Mole concept calculation with examples.

The mole concept is the most relevant topic in chemistry. It is a concept that students must grasp to do most of the chemistry calculations. In this article, we are going to find how many atoms are in a mole of a substance, how to calculate moles of a substance, and how and how to calculate the mass of a substance using molar mass.

The molar mass of compounds

To know how to calculate the mole concept, one must know how to find the molar mass of a given molecule. Molar mass is the mass of a substance per mole. These values can be obtained using a periodic table.

For example, water (H2O) has a molar mass of 18.02g/mol, whereby the atomic mass of oxygen is 16 amu, and the mass of hydrogen is 1.01amu, but since they are two, it becomes 2.02amu. Thus the total is 18.02g/mol.

When you add amu of atoms, the final answer is g/mol for a molecule or compound.

If the molar mass and mass of a substance are known, you can find its moles. Consequently, you can find the molar mass of substances using the mass and moles of that compound.

The formula for molar mass is : molar mass= mass/moles ,

 for moles is : moles=/mass/molar mass.

History of the mole concept.

The mole concept was first introduced in 1894 by a German chemist called Wilhelm Ostwald, in 1894. He defined one mole as the molecular weight of a substance in units of mass grams.

Avogadro’s constant

The Avogadro’s constant was introduced by a scientist by the name of Avogadro. He discovered that different gases contain an equal number of molecules at constant pressure, volume, and temperature.

The constant is 1mole= 6.022×10²³molecules/atoms

Application of mole concept.

The mole concept is applied in the following areas.

Converting mass to moles.

To solve it, we must know the substance’s molar mass and its mass. An example is: How many moles of potassium are present in 64g of potassium metal.

We know the molar mass of potassium (K) is 39g/mol

Moles=mass/molar mass: 64g/39g/mol=1.64moles

Converting moles to mass

Here, we have to multiply the moles given by the molar mass of that molecule. Example of such calculation: how many grams are in 0.5moles of oxygen gas?

Mass=moles x molar mass: 0.5moles x32g/mol=16g

Converting moles to the number of atoms or molecules.

In order to do this calculation, we must know that; 1mole= 6.022×10²³molecules/atoms. Let’s use the examples above.

1)how many atoms are present in 1.64moles of potassium?

• 1mole= 6.022×10²³atoms
• =1.64moles x (6.022×10²³atoms/1mole) =9.88×10²³atoms
• 2) How many molecules are in 0.5moles of oxygen?
• 1mole= 6.022×10²³molecules
• =0.5moles x (6.022×10²³molecules/1mole) =3.01×10²³atoms

Converting mass to atoms.

When you are asked to convert mass to atoms. The first thing is to convert mass to moles. Then you convert moles to the number of atoms.

Example: convert 3g of Calcium to the number of atoms.

• Moles of Calcium = mass/molar mass=3g/40g/mol=0.075moles
• Number of atoms =moles x Avogadro’s number =0.075moles x (6.022×10²³atoms/1mole) =4.52×10²²atoms

Converting atoms to moles

Example: How many atoms are in 1mol of a sodium atom.

To solve this problem, we use Avogadro’s constant, then we multiply by moles the sodium given.

Number of atoms = 1mol x (6.022×10²³atoms/1mol)=6.022×10²³atoms

The following table is designed to help you with mole concept calculation.

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