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Molarity of Pure Substances

23rd Nov 2019 @ 2 min read

Analytical Chemistry

The molarity (aka molar concentration) of a pure substance is the ratio of the density of the pure substance to its molar mass. It is calculated by the following formula.

Molar/Molar concentration of pure Compounds formula

Here, c is the molarity, ρ is the density, and M is the molar mass of pure compound.


Molarity is the number of moles of a substance (usually a solute) present in a litre of the solution.

Molarity formula

Here, ci is the molarity of a solute, mi is moles of the solute, and V is the volume of the solution.

In a pure substance, there is no solute or solvent, only one chemical. Thus, ci = c and ni = n.


Volume is mass divided by density; Volume is mass by density.

Molar concentration of pure substance formula derivation

Here, Molar mass is mass divided by density. is molar mass (mass divided by the number of moles).

Note: The term the molarity of a pure substance is inappropriate because molarity is a property of solutions. A solution is a homogeneous mixture of two or substances. And pure substances are not solutions. Molar density is the appropriate term to use here, not molarity or molar concentration. The ratio of density of a substance to the molar mass of the substance gives its molar density.

molar density formula

In short, molar density and molarity are the same for pure substances ignorantly.

Molar density = molarity for pure substance


The molar concentration (or molar density correctly) of some common chemicals is calculated below.

Example 1: Water

ρH2O = 1000 g L−1 and MH2O = 18.0 g mol−1.

Molarity/Molar concentration of H2O (water)

Example 2: Ethanol

ρC2H6O = 789 g L−1 and MC2H6O = 46.1 g mol−1.

Molarity/Molar concentration of ethanol

Example 3: Methanol

ρCH4O = 792 g L−1 and MCH4O = 32.0 g mol−1.

Molarity/Molar concentration of methanol

Example 4: Benzene

ρC6H6 = 876 g L−1 and MC6H6 = 78.1 g mol−1.

Molarity/Molar concentration of benzene

Example 5: Acetic acid

ρC2H4O2 = 1050 g L−1 and MC2H4O2 = 60.1 g mol−1.

Molarity/Molar concentration of acetic acid

Example 6: Glycerol

ρC3H8O3 = 1260 g L−1 and MC3H8O3 = 92.1 g mol−1.

Molarity/Molar concentration of glycerol

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