Relative Atomic Mass Unit: Gram Molar Mass, Atomic Mass Formula, Unit, Molecular mass and Examples

Do you know that an adult blue whale weighs around 2,00,000 kg? How about the weight of the sun? The estimated weight is around 2*10³⁰kg. How about the weight of an ant? They weigh close to 10^-4kg.

Now if you observe these numerical values, what strikes out the most is that the magnitude difference is quite huge. So what would be the desired outcome if we want to find and measure the weight of an atom? We could use the existing metric system but the practicality of using those measurements will be rendered useless owing to the very small weight of an atom.

So what do you think should be done in order to overcome this problem?

That's right. Develop a system which accounts for such small values. 

In the same line of thinking, it was John Dalton who first suggested a means of expressing relative atomic mass (the measurement term) in 1803. He proposed the use of hydrogen-1 (protium). In short mass of the proton is regarded as the basic unit of mass. So, the simplest unit of mass (mass of a proton) is considered an atomic mass unit (amu or ‘u’).

1 amu = 1.66 10^-24 g

Table of contents

• Relative atomic mass (R.A.M)
• Atomic mass
• Atomic mass unit
• Gram atomic mass (GMM)
• Molar mass
• Molecular mass
• Gram molecular mass
• Practice problems
• Frequently asked questions-FAQs

Relative Atomic mass (R.A.M)

Relative atomic mass =

Relative atomic mass = =total number of nucleons

Relative atomic mass = total number of nucleon

Note: Relative atomic mass is unitless.

Atomic mass

Mass of 1 atom and expressed in ‘amu’ or ‘u’

Mass of one atom = Mass of total proton + mass of total neutron + mass of total electron

During the calculation of atomic mass (mass of one atom) we neglect the mass of the electron and for numerical simplicity we consider the mass of one proton is equal to the mass of one neutron. 

We can say, Mass of a proton = mass of a neutron = mass of a nucleon

Mass of one atom = Mass of total protons + mass of total neutrons = mass of total nucleons

Mass of one atom = total number of nucleons mass of one nucleon

(total number of nucleon = sum of the number of neutrons and protons)

Mass of one atom = total number of nucleons 1.6610^-24g

Mass of one atom = total number of nucleon  amu

Mass of one atom = R.A.M  amu

Atomic mass = R.A.M  amu

Example. Calculate the atomic mass of N¹⁴ atom

A. 14 amu
B. 14 
C. 14 g
D. All of these

Answer: (A)

number of proton = 7

Number of neutron = 14 - 7 = 7

Total number of nucleons = 14

Atomic mass of N atom = 14 amu

Atomic mass unit

Gram atomic mass (G.A.M)

Atomic mass of an atom expressed in grams is called gram atomic mass or simply we can say, the mass of 6.022  10²³ atoms (Avagadro’s Number) is the gram atomic mass.

Example. Calculate gram atomic mass of N14 atom.

A. 14 amu
B. 14 kg
C. 14 g
D. All of these

Answer: (C)

Solution: we know, the atomic mass of N atom = 14 amu

Note:

Molar mass

Mass of 6.022 10²³ entities of any particles are considered as the molar mass of that particular substance

.

Molecular mass

Simply put mass of one molecule is called molecular mass.

Example. Calculate the molecular mass of water (H₂O)

A. 18
B. 18 amu
C. 18 g
D. All of these

Answer: (B)

Gram molecular mass (G.M.M)

Mass of 1 molecule expressed in grams or mass of 6.022 10²³ molecules.

Example. . Calculate gram molecular mass of water (H₂O)

A. 18
B. 18 amu
C. 18 g
D. All of these

Answer: (C)

Atomicity

It is defined as the total number of atoms present in one molecule is called its atomicity.

E.g- Potassium Permanganate (KMnO₄) has 1 atom of potassium + 1 atom of Manganese + 4 atoms of oxygen.

So, atomicity = 6

Practice problems

Q1. Calculate the molecular mass of O₃

A. 48
B. 48 amu
C. 48 g
D. All of these

Answer: (B)

Q2. Calculate the relative molecular mass of CO₂

A. 44
B. 44 amu
C. 44 g
D. All of these

Answer: (A)

Q3. Calculate the molecular mass of NH₃

A. 17
B. 17 amu
C. 17 g
D. All of these

Answer: (C)

Q4. Find atomicity of CuSO₄.5H₂O

A. 7
B. 10
C. 17
D. 21

Answer: (D)

Frequently asked question-FAQs

Question 1. Can relative atomic mass be defined in respect of other elements than carbon?
Answer: yes, mathematically we define relative atomic mass with respect to any other element but according to the scientific community carbon is taken as standard. 

Standard definition; Relative atomic mass =

In respect of oxygen; Relative atomic mass =

Question 2. What is the difference between atomic weight and atomic mass?
Answer: Atomic mass is generally used for single isotopes but atomic weight is generally the weighted average of isotopic abundance in nature. Atomic mass is always a whole number because it has a relation with the number of protons and neutrons in the nucleus. 

Question 3. Can relative atomic mass of any element be a fraction?
Answer: No, relative atomic mass can never be fractions because it is simply the total number of protons and nucleons. Protons and neutrons are never present in fractions.

Question 4. If relative atomic mass can not be a fraction why somewhere is fractional atomic mass or molar mass mentioned, e.g- Cl^35.5
Answer: Cl^35.5 written because of its isotopes, Cl present in nature in different isotopes in different natural abundance (%). So, due to their isotopes, they have fractional mass. We can call it weighted average atomic mass.

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