 16 u)

Ans: Molecular mass of CO2 = 12 + 16 × 2

= 12 + 32

= 44 u

Gram molecular mass of CO2 = 44 g

1 mole = Gram molecular mass of CO2

1 mole = 44 g

44g = 1 mole

1 g = 1 / 44

22 g = 1/44 × 22

= 1/5

= 0.5 mole

Q5. What is the mass of 0.5 mole of water (H2O).

Ans. Atomic mass of hydrogen atom = 1 u

Atomic mass of oxygen atom = 16 u

Atomic mass of H2O = 1× 2 + 16

= 18 u

Gram molecular mass of H2O = 18 g

1 mole = Gram molecular mass of H2O

1 mole = 18 g

0.5 mole = 0.5 × 18 g

= 9 g

Q6. Calculate the no. of aluminium ions present in 0.051 g of aluminium oxide(Al2O3). (Atomic masses: Al = 27 u; O = 16 u)

Ans: 1 mole of Al2O3 = 102 g

Now, 1 mole of Al2O3 contains 2 moles of Al

Mass of Al in 1 mole of Al2O3 = Mass of Al × 2

= 27×2

= 54 g

102 g of Al2O3 contains = 54 g of Al

0.051 g of Al2O3 contains = 54/102 × 0.051g

= 0.027 g

The atomic mass of aluminium is 27 u. This means 1 mole of aluminium atoms or ions has a mass of 27 g and it contains 6.022×1023.

27 g = 6.022×1023.

0.027 g = 6.022×1023 / 27 × 0.027

= 6.022×1020.

Thus, the no. of aluminium ions in 0.051 g of Al2O3 is 6.022×1020.

1 mole of C6H12O6 = 180g

180 g = 1 mole

1 g = 1 / 180

100 g = 1 / 180 × 100

= 0.55 mole

Q. Calculate the no. of molecules in 4 g of oxygen?

Ans.

6.022×1023 molecule = Gram molecular mass of O2

mass of 1 atom of O = 16g

6.022×1023 molecules = 32 g

6.022×1023/ 32 = 1 g

6.022×1023/ 32 × 4 = 4g