What is Molar Mass Definition and Absorptivity?

Molar mass is the mass of one mole of a substance, expressed in grams per mole. It is a physical property commonly used in chemistry to relate the amount of a substance to its mass.

One mole of a substance is defined as the amount of that substance that contains as many elements (atoms, molecules, ions, etc.) as there are atoms in exactly 12 grams of carbon-12. This number is called Avogadro's number (6.022 x 10 ^ 23 units per mole).

The molar mass of a substance is equal to its atomic mass, molecular mass, or formula weight in grams per mole. For example, the molar mass of water (H2O) is approximately 18.015 grams per mole, which is the sum of the atomic masses of two hydrogen atoms and one oxygen atom.

Molar mass is an important concept in chemistry. It is used to convert between mass and volume (moles) of a substance, which is essential for many calculations such as determining the limiting reactant in a chemical reaction or solution.

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Molar Mass Calculations 

To calculate the molar mass of a substance, the atomic masses of all the atoms in its chemical formula have to be added. Below are the steps to calculate the molar mass:

How to Calculate Molar Mass

  • Determine the chemical formula of the substance.
  • Finding the atomic mass of each element in a chemical formula. These are found in the periodic table.
  • The atomic mass of each element in a chemical formula is multiplied by the number of atoms of that element.
  • Multiply the products of the atomic masses of each element by the number of atoms of that element and add them all up. As mentioned in the example.


The chemical formula of water is H2O.

The atomic mass of hydrogen (H) is 1.008 and the atomic mass of oxygen (O) is 15.999.

There are 2 hydrogen atoms in H2O, so you would multiply the atomic mass of hydrogen (1.008) by 2. There is 1 oxygen atom in H2O, so you would multiply the atomic mass of oxygen (15.999) by 1.

For H2O, this would be: (2 x 1.008) + (1 x 15.999) = 18.015 g/mol.

Molar Mass Unit

The unit of molar mass is gram per mole (g/mol). It represents the mass of one mole of a substance, which is equal to its molecular or atomic weight. Molar mass is useful in chemical calculations.

Molar Absorptivity

Molar absorptivity, also known as molar extinction coefficient or molar absorptivity index, is a measure of how strongly a substance absorbs light at a specific wavelength. It is represented by the symbol ɛ (epsilon) and has units of L mol^-1 cm^-1.

Molar absorption is a characteristic property of a substance and is related to its molecular structure and electronic transitions. It is a measure of the efficiency of light absorption by a solution of a substance and is used to determine the concentration of a substance in solution using spectrophotometry.

The Beer-Lambert law

The Beer-Lambert law gave the equation relating molar absorption to the concentration of the substance in solution and the thickness of the sample which is as follows:

A = ɛ l c


A = is the absorbance.

ɛ = is the molar absorptivity.

l = is the path length of the sample.

c = is the concentration of the absorbing species.

Molar absorbance is an essential parameter in analytical chemistry and biochemistry, where it is used in various applications such as determining the concentration of a substance in solution, studying enzyme kinetics, and characterizing proteins and nucleic acids.


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