In the world of chemistry, there are numerous ways to express the concentration of a solution. One such method is through the use of a mole fraction. This unit of concentration measures the relative amount of solute and solvents present in a solution and is symbolized by “X.”
Simply put, the mole fraction is the ratio of the number of moles of a particular component to the total number of moles in the solution. This is a very useful tool when dealing with solutions, as it allows us to accurately determine the concentration of each component.
The Formula for Mole Fraction
Let's assume we have a solution composed of two substances, A and B. In this case, the mole fraction of each substance can be calculated using the following formulas:
And
It's important to remember that:
1. The sum of all mole fractions in a given mixture equals one. (XA + XB = 1)
2. The mole fraction multiplied by 100 gives the mole percentage.
3. The mole fraction is a unitless and dimensionless expression.
You don't need to know the density of the phase to calculate the mole fraction.
In an ideal gas mixture, the mole fraction is represented by the ratio of partial pressure to the total pressure of the mixture.
Example Problems
Example 1: If we dissolve 0.025 moles of NaCl in 200 grams of pure water, what are the mole fractions of NaCl and H2O?
Example 2: Find the mole fraction of benzene in a solution containing 2 moles of benzene, 3 moles of carbon tetrachloride, and 5 moles of acetone.
Example 3: A solution is prepared by dissolving 30.0 grams of formaldehyde (CH2O) in 4.00 moles of carbon tetrachloride (CCl4). What is the mole fraction of formaldehyde?
Example 4: Determine the mole fractions of each component in a solution prepared by mixing 35.0 grams of ethanol and 35.0 grams of water.