Mole

In performing a chemical reaction, it is indeed necessary for a chemist to determine what amount of substance is to use as reactant and what amount of product and byproduct and unwanted substances are produced. Particularly, in chemical industries this calculation is a must for economic considerations. This is why the number of molecules and the number of atoms and ions in a molecule of reactant and product of a chemical reaction are to be calculated. Molecules, ions and atoms are so small particles that, they cannot be calculated as pairs, fours, dozens, hundreds, thousands even not in corers.

When objects are very small, it is often inconvenient or inefficient, or even impossible to deal with the objects one at a time. For these reasons, we often deal with very small objects in groups, and have even invented names for various numbers of objects. The most common of these is “dozen” which refers to 12 objects. We frequently buy objects in groups of 12, like doughnuts or pencils. Even smaller objects such as straight pins or staples are usually sold in boxes of 144, or a dozen. A group of 144 is called a “gross.”

This problem of dealing with things that are too small to operate with as single items also occurs in chemistry. Atoms and molecules are too small to see, let alone to count or measure. Chemists needed to select a group of atoms or molecules that would be convenient to operate with.

Avogadro's number

In chemistry, it is impossible to deal with a single atom or molecule because we can’t see them or count them or weigh them. Chemists have selected a number of particles with which to work that is convenient. Since molecules are extremely small, you may suspect that this number is going to be very large and you are right. The number of particles in this group is 6.02 x 10²³ particles and the name of this group is the mole (the abbreviation for mole is mol). One mole of any object is 6.02 x 10²³ of those objects. There is a very particular reason that this number was chosen and we hope to make that reason clear to you.

When chemists are carrying out chemical reactions, it is important that the relationship between the numbers of particles of each reactant is known. Chemists looked at the atomic masses on the periodic table and understood that the mass ratio of one carbon atom to one sulfur atom was 12 amu to 32 amu. They realized that if they massed out 12 grams of carbon and 32 grams of sulfur, they would have the same number of atoms of each element. They didn’t know how many atoms were in each pile but they knew the number in each pile had to be the same. This is the same logic as knowing that if a basketball has twice the mass of a soccer ball and you massed out 100 lbs of basketballs and 50 lbs of soccer balls, you would have the same number of each ball. Many years later, when it became possible to count particles using electrochemical reactions, the number of atoms turned out to be 6.02 x 10²³ particles.

Chemists use a large number to count molecules, ions and atoms. The value of this number is 6.02 x 10²³. According to the name of Italian scientist Amedeo Avogadro this number is called the Avogadro number or Avogadro constant. The amount of a substance containing 6.02 x 10²³ numbers of molecules, ions and atoms are called mole. In chemistry, calculation of molecule, atom, ion, reactant and product is known as Stoichiometry.

Example of mole

The word mole is used with different meanings in biology and chemistry. In biology the mole means small livings with hair and in chemistry the word mole means certain amount of a chemical substance. Mole is the unit of measurement of chemical substance. As fours and dozens unit are used to count eggs or bananas, similarly mole unit is used to count the particles of chemical substance. For example, one mole of water means 6.02 x 10²³ number of water molecules. Though one dozen eggs or bananas can be measured by counting but one mole of substance cannot be calculated by counting. This amount of chemical substance is measured in mass. That is, there is a relation of unit of mass gram/milligram with mole. The atomic mass or molecular mass of a chemical substance when expressed in gram unit is called the mole.

The amount of a chemical substance which contains the Avogadro number (6.02 x 10²³) of molecules, ions or atoms is called the mole of the substance. The number is so large that if all people of the world start to count them at a time, the sum of their counts for the whole life will not be the same as this number.

Avogadro number can be represented as 602,000,000,000,000,000,000,000.

Relative atomic mass of carbon is 12. That is, there are 6.02 x 10²³ numbers of atoms in one mole carbon, mass of which is 12 g.

Relative molecular mass of water is 18. That is, there are 6.02 x 10²³ numbers of molecules in one mole water, mass of which is 18 g.

Similarly,

1 mole hydrogen atom	=1.008g	= 6.02 x 1023 number of atoms.
1 mole oxygen atom	=16g	= 6.02 x 1023 number of atoms.
1 mole oxygen molecule	= 32g	= 6.02 x 1023 number of molecules.
1 mole carbon dioxide molecule	= 44g	= 6.02 x 1023 number of molecules.

Molar volume

The volume of one mole of substance is called the molar volume. Molar volume of Solid, liquid and gaseous substances are different. But in standard condition, the volumes of one mole of different gases are equal. The volume of a substance is pressure and temperature dependent. The volume of substance increases/decreases with the increase/decrease of temperature. On the other hand, the volume of gas decreases with the increase of temperature. The volume of a substance changes in large amount with the change of pressure and temperature. So in calculating the volume of gaseous substance, it is required to mention the pressure and temperature. You will learn here only the molar volume of a substance only at standard temperature and pressure. 25⁰ C temperatures and 1 atmospheric pressure are called the standard temperature and pressure. In standard condition molar volume of any gaseous substance is 22.4 liter.

In standard condition volume of 1 mole or 44g Carbon dioxide is 22.4 liter. Similarly, in standard condition volume of 1 mole or 32g Oxygen is 22.4 liter. In standard condition volume of 1 mole or 2g hydrogen is 22.4 liter.

Volume of one mole of different substance

Mole and molecular formula

The number of atoms of an element that can combine with a certain number of atoms of an element can be known from the molecular formula of an element. For example, CO₂ molecule is composed of carbon and oxygen elements. CO₂ molecule is formed by combining one atom of carbon with two atoms of oxygen. In terms of mole, CO₂ molecule is formed by combining one mole of carbon atom with two moles of oxygen atom. Atomic formula of a substance can be determined by calculating the number of mole from the mass of the elements.

It is seen from experiment that, 3gm carbon combines with 8gm oxygen and forms carbon dioxide. Molecular formula of the formed molecule can be determined as follows.

Certain amount of atom of an element combines with how much amount of atoms of another element can be determined from the molecular formula of an element using the concept of mole. In hydrochloric acid (HCl_(g)) molecule one mole of hydrogen atom combines with one mole of chlorine atom. That is, 1.008 or 1g hydrogen joins with 35.5g chlorine. If 1gm hydrogen and 85g chlorine is kept together in any container, on appropriate condition, 1g hydrogen will combine with maximum of 35.5g chlorine. The excess chlorine will remain in the container.

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