# Problems / Numericals based on Mole Concept (Atomic Mass/ Molar Mass/ Avogadro number)

Formulas in brief –

•  Mass = Molar mass x Number of moles
•  Number of moles = Given number of particles / Avogadro number
•  Number of particles = (Given mass / molar mass) x Avogadro number          (from 1 and 2)
•  If one carbon atom has a mass of 12 atomic mass units and one magnesium atom has a mass of 24 atomic mass units, then as a magnesium atom is twice as heavy as a carbon atom. It follows that this ratio will be maintained for any number of atoms.
• Equal masses of carbon and magnesium contain different numbers of atoms.

6 g of carbon contains 6 / 12 moles of carbon = 0.5 moles
6 g of magnesium contains 6 / 24 moles of magnesium =0.25 moles

• 1 mole = 6.022 x 1023atoms or molecules or formula units of that substance.

1 mole of water (H2O)contains 2 moles of hydrogen atoms and 1 mole of oxygen atoms. It is a simple matter of multiplying the moles of the compound by the atoms or ions that make it up.

What is Matter? What is its Characteristics?

States of Matter

Law of Conservation of Mass

Problems based on Law of Conservation of Mass

Law of Definite Proportions / Constant Composition

Problems based on law of definite proportions or constant composition

Mole Concept – Importance and formulas

Quiz – Mole Concept and Stoichiometry – 1

Problem based on formula no. – 1
Mass = Molar mass x Number of moles
Calculation of mass from mole of any fundamental unit like atom, molecule and vice – versa.

Note that if it is not mention atom or molecule before mole, it always means one mole of that substance in its natural form.

Example 1- How many grams are there in 5.5 mol of sulphur?

Solution – For converting mass into mole and vice versa, we always need the molar mass.
Molar mass of sulphur = 32.0 g mol–1
Number of moles of sulphur = 5.50 mol
Therefore, mass of sulphur (in grams)  = molar mass x number of moles

= 5.50 mol × 32.0 g mol–1= 176.0 g sulphur

Example 2- Calculate number of moles present in 64 g of oxygen.

Solution – Molar mass of oxygen = 32 g mol–1

Oxygen in natural form will be molecular oxygen, O2

Therefore, number of moles of oxygen = 64 g / 32 g mol–1  = 2.0 moles

Example 3- Calculate the number of moles present in 108 g of aluminium.

Solution –      Atomic mass of Al = 27gm

So,      27g of aluminium  = 1 mole of aluminium

Or, by using formula , Number of moles = given mass / atomic mass

Or,                      Number of moles = 1/27 x 108 =  4 moles of aluminium

Thus,  108g of aluminium  = 4 moles of aluminium

Example 4 – Calculate the mass of 0.5 mole of iron.

Solution –      Atomic mass of iron = 55.9 g

Mass of the 1 mole of iron = 55.9 g

Or, by using formula,  mass = atomic mass x number of moles

Mass of the 0.5 mole of iron = 55.9 x 0.5 g = 27.95 g

Now, Check your understanding –

1. Atomic number (Z) of Iron, Fe is 26, Atomic mass 56.
Using the above information answer the following-
(a) 1 mole of iron weighs __________ grams.
(b) 7.4 moles of iron weighs ________________ grams.

2. 1 litre of an aqueous solution of NaCl contains 117g of the dissolved salt. Calculate the number of moles of the solute in the solution. (Na = 23u , Cl = 35.5)

3. Two bottles A and B contain 1000 ml. water solutions of 60g NaOH and 117g NaCl respectively. Which bottle has more concentrated solution?

Problem based on formula no. – 2
Number of moles = Given number of particles / Avogadro number
Calculation of mole from number of particles and vice – versa.

Example 5 – Calculate the number of moles of 24.088 x 1023 numbers of sodium atom.

Solution –      Given number of particles =  24.088 x 1023
Avogadro number =   6.022 x 1023
Number of moles = Given number of particles / Avogadro number
24.088 x 1023   / 6.022 x 1023

=  4 moles

Example 6 – Calculate the number of particles in 0.5 moles of N atom.

Solution –                              Number of moles =  0.5 moles
Avogadro number =   6.022 x 1023
Number of particles = Number of moles x Avogadro number
0.5 x 6.022 x 1023

=  3.011 x 1023

Now, Check your understanding –

Atomic number (Z) of Iron, Fe is 26, Atomic mass 56.
Using the above information answer the following-
(a) 7.4 moles of iron contains ________________ atoms.
(b) 1.2 x 1024 atoms of iron would be _____________ moles of iron.

Problem based on formula no. – 3
Number of particles = (Given mass / molar mass) x Avogadro number
Calculation of number of particles from mass and vice – versa.

Example 7 – Calculate the number of molecules in 22 g of CO2

Solution –      Gram molecular mass or molar mass of CO2 = 44 g

Number of particles = (Given mass / molar mass) x Avogadro number                                   =  ( 22 g / 44 g ) x 6.022 x 1023 = 3.011 x 1023  molecules

Example 8 – Calculate the mass of 18.066 x 1023  molecules of SO2

Solution –      Gram molecular mass or molar mass of SO2 = 64 g
Mass = (Number of particles x molar mass) / Avogadro number
= (18.066 x 1023 x 64) /  6.022 x 1023

=  192 g

Example 9 – Calculate the mass of glucose in 2 x 1024 molecules.

Solution –     Gram molecular mass or molar mass of glucose = 180 g

Mass = (Number of particles x molar mass) / Avogadro number
= (2 x 1024  x 180) /  6.022 x 1023

=  597.8 g

Example 10 – How many atoms are there in 24 g of carbon?

Solution –        24 g of carbon = 24 / 12 moles = 2 moles

1 mole of atoms = 6.022 x 1023

Therefore 2 moles of carbon contains 2 x 6.022 x 1023 atoms = 1.204 x 1024 atoms.

Now, Check your understanding –

Atomic number (Z) of Iron, Fe is 26, Atomic mass 56.
Using the above information answer the following-
(a) 1 gram of iron contains ________________ atoms.
(b) 1 atom of iron weighs _________________ grams.

Thus, One mole of oxygen stands for –
1.  6.022 x 1023 molecules of oxygen
2.  2 x 6.022 x 1023 atoms of oxygen
3. 1 gram molecule of oxygen
4. 2 gram atoms of oxygen
5. 32 grams oxygen

Don’t Miss –

QUIZ – Mole Concept and Stoichiometry – 1

### 17 thoughts on “Problems / Numericals based on Mole Concept (Atomic Mass/ Molar Mass/ Avogadro number)”

1. Quiz was too good but due to some reasons I didn’t saw the results of the quiz or I think the website didn’t make me to see the result

1. Thanks and keep visiting.
It should be available in your email. Also you can find the answers with result after submitting your answers.

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