Mole Concept, Molar Mass and Percentage Composition

Mole Definition 

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.

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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.


What Is Molar Mass?

Related to the concept of moles, the molar mass of a substance refers to the total mass of one mole of a substance (i.e. molar mass = mass of one mole which is equal to 6.02214076×1023  number of particles of that substance ).

We will also talk about the relation between atomic mass and molar mass.

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).

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What is Molecular Mass?

Molecular mass is used to define the mass of a single molecule of a compound. 

So when we calculate the molecular mass of a compound such as methane or even dihydrogen we first see the number of different atoms bonding in the molecules and add the respective atomic mass units to give the molecular mass.

Molecular mass is expressed in terms of Daltons (Da) and 1 Da = 1 u 

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Define 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 = \[\frac{\text{Moles of Solute}}{\text{Moles of Solute + Moles of Solvent}}\]

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 is Relative Molecular Mass?

To discuss relative molecular mass, we should know about relative atomic mass first.

Relative atomic mass was founded to serve as a solution for establishing a single atomic mass for elements having several isotopes. Hence, we find the relative atomic mass of an element by finding an average of weighted isotopic mass according to their frequency.

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And similar to molecular mass, the relative molecular mass is just the sum of the relative atomic mass of the different number of elements present in a compound.

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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.


Did You Know 

The number of atoms or other particles in a mole is equal for all substances. For example, one mole of carbon has 6.02214076 × 1023 atoms & mass = 12g. Similarly, a mole of oxygen also has the same number of atoms as carbon, however, it has a higher mass at 16 g.

Molality: Similar to molarity this property is also dependent on the number of moles of solute. Molality is the moles of solute that are present per kilogram of solvent. When we use  “liter”, it is a measure of volume which is liable to change at different temperature and pressure and therefore, molality serves as a better and more reliable indicator of concentration in some cases.

FAQs (Frequently Asked Questions)

Q1. How are Molarity and Normality Related?

When we calculate molarity we use the gram molecular weight of an element or a compound. That is to determine the number of moles present we divide the given mass of substance dissolved by its gram molecular weight. Similarly, in Normality, we divide the given dissolved mass by its equivalent weight which is just its gram molecular weight divided by its valence factor. 

Valence factor refers to the number of electrons it can donate or accept accordingly. In the case of sodium hydroxide (NaOH) the gram equivalent weight and the molecular weight are equal as the valence factor is equal to one.

Q2. What are Colligative Properties and Give their Examples?

Colligative properties are characteristic of substances that are only dependent on the number of particles present and independent of the nature of particles. In other words, the colligative properties of a solution are exhibited by the concentration of solute particles and hence are related to molarity, normality, mole fraction, molality.

Some very common examples are elevation of the boiling point of a solution compared to a pure solvent, decrease in the freezing point of a solution, and finally the increase in the osmotic pressure of a solution with the addition of a solute.