[Chemistry Class Notes] Mole Concept, Molar Mass and Percentage Composition Pdf for Exam

Everything that surrounds us is composed of millions and millions of tiny particles, known as atoms. While these are invisible to the naked eye, they are the fundamental building blocks of this universe. Studying these substances and how their composition plays a vital role in our everyday existence is a very important part of applied sciences. 

The mole concept comes into play when we talk about atoms and their properties. 

What is a Mole? 

To introduce the concept of a mole, first, we will talk about the Avogadro’s number/constant which is equal to 6.02214076 × 10²³. 

When we talk about the number of moles of a substrate we divide the number of total particles of a molecule/compound/atom by Avogadro’s constant.

Hence, 6.02214076×10²³  molecules of an oxygen molecule (O₂) will constitute one mole of oxygen & similarly 6.02214076×10²³ atoms of carbon will constitute one mole of carbon. In other words, when we say one mole of a substance is present in a sample that means that 6.02214076×10²³ number of that specific element is present in the sample. 

Therefore, a mole is a unit just like kilograms or centimetres, which refers to the specific measure of how many atoms or molecules are contained in a substance. 1 mole of one element contains the same quantity of atoms as 1 mole of any other element. However, the mass of 1 mole is different for every element. 

What Do We Understand by Molar Mass? 

The molar mass of an element means the total mass of one mole of that element. Relative molar mass refers to the smallest mass unit of any compound in comparison to one-twelfth one of the mass of one Carbon-12 atom. This means that the number of atoms that are present on 12 grams of one Carbon-12 atom is the same as the number of moles present in one mole of it. 

Using a mass spectrometer, it has been found that the mass of a Carbon-12 atom is 1.992648×10^-23 g. In any element, the total number of sub-particles present, such as atoms and molecules, is collectively referred to as one mole.

Atomic mass strictly refers to the mass of one single atom of an element and is expressed in Atomic Mass Units (u). 

One atomic mass unit =1.9944235 × 10^-23 g

One molar mass of an element would be the mass of one mole of that element’s atoms.

So numerically, one molar mass = mass of (6.02214076×10²³ * Atomic mass of that element).  

What Do We Understand by Molecular Mass? 

This term is used to refer to the mass of a single molecule of any given compound or element. The molecular mass of an element or a compound is calculated by first calculating the number of atoms that are bonding in the molecules and adding that with the respective atomic mass units. 

Molecular mass is denoted by Daltons (Da), where 1 Da is equal to 1 u. 

What is Mole Fraction?

Mole fraction is also a method to express the concentration of a substance in a mixture. Of a solution mixture containing different elements in different quantities, we express the concentration of a specific element(X) by dividing the number of its moles present by the total number of moles that includes the number of moles of other constituents and the moles of (X) included. 

Mole fraction of solute = Moles of Solute/ Moles of (solute + solvent) 

Mole fraction is just a ratio and is hence dimensionless.

Percentage Composition

Another indicator of concentration, percentage composition also depicts the proportion contributed to the total mass of a mixture by its mass. The formula for percentage composition is given by:

Percentage composition = (Mass of the individual component/mass of the mixture) * 100.

Taking Methane as an example, methane’s percentage composition is  74% carbon and 25% hydrogen, though there are more hydrogen atoms. 

What Does Relative Molecular Mass Indicate? 

To understand this, it is first important to know what relative atomic mass refers to. This simply means having a single and uniform atomic mass for elements that may have many different isotopes, such as Carbon. The relative atomic mass of an element can be found out by calculating the average of the weighted isotope mass, as per their frequency. 

Therefore, the relative molecular mass refers to the total relative atomic masses of all the elements that are present in one compound. 

Molarity Meaning

Molarity is a very common indicator of concentration in which we determine the number of moles of a chemical species (solute) that has been dissolved in a liter of solvent. To determine Molarity we first find out the number of moles that are present inside the solution. 

We go about it by dividing the quantity of a substance that has been dissolved in 1 L of solvent and dividing it by the molar mass of that substance which will give us the Molarity.