Atomic Mass And Molecular Mass: Definition, Formula and Examples

Atomic mass is generally called the atomic weight of any chemical element. it is the mass of a chemical element and is measured in the atomic mass unit. The mass shows the number of protons and neutrons in the nuclei of an atom. It was discovered by the scientist John Dalton at the start of the 19th century. Dalton said that each atom has a different atomic weight as their compositions are different. And this theory of Dalton builds the idea of atomic theory.

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This Story also Contains

1. Atomic Mass And Molecular Mass And Dalton's Atomic Theory
2. Some Solved Examples
3. Summary

Molecular mass is the mass of molecules that are formed by the combination of elements. The whole mass of the molecule including all of its elements is called the molecular mass. This theory was evolved from Dalton's work but further established by another scientist named Dmitri Mendeleev along with another chemist. Mendeleev developed the periodic table which helps us in understanding the relationship between the elements and their compounds. The knowledge of molecular mass is important for calculating the stoichiometry of chemical reactions and also the properties of the substance.

Atomic Mass And Molecular Mass And Dalton's Atomic Theory

Atomic Mass :

One atomic mass unit is defined as a mass exactly equal to one-twelfth the mass of one carbon-12 atom.

And 1 amu = 1.66056 x 10^-24 g

Mass of an atom of hydrogen = 1.6736 x 10^-24 g

Thus, in terms of amu, the mass of hydrogen atom

=1.6736×10−24gm1.66056×10−24gm

= 1.0078 amu

= 1.0080 amu

Similarly, the mass of oxygen - 16 (¹⁶O) atom would be 15.995 amu.

Today, 'amu' has been replaced by 'u' which is known as unified mass.

When we use atomic masses of elements in calculations, we actually use average atomic masses of elements which are explained.

Average Atomic Mass :

Many naturally occurring elements exist as more than one isotope. When we take into account the existence of these isotopes and their relative abundance (percent occurrence), the average atomic mass of that element can be computed.

Average Atomic Mass =Σ( Mass of Isotopes )i×( \%abundance )i100

In the periodic table of elements, the atomic masses mentioned for different elements actually represented their average atomic masses.

Molecular Mass :

Molecular mass is the sum of the atomic masses of the elements present in a molecule. It is obtained by multiplying the atomic mass of each element by the number of its atoms and adding them together.

Formula Mass :

It may be noted that in sodium chloride, One Na⁺ is surrounded by six CI^- and vice-versa. A formula such as NaCI is used to calculate the formula mass instead of molecular mass as in the solid-state sodium chloride does not exist as a single entity.

Thus. formula mass of sodium chloride = atomic mass of sodium + atomic mass of chlorine

= 23.0 u + 35.5 u = 58.5 u

Dalton's Atomic Theory

1. Matter consists of indivisible atoms.

2. Atom is indivisible and cannot be broken down.

3. All the atoms of a given element have identical properties, including identical mass. Atoms of different elements differ in mass.

4. Compounds are formed when atoms of different elements combine in a fixed ratio.

5. Chemical reactions involve the reorganization of atoms. These are neither created nor destroyed in a chemical reaction.

Recommended topic video on(Atomic Mass And Molecular Mass )

Some Solved Examples

Example.1 What is the standard for the present system of atomic masses?

1)H - 1

2) (correct)C - 12

3)He - 4

4)O - 16

Solution

The present system of atomic mass is based on Carbon - 12 as the standard and has been agreed upon in 1961.

Hence, the answer is an option (2).

Example.2 What is the mass equivalent to 1 amu?

1) (correct)112th of C-12 isotope

2)116th of O-16 isotope

3)H-1 isotope

4)14th of the He atom

Solution

1 amu = 112 thof the mass of C-12 atom.

Hence, the answer is an option (1).

Example. 3 What is the mass (in u) of one Nitrogen atom in u if its mass in g is 2.32597×10−23 g?

1)14

2)15

3)13.89

4) (correct)14.007

Solution

As we learned in

Relation between amu and Gram -

¹²C is assigned a mass of exactly 12 atomic mass units (amu).

1 amu = 1.66056×10^–24 g

mass in u=2.32597×10−231.66056×10−24=14.007u

Hence, the answer is (14.007u).

Example.4 What is the molecular mass of H₂SO₄?

(response should be like 67 or 70)

1)96u

2)100u

3)102u

4) (correct)98u

Solution

The molecular formula of H₂SO₄:
2H = 2u, S =32u, 4O = 64u

Thus, Molar mass of H₂SO₄ = 2 + 32 + 64 = 98u

Hence, the answer is (98u).

Example.5 5 moles of AB2 weigh 125×10−3 kg and 10 moles of A₂B₂ weigh 300×10−3 kg The molar mass of A(M_A) and molar mass of B(M_B) in kgmol−1 are :

1)MA=50×10−3 and MB=25×10−3

2)MA=25×10−3 and MB=50×10−3

3) (correct)MA=5×10−3 and MB=10×10−3

4)MA=10×10−3 and MB=5×10−3

Solution

Let the atomic mass of A = x & atomic mass of B = y

∴ For AB2 :

5(x+2y)=125g⇒x+2y=25g⋯(I)

& For A2B2 :

10(2x+2y)=300g

⇒x+y=15g⋯⋯(II)

Solving equations (I) & (II), simultaneously, we get:

x=5×10−3 kg & y=10×10−3 kg

Thus, MA=5×10−3 kg

MB=10×10−3 kg

Hence, the answer is the option (3).

Example.6 Given the reaction :

2A+6B+3C+3D→Ax−yBxC3+ByDz

What are the values of x, y & z?

1)3, 2, 3

2)2, 4, 3

3)4, 2, 1

4) (correct)4, 2, 3

Solution

The number of moles of atoms of reactant is equal to the number of moles of atoms of the product.

So, Balancing A

x−y=2−(1)

In B,

x+y=6−(2)

In D,

z=3

from (1) & (2),

x=4

y=2

Hence, the answer is the option (4).

Summary

Atomic mass is referred to as the average mass of the elements and basic framework of the periodic table as it helps to classify and in identification of the elements in the periodic table and it is also essential for calculating the stoichiometry of the reactant and the product which determine their quantity and by the help of atomic mass, we can calculate the formula mass of any compound to understand their property.