The key difference between mole fraction and weight percent is that mole fraction gives the composition of a substance in a mixture by means of moles while weight percent gives the composition by means of mass.
Mole fraction and weight percent or mass fraction are two different ways of expressing the composition of a component in a mixture. There, these two values are not equal, but the method of determination is similar because, in both phenomena, we need to divide the mole number or mass of the desired component by the total mole number or total mass of the mixture to get the value. Generally, weight percent is expressed as a percentage value.
CONTENTS
1. Overview and Key Difference
2. What is Mole Fraction
3. What is Weight Percent
4. Side by Side Comparison – Mole Fraction vs Weight Percent in Tabular Form
5. Summary
What is a Mole Fraction?
Mole fraction of a component in a mixture is the value given by dividing the number of moles of a component by the total mole number of a mixture. The value gives how much of a particular component is present in a mixture (the composition of the component by means of moles). Usually, we give this value as a fraction or in one or two decimal places. Since it is a ratio, the mole fraction is unitless.
We can determine the mole fractions of each component in a mixture, and by adding these fraction values together, we get 1. Let us consider an example; if there is a mixture containing 2 moles of sodium chloride and 5 moles of water in a salt solution, the total number of moles equals 2+5=7. Here, if we need to calculate the mole fraction of sodium chloride,
Mole fraction of sodium chloride = moles of sodium chloride/total moles in the mixture
=2/(2+5)
=2/7
=0.28
Figure 01: Sodium Chloride Mole Fraction in Salt Water
What is Weight Percent?
Weight percent or mass percent is the percentage of the mass of a component in a mixture. We call it the mass fraction as well. Further, this term describes the ratio between the mass of the desired component and the total mass of the mixture multiplied by 100 to get a percentage value. The formula for this determination is as follows:
Weight percent = (mass of the component/total mass of the mixture)*100
The denotation of the mass percent is (w/w)%.
Although the above formula gives a general idea about the mass percent, the application of the formula varies depending on the type of components and the mixture that we are dealing with. For example,
- For a chemical element in a compound, the mass percent is calculated as follows:
Weight percent = (mass of the element per mole/mass of a mole of the compound)*100
- For a solute in a solution,
Weight percent = (grams of solute/grams of solute plus solvent)*100
What is the Difference Between Mole Fraction and Weight Percent?
Mole fraction and weight percentage are two different ways of expressing the concentration of a component in a mixture. The key difference between mole fraction and weight percent is that the mole fraction gives the composition of a substance in a mixture by means of moles, while the weight percent gives the composition by means of mass. Furthermore, the value of mole fraction is given as a fraction or in one or two decimal places while weight percent is given as a percentage value.
Below infographic summarizes the difference between mole fraction and weight percent.
Summary – Mole Fraction vs Weight Percent
Mole fraction and weight percentage are two different ways of expressing the concentration of a component in a mixture. The key difference between mole fraction and weight percent is that the mole fraction gives the composition of a substance in a mixture by means of moles, while the weight percent gives the composition by means of mass.
Reference:
1. Helmenstine, Anne Marie. “What Is a Mole Fraction?” ThoughtCo, May. 6, 2019, Available here.
Image Courtesy:
1. “Salt mole fraction” By Jzana – Own work (CC BY-SA 3.0) via Commons Wikimedia