Molar mass

The molar mass M (also molar mass; unusual material volume density) is the ratio of the mass m and the molar amount of substance n: M = m / n The SI unit is kg / mol; in chemistry is g / mol usual.

The numerical value of the molar mass of a chemical compound is given by the sum of the average atomic masses of the chemical elements involved in the connection of one mole (approximately 6.1023 ) or particles of one mole of the structural elements of the connection. In the case of molecular compounds is the empirical formula of the molecule and uses the empirical formula of the compound used to calculate the molar mass in the case of non-molecular compounds (metals and ionic compounds ). The numerical value of the molar mass in g / mol corresponding to the molecular weight.

The molar mass is a material constant and thus an intensive quantity in contrast to all extensive quantities.

Definition

The individual symbols stand for the following sizes:

Calculation

The molar mass of a compound can be calculated if one knows their sum formula: For each element takes you from the molecular formula, the index number - it is in the empirical formula after the element symbol. For each element you have to then take out the molar mass, for example, from tables - its numerical value is equal to the relative atomic mass. Is then obtained as the sum of the molar mass of the molecular weights of the elements that make up the connection:

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

Example of water ( H2O):

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

Determination

For the determination of the molecular weights of the molecules Avogadro's law was determinative. In the gaseous state, many molecules absorb at the same temperature equal to a nearly identical volume one.

For simple molecules, such as chlorine, hydrogen, hydrogen chloride, oxygen, water vapor, could be determined from the ratios of the gases by a weighing electrolysis.

With the method according to Bunsen can be molar masses of gases across the Ausströmungszeiten determine.

For complex organic molecules are first used also the Avogadro's law, by the evaporated pure organic substances and the displaced water volume specific. The method was first applied by Joseph Louis Gay -Lussac, later improved by Victor Meyer. An older method is to Dumas, wherein the materials were also evaporated.

Many methods for determining molecular weight are based on the measurement of effects, the size of which is dependent on the number of causing the particles, but not the mass ( colligative effects). These methods include the measurement of the freezing point depression (cryoscopy ), boiling point of the measurement of the elevation (ebullioscopy ) and osmometry.

For non-evaporable molecules was once used the freezing point depression or osmotic pressure of solutions. The latter method developed Jacobus Henricus van't Hoff.

In mass spectrometry results in the relative molar mass of the molecular peak. In high-resolution mass spectrometry, the molecular weight to four decimal places can be determined, so that the molecular formula can be determined.

Related sizes

  • Atomic mass unit
  • Average molecular weight
  • Molar volume ( molar volume )
  • Molecular mass or molecular mass or molecular mass (formerly molecular weight)
System of measurement Quotient
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