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Fraction molaire

La fraction molaire: x, ou le pourcentage molaire (x%) est une grandeur utilisée en chimie et en métallurgie pour exprimer la composition d'un mélange ou d'un alliage (c'est-à-dire les proportions de chaque composant dans le mélange). Lorsque les espèces chimiques sont des atomes, on utilise aussi le terme pourcentage atomique.

La fraction molaire d'un constituant i est égale au rapport du nombre de moles de ce constituant ni sur le nombre total de moles du mélange ntot. Elle est donc une grandeur sans dimensions.
x_i =\frac {n_i}{n_{tot}} ce qui donne en pourcentage x_i% =\frac {n_i}{n_{tot}}.100
La somme des fractions molaires des constituants du mélange est égale à l'unité.
\sum x_i = 1

Cette grandeur permet de décrire les rapports entre les nombres d'atomes de chaque espèce; elle est intéressante pour les mélanges ou pour les alliages intermétalliques.

Autres façons d'exprimer la composition

Il existe plusieurs autres façons d'exprimer la composition ou la concentration:

molarité

molalité

ppm, ppb

pression partielle

CH ou DH: dilution Centésimale ou Décimale, Hahnemanienne, réservée à l'homéopathie.

Voir aussi

  • Portail de la chimie Portail de la chimie
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