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×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.

The atomic mass constant is used to define the atomic mass unit (or the unified atomic mass). It equals one atomic mass unit. The atomic mass unit is the standard unit used to quantify mass on the atomic scale. Also, the atomic mass unit, the unified mass unit, and the dalton are all synonymous units and are interchangeably used.

The ideal gas constant is also known as the universal gas constant or the molar gas constant or simply the gas constant. It is a very important constant in chemistry and physics. It is denoted as *R*. The dimension of the gas constant is expressed in energy per unit mole per unit temperature. The value of the gas constant in SI unit is 8.314 J mol^{−1} K^{−1}. The gas constant has the same unit as of entropy and molar heat capacity.

The Boltzmann constant is a very important constant in physics and chemistry. The constant relates the average kinetic energy of molecules of a gas with thermodynamic temperature. The Boltzmann constant is denoted as *k*_{B} or *k*. The dimension of the Boltzmann constant is energy per thermodynamic temperature. The SI unit is J K^{−}^{1}, which is the same as of entropy. The value of the Boltzmann constant is 1.380 649 × 10^{−}^{23} J K^{−}^{1}.

An amagat is a unit of number density. It is similar to Loschmidt constant which is the molecular density quoted at STP (*T* = 273.15 K, *P* = 1 atm). 1 amagat is defined as the number of molecules of an ideal gas per unit volume at STP. 1 amagat is equivalent to Loschmidt constant. The symbol used for amagat is amg or Am (rarely). It is a non-SI unit.

The number is named after Émile Hilaire Amagat. He was a French physicist and also known for Amagat’s law of additive volume.

The Loschmidt constant is also called as the Loschmidt number. The symbol used for the Loschmidt constant is *n*_{0}. It is the number of molecules of an ideal gas per unit volume. The Loschmidt constant has the unit of the reciprocal cubic metre. At STP (*P* = 1 atm, *T* = 273.15 K), The value of the constant is 2.686 781 1 × 10^{25} m^{−3}. The constant is a measure of number density.

The Avogadro constant or (the Avogadro number earlier) is the number of elementary units in one mole of any substance. The Avogadro constant is denoted as *N*_{A}. It has the dimension of the reciprocal amount of substance (mol^{−1}). The approximate value of *N*_{A} is 6.022 × 10^{23} mol^{−1}. This means one mole of any substance contains 6.022 × 10^{23} elementary particles. The Avogadro constant is named after Italian scientist Amedeo Avogadro.

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