Molar fraction
The mole fraction is a chemical unit used to express the concentration of a solute in a solution. It is defined as the quotient between the moles of solute and the total moles of the solution, which is calculated by adding the moles of solute(s) and solvent:
<math alttext="{displaystyle x_{i}={frac {n_{i}}{n_{t}}}xi=nint.1{displaystyle x_{i}={frac {n_{i}}{n_{t}}}}{displaystyle x_{i}}={frac {n_{i}}}}}{n_{n}}{n}}{n}}{n}}}}}{<img alt="{displaystyle x_{i}={frac {n_{i}}{n_{t}}}
It can also be expressed as a percentage (called a "mole percentage"):
The sum of the mole fractions of all the components of a mixture is unity:
␡ ␡ i=1nxi=x1+x2+...+xn=1{displaystyle sum _{i=1}^{n}x_{i}=x_{1} +x_{2} +...+x_{n}=1}
For example, in a binary mixture of 6 moles of ethanol and 4 moles of water, giving a total of 10 moles, the mole fraction of ethanol is 6/10 = 0.6; while the mole fraction of water is 4/10 = 0.4.
Since the volume of a solution depends on temperature and pressure, when these change the volume does too, which affects other ways of expressing concentration, such as molarity. Instead, the mole fraction is independent of temperature and pressure.
In addition, it should be noted that in ideal gases the partial volume of each of the components is proportional to the number of moles of it, and the same happens with the volume of the mixture and the total number of moles. Consequently, the mole fraction of a component is equal to the fraction corresponding to its partial volume with respect to the total. Similarly, the ratio of the partial pressure of a component of an ideal gas mixture to the total pressure is equal to the mole fraction of that component (Dalton's law of partial pressures).