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Molar Mass Constant

26th Jun 2019 @ 2 min read

Physical Chemistry

The molar mass constant is a quantity similar to the atomic mass constant. The constant, as from the name, is related to the molar mass of carbon-12.

Definition of Molar Mass Constant

It is defined as the one-twelfth of the molar mass of carbon-12.

Symbol

The molar mass constant is denoted by Mu.

Unit

The unit of the molar mass constant is g mol−1.

Formula

According to the above definition, the formula of the constant is as follows:

where 12C is the molar mass of carbon-12.

Value of Molar Mass Constant

The approximate value of Mu is 1 g mol−1. The more precise value, as per Committee on Data for Science and Technology (CODATA), is given below.

As we can see the above-mentioned value is very close to one. Thus, we can say for practical purposes Mu ≈ 1.

Molar Mass Constant, Atomic Mass Constant, and Avogadro Constant

The molar mass constant, as mention earlier, is given as:

Here, M(12C) is the molar mass of carbon-12, or in other words, the mass of carbon-12 in one mole.

The molar mass of carbon-12 can be expressed as the mass of one carbon-12 atom (m(12C)) multiplied by the Avogadro’s constant (NA).

Using the above two equations , we get,

From the definition of the unified mass unit (u) and the atomic mass constant (mu), we have the below relationship.

From the above two equations, we have

Using Mu ≈ 1 g mol−1,

Relation with Relative Atomic Weight

Relative atomic weight is a dimensionless quantity and it is denoted as Ar. The relative atomic weight of carbon-12 is 12.

We can replace the numeral 12 in the equation Mu = 112 M(12C) by Ar(12C). Thus, we have

From the above equation, the molar mass constant is the ratio of the molar mass of carbon-12 to the relative atomic weight of carbon-12.

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