Atoms and Molecuels

Molecular Mass

Definition of Molecular Mass: Molecular Mass is the sum of the atomic masses of all the atoms in a molecule of the substance.

Explanation:

A molecule of an element may contain more than one atom of same or different elements.

For example:

Oxygen molecule (O₂): Oxygen molecule contains two atoms of oxygen.

Hydrogen molecule (H₂): Hydrogen molecule contains two atoms of hydrogen.

Molecule of water (H₂O): One molecule of water contains two hydrogen atoms and one oxygen atom.

The total atomic mass of all atoms in a molecule is called the Molecular Mass.

How to calculate the molecular mass of a molecule

Step: (a) Write the molecular formula of the substance

Step: (b) Add the atomic mass of all of the atoms present in molecule of the substance

Step: (c) Result of sum of atomic mass of all of the atoms present in the molecule of the substance is the molecular mass of the given substance.

Example:

Calculate the Molecular mass of oxygen (O₂)

Chemical Formula of molecule of oxygen : O₂

We know that atomic mass of one atom of oxygen = 16

Thus, molecular mass of oxygen molecule (O₂) = 16 – 2 = 32 u

Thus, molecular mass of molecule of oxygen = 32 u

Calculate the Molecular mass of Hydrogen molecule (H₂)

Chrmical Formulua of molecule of hydrogen : H₂

The atomic mass of hydrogen = 1 u

Thus, molecular mass of hydrogen molecule = 1 u × 2 = 2 u

Calculate the Molecular mass of molecule of ammonia (NH₃)

Chemical formula of molecule of ammonia: NH₃

Molecule of ammonia contains one atom of nitrogen and three molecule of hydrogen.

Thus, molecular mass of ammonia = atomic mass of one atom of nitrogen + atomic mass of three atoms of hydrogen.

We know that, atomic mass of atom of nitrogen (N) = 14 u

And, atomic mass of atom of hydrogen (H) = 1 u

Since, Chemical formula of molecule of ammonia: NH₃

Thus, molecular mass of ammonia = atomic mass of one atom of nitrogen + atomic mass of three atoms of hydrogen.

= 14 u + (1 u × 3)

= 14 u + 3 u

Thus, molecular mass of ammonia (NH₃) = 17 u

Calculate the molecular mass of Carbon dioxide (CO₂)

Chemical formula of carbon dioxide : CO₂

From chemical formula of carbon dioxide it is clear that, one molecule of carbon dioxide is formed by the combination of one atom of carbon and two atoms of oxygen.

Thus, molecular mass of carbon dioxide = atomic mass of one carbon atom + atomic mass of two oxygen atoms.

We know that, atomic mass of atom of carbon = 12 u

And atomic mass of atom of oxygen = 16 u

Thus, molecular mass of carbon dioxide = 12 u + (16 u × 2)

= 12 u + 32 u = 44 u

Thus, molecular mass of carbon dioxide = 44 u

Calculate the molecular mass of ammonium nitrate (NH₄NO₃)

Chemical formula of ammonium nitrate : NH₄NO₃

This means one molecule of ammonium nitrate is formed by the combination of one atom of nitrogen, four atoms of hydrogen, one atom of nitrogen and three atoms of oxygen.

Atomic mass of nitrogen = 14 u

Atomic mass of Hydrogen = 1 u

Atomic mass of oxygen = 16 u

Thus, molecular mass of ammonium chloride (NH₄NO₃)

= atomic mass of one atom of nitrogen + (atomic mass of atom of hydrogen × 3) + atomic mass of one atom of nitrogen + (atomic mass of oxygen atom × 3)

= 14 u + (1 u × 3) + 14 u + (16 u × 3)

= 14 u + 1 u + 14 u + 48 u

= 77 u

Formula Unit

Ionic compounds do not exist as single molecule. They are made up of a large number of positively and negatively charged ions joined together. They exist as a large solid lattice. Thus, for convenience, the lowest ratio of components of ionic compouns is written as formula unit. Formula unit is the smallest representative unit of ionic compound.

Thus, formula unit is the chemical formula of an ionic compound.

Formula Unit Mass:

Formula unit Mass is used instead of molecular mass for ionic compounds.

The formula unit mass is the sum of atomic masses of the atoms represented by its formula.

Hence, the formula mass of an ionic compound is the relative mass of the sum of its atoms, which are indicated in formula unit.

The only difference is that at the place of molecular mass word formula unit is used for those substances whose constituent particles are ions.

Example: NaCl, KCl, CaCl₂, etc. are formula unit of some of the ionic compounds.

Calculate the Formula unit Mass of Sodium Chloride (NaCl)

We know that atomic mass of sodium (Na) = 23 u

And atomic mass of chlorine = 35.5 u

Chemical formula of sodium chloride : NaCl

Thus, formula unit mass of sodium chloride (NaCl) = atomic mass of sodium (Na) + atomic mass of chlorine (Cl)

= 23 u + 35.5 u = 78.5 u

Thus, formula unit mass of sodium chloride (NaCl) = 28 u

Calculate the formula unit mass of calcium chloride (CaCl₂)

Chemical formula of calcium chloride: CaCl₂

Atomic mass of calcium (Ca) = 40 u

Atomic mass of chlorine (Cl) = 35.5 u

Therefore, Formula unit mass of calcium chloride (CaCl₂) = atomic mass of calcium + atomic mass of chlorine × 2

= 40 u + 35.5 × 2

= 40 u + 71 u = 111 u

Thus, Formula unit mass of calcium chloride = 111 u

Mole Concept

"Mole" is a SI unit uned in chemistry. Its unit symbol is "mol".

"Moles" is a Latin word which means "heap or pile".

"Mole" is used to measure the quantity of substance in terms of the number of molecules or atoms or ions rathter than its mass.

The word "mole" was introduced in 1896 by Wilhelm Ostwald. This unit mole was accepted in 1967.

The number of particles (atoms, molecules or ions) in 1"Mole" of anysubstnce is fixed.

Avogadrao's number or Avogadrao's constant

1 mole of any substance = 6.0022 × 10²³

The value "6.0022 × 10²³" has been obtained experimentally and is called Avogadrao's number or Avogadrao's constant.

Avogadrao's number or Avogadrao's constant is represented by the symbol `N_o`.

It (Avogadrao's number or Avogadrao's constant) had been named in the honour of Italian scientist 'Amedeo Avogadrao' who discovered it first.

Gram Atomic Mass

One "Mole" of any species (atoms, molecules or ions, or particles) in number having mass equal to its atomic or molecular mass in grams.

This means that 1 mole of a substance = the relative atomic mass or relative molecular mass of the respective substance in grams.

The atomic mass of an element gives the mass of one atom of that element in atomic mass unit (u).

To get the mass of 1 mole of atom of that element, that is, molar mass, the same numberical value is taken but unit is changed from "u" to "g".

Molar mass of atoms is also known as Gram Atomic mass.

Example:

Atomic mass of hydrogen = 1 u

Thus, gram atomic mass of hydrogen = 1 g

1 atom of hydrogen = 1 u of hydrogen

Thus, 1 atom of hydrogen = 1 u of hydrogen

= 1 g of hydrogen

= 1 mol of hydrogen

= 6.022 × 10²³ atoms of hydrogen.

[Because one mole of anything = 6.022 × 10²³]

Similarly,

1 atom of oxygen = 16 u of oxygen

= 16 g of oxygen

= 1 mole of oxygen atoms

= 6.022 × 10²³ atoms of oxygen.

[Because one mole of anything = 6.022 × 10²³]

Gram Molecular Mass

When molecular mass is expressed in gram, it is called Gram Molecular Mass.

To express a molecular mass in gram molecular mass, only unit of moleulcar mass "u" is changed to "g".

Example:

1 molecule of water = 18 u of water

= 18 g of water

= 1 mole molecule of water

= 6.022 × 10²³ molecules of water.

[Because one mole of anything = 6.022 × 10²³]

Thus, 1 mole = 6.022 × 10²³ number

= Relative mass in grams.

Calculation based on Mole Concept

If, Number of moles =n

Mass = m

Molar mass = M

Number of particles = N

And Avogadro's number of particles `=N_o`

Therefore, the number of moles `={text(Given Mass)}/{text(Molar mass)}`

And the number of moles `={text(given number of particles)}/{text(Avogadro number)}`

Example problem: (1) Calculate the number of moles present in 32 gram of oxygen molecule.

Solution:

Given, Molecular Mass of oxygen = 32 u

Molar mass of oxygen = 32 g

Therefore, no. of moles = ?

The number of moles `= {text(given mass)}/{text(molar mass)}`

`=(32\ g)/(4\ g) = 8`

Thus, number of moles = 8 Answer

Example Problem: (2) Calculate the number of moles present in 8 gram of hydrogen atoms.

Solution:

Give, Mass of hydrogen atoms = 8 g

We know that atomic mass of hydrogen atom = 1 u

Thus, molar mass of hydrogen atom = 1 g

The number of moles `= {text(given mass)}/{text(molar mass)}`

`=(8\ g)/(1\ g) = 8`

Thus, number of moles = 8 Answer

Example problem: (3) What is the number of moles in 64 g of oxygen molecules.

Solution:

Molecular mass of molecules of oxygen = 32 u

Thus, molar mass of molecules of oxygen = 32 g

Given, mass of oxygen molecules = 64 g

Thus, number of moles = ?

The number of moles `= {text(given mass)}/{text(molar mass)}`

`=(64\ g)/(32\ g) = 2`

Thus, number of moles = 2 Answer

Example Problem: (4) If Number of helium atom is 18.066 × 10²³, then find the number of moles of helium atoms.

Solution:

Given, number of particles of helium atom = 18.066 × 10²³

We know that Avogadro's number = 6.022 × 10²³

Thus, no. of moles of helium atom = ?

The number of moles `={text(given number of particles)}/{text(Avogadro number)}`

`=(18.066 xx 10^23)/(6.022xx10^23)=3`

Thus, number of moles of helium atom = 3 Answer