Molar mass

The molar mass M(also obsolete molar mass or molar weight; uncommonly substance quantity-related mass) of a substance is the mass per quantity of substance or, in other words, the proportionality factor between mass mand quantity of substance n:

{\displaystyle m=M\cdot n}.

It is an intensive quantity. The SI unit is kg/mol; in chemistry g/mol is common.

The molar mass of a chemical compound is the sum of the molar masses of the chemical elements involved in the compound multiplied by the respective stoichiometry factor. Stoichiometry factors are the numbers in the molecular formula of a molecule or, in the case of non-molecular compounds (metals and ionic compounds), the ratio formula.

The molar mass of an isotopically pure substance is constant. The numerical value of the molar mass in g/mol is the relative molecular mass and is equal to the numerical value of the molecular mass in the atomic mass unit (u or dalton if the compound consists of nuclide-pure elements).

Definition

M={\frac {m}{n}}=N_{{\mathrm {A}}}\cdot m_{M}

The individual formula symbols stand for the following quantities:

  • m- mass
  • n - amount of substance
  • N_{{\mathrm A}}- Avogadro constant
  • m_{M}- particle mass
The relationship between mass, amount of substance, volume and number of particles.Zoom
The relationship between mass, amount of substance, volume and number of particles.

Calculation

The molar mass of a compound can be calculated if its molecular formula is known: For each element, the index number is taken from the molecular formula - it is given in the molecular formula after the element symbol. For each element, the molar mass must then be taken from tables, for example - its numerical value is equal to the relative atomic mass. Then you get the molar mass as the sum of the molar masses of the elements that make up the compound:

The molar mass of a compound is equal to the sum of the molar masses of the elements multiplied by their index numbers.

Example of water (H2O):

M_{\mathrm{H_{2}O}} = 2 \ M_\mathrm{H} + M_\mathrm{O} = 2 \cdot 1{,}00794\,\frac{\mathrm{g}}{\mathrm{mol}} + 15{,}9994\,\frac{\mathrm{g}}{\mathrm{mol}} = 18{,}01528\,\frac{\mathrm{g}}{\mathrm{mol}}

From the molar masses of the chemical elements one can calculate the molar masses of all compounds.

Item

Element symbol

Atomic number

Molar mass

Hydrogen

H

1

1.00794 g/mol

Carbon

C

6

12.0107 g/mol

Oxygen

O

8

15.9994 g/mol

 

Link

Sum formula

atomicity

Molar mass

Hydrogen

H2

2

2.01588 g/mol

Oxygen

O2

2

31.9988 g/mol

Water

H2O

3

18.01528 g/mol

Methane

CH4

5

16.043 g/mol

Acetylsalicylic acid

C9H8O4

21

180.16 g/mol

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