What is Mole Fraction?
The mole fraction (χ) in chemistry is a unit of concentration that expresses the ratio of the number of moles of a particular component to the total number of moles of all components in a mixture. It is a dimensionless quantity, meaning it has no units, as it is a ratio of two amounts of substance (moles).
Calculating Mole Fraction
To calculate the mole fraction of a component, you divide the moles of that component by the sum of the moles of all components present in the mixture. For a mixture with components A, B, and C, the mole fraction of A (χ_A) would be moles A / (moles A + moles B + moles C). The sum of all mole fractions in a mixture must always equal 1.
A Practical Example
Consider a gas mixture containing 0.5 moles of nitrogen (N₂) and 1.5 moles of oxygen (O₂). The total moles in the mixture are 0.5 + 1.5 = 2.0 moles. The mole fraction of nitrogen (χ_N₂) is 0.5 moles / 2.0 moles = 0.25. The mole fraction of oxygen (χ_O₂) is 1.5 moles / 2.0 moles = 0.75. Notice that 0.25 + 0.75 = 1.0.
Importance and Applications
Mole fraction is particularly useful in physical chemistry and engineering, especially when dealing with gas laws (like Raoult's Law and Dalton's Law of Partial Pressures) and colligative properties of solutions. It directly relates to the partial pressure of a gas in a mixture and is a fundamental concept for calculating thermodynamic properties of mixtures, such as entropy and free energy.