# The Concept Of Mole

The quantity of a given substance that contains as many units or molecules as the number of atoms in 12 grams of carbon-12 is called a

**mole**. For example, one mole of glucose contains the same number of glucose molecules as in 12 grams of carbon-12. The mass of one mole of a substance is called its**molar mass**. The number of atoms in 12 grams of carbon-12 is represented by the**Avogadro number (6.023 x 10**. So one mole of any substance contains Avogadro number of units in them.^{23})To understand this concept thoroughly, try different possible scenarios where the Avogadro number can be used. Here are some. One mole of hydrogen atoms contains Avogadro number of hydrogen atoms. One mole of hydrogen molecules contains Avogadro number of hydrogen molecules. A mole of water contains 6.023 x 10

^{23}water molecules. These are all different ways of expressing the same concept.

The molar mass of a substance is equal to the molecular weight of that substance. The molecular weight (formula weight) of water is 18 amu. Since this is the molar mass, we can express it as 18 grams/mol.

Example 1-3

Calculate the mass of one molecule of sodium hydroxide (NaOH).

Solution

We know that the formula weight of sodium hydroxide is 40 g/mol.

Sodium 23

Oxygen 16

Hydrogen

__1__40 g/mol

We also know that one mole of NaOH contains Avogadro number of molecules. So the mass of the NaOH molecule can be found by the following method:

Mass of one molecule of sodium hydroxide

Example 1-4

Calculate the number of moles in 109.5 grams of hydrogen chloride.

Solution

Just like the last example, we have to find the molar mass of the molecule. The molar mass is 35.5 + 1 = 36.5 g/mol.

So, 109.5 grams of HCl correspond to 3 moles of HCl.

You should be able to do these types of conversions back and forth, from grams to moles and moles to grams.

Try the next problem to see whether you have mastered the idea.

Problem 1-2

Calculate the number of grams in 8 moles of sulfur dioxide.

Solution

If your answer is close to 512.8 g, you solved the problem correctly.