मोल अंश

रसायन विज्ञान में, किसी मिश्रण के किसी अवयव का मोल अंश या मोल प्रभाज (mole fraction या molar fraction ; संकेत : <math>x_i</math>) निम्नलिखित प्रकार से पारिभाषित है-

<math>x_i = \frac{n_i}{n_{tot}}</math>

जहाँ,

<math>n_i</math> = अवयव के मोलों की संख्या

तथा,

<math>\sum_{i=1}^{N} n_i = n_{tot} ; \ ; \sum_{i=1}^{N} x_i = 1</math>

अर्थात सभी अवयवों के अणु-अंशों का योग 1 होता है।

अणु-अंश के गुण

अणु-अंश का उपयोग प्रायः प्रावस्था आरेख (phase diagram) के निर्माण में किया जाता है। इसके अनेक लाभ हैं-

• यह ताप पर निर्भर नहीं है। (such as molar concentration) and does not require knowledge of the densities of the phase(s) involved
• a mixture of known mole fraction can be prepared by weighing off the appropriate masses of the constituents
• the measure is symmetric: in the mole fractions x=0.1 and x=0.9, the roles of 'solvent' and 'solute' are reversed.
• In a mixture of ideal gases, the mole fraction can be expressed as the ratio of partial pressure to total pressure of the mixture

मोल अंश से सम्बन्धित अन्य राशियाँ

Mass fraction

The mass fraction <math>w_i</math> can be calculated using the formula

<math>w_i = x_i \cdot \frac {M_i}{M}</math>

where <math>M_i</math> is the molar mass of the component <math>i</math> and <math>M</math> is the average molar mass of the mixture.

Replacing the expression of the molar mass:

<math>w_i = x_i \cdot \frac {M_i}{\sum_i x_i M_i}</math>

अणु प्रतिशत (Mole percentage)

Multiplying mole fraction by 100 gives the mole percentage, also referred as amount/amount percent (abbreviated as n/n%).

Mass concentration

The conversion to and from mass concentration <math>\rho_i</math> is given by:

<math>x_i = \frac{\rho_i}{\rho} \cdot \frac{M}{M_i}</math>

where <math>M</math> is the average molar mass of the mixture.

<math>\rho_i = x_i \rho \cdot \frac{M_i}{M}</math>

Molar concentration

The conversion to molar concentration <math>c_i</math> is given by:

<math>c_i = \fracसाँचा:x i \cdot \rhoṃ = x_i c </math>

or

<math>c_i = \fracसाँचा:x i \cdot \rhoसाँचा:\sum i x i M i </math>

where <math>M</math> is the average molar mass of the solution, c total molar concentration and <math>\rho</math> is the density of the solution .

Mass and molar mass

The mole fraction can be calculated from the masses <math>m_i</math> and molar masses <math>M_i</math> of the components:

<math> x_i= \frac{{\fracसाँचा:m iसाँचा:M i}}{{\sum_i \fracसाँचा:m iसाँचा:M i}}</math>

Spatial variation and gradient

In a spatially non-uniform mixture, the mole fraction gradient triggers the phenomenon of diffusion.