# Convert Composition Expressed in Weight Fraction to Mol Fraction

In this post, I want to share you a simple calculation that may be useful in the future of your process engineering career 😊. This post explains how to convert composition expressed in weight fraction to mol fraction.

Before we jump into the calculation method, let’s first understand that it is important to state the basis of percentage, whether it is expressed in weight basis, molar basis, or volume basis. The abbreviations w/w and v/v are used to designate weight basis and volume basis.

Let’s use examples on how to convert composition expressed in weight fraction to mol fraction.

In this example, we want to express in mol fraction a technical grade hydrochloric acid which has strength of 28% w/w.

Solution

When the known concentration is expressed in weight basis, we need to assume basis of calculation in weight basis also.

For example, basis of calculation is 100 gr of 28% w/w HCl

• Molecular weight of water = 18
• Molecular weight of HCl = 36.5
• Mass of HCl = 28% x 100 gr = 28 gr
• Mass of water = (100% – 28%) x 100 gr = 72 gr
• Mol HCl = 28 / 36.5 = 0.77
• Mol water = 72 / 18 = 4
• Total mol = 0.77 + 4 = 4.77

Therefore,

• Mol fraction of HCl = mol HCl / total mol = 0.77 / 4.77 = 0.16
• Mol fraction of water = mol water / total mol = 4 / 4.77 = 0.84
• Check, total mol fraction = 0.16 + 0.84 = 1