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What is a Mole? gram atom-gram molecule-gram formula and gram ion

March 10, 2023
written by shahzad haider


When the atomic mass of an element, the molecular mass of a molecular substance, and the formula mass of an ionic compound are expressed in grams is called Mole.

Mole can also be defined as,

The amount of substance that contains 6.02 x 1023 particles (atoms, ions, molecules) is called a mole.


The unit “mole” was introduced into chemistry around 1900 by Ostwald, and he originally defined this unit in terms of a gram.

The name mole is an 1897 translation of the German unit Mol, coined by the chemist Wilhelm Ostwald in 1894 from the German word Molekul (molecule). The related concept of equivalent mass had been in use at least a century earlier.


We know that an atom is an extremely small particle. The mass of an individual atom is an extremely small quantity. It is not possible to weigh individual atoms or even a small number of atoms directly. Therefore, we use the atomic mass unit (amu) to express the atomic masses. The atomic mass may be given in any unit of measurement. i.e gram, kg, pound, and so on. Mole is expressed by “Mol”

What is a gram atom?

When the atomic mass of an element is expressed in grams is called one gram atom. It is also called one gram mole or simply a mole of that element.

Number of gram atoms = Mass of an element in grams/Molar mass of an element

For example

1 gram atom of hydrogen   = 1.008 g

1 gram atom of carbon        = 12.00 g

1 gram atom of uranium      = 238.0 g

It means that one gram of atom of different elements has different masses in them.

One mole of carbon is 12 g, while one mole of magnesium is 24 g. It shows that one atom of magnesium is twice heavier as one atom of carbon.

What is a gram molecule?

The molecular mass of a substance expressed in grams is called a gram molecule or gram mole or simply the mole of the substance.

Number of gram molecules = Mass of molecular substance in grams/Molar mass of a substance

For example

1 gram molecule of water = 18.0 g

1 gram molecule of H2SO4 = 98.0 g

1 gram molecule of sucrose = 342.0 g

It means that one-gram molecules of different molecular substances have different masses.

What is a gram formula?

The formula unit mass of an ionic compound expressed in grams is called the gram formula of the substance.

Since ionic compounds do not exist in molecular form therefore the sum of atomic masses of individual ions gives the formula mass. The gram formula is also referred to as a gram mole or simply a mole.

Number of gram formulae = Mass of the ionic substance in grams / Formula mass of the ionic  substance

For example

1 gram formula of NaCl     = 58.50 g

1 gram formula of Na2CO3 = 106 g

1 gram formula of AgNO = 170 g

What is a gram ion?

When the mass of ionic specie is expressed in grams is called a gram ion or one mole of ions.

For example

1 gram ion of OH   = 17 g

1 gram ion of SO42 = 96 g

1 gram ion of CO32 = 60 g

Solved Problem 1

Calculate he gram atoms (moles) in

0.1 g of sodium.

0.1 kg of silicon


 No of gram atoms = Mass of element in gram/ Molar mass

Mass of sodium = 0.1 g

Molar mass = 23 g/mol

Number of gram atom of sodium = 0.1/23 = 0.0043 mol

                                                             = 4.3 x 10-3 mol

First of all convert the mass of silicon into grams.

Mass of silicon = 0.1 kg = 0.1 x 1000 = 100 g

Molar mass = 28.086 g/mol

Number of gram atoms of silicon = 100/28.086       

                                                            = 3.56 mol

Solved Problem 2

Calculate the mass of 10-3 mol of MgSO4.


MgSO4 is an ionic compound. We will consider is formula mass in place of molecular mass.

Moles of the substance  = Mass of the ionic substance/formula mass of the ionic substance

Formula mass of MgSO4 = 24+ 96 =120 g/mol

Number of moles of MgSO4 = 10-3

Applying the formula

          10-3  = Mass of MgSO4/120

    Mass of MgSO= 120 x 10-3  = 0.12 g

What is the SI unit of mole?

The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly    6.02×1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol–1 and is called the Avogadro number.

What is 1 mole equal to in grams?

One mole consists of Avogadro’s number of atoms. If you know the quantity of mole, it can be converted into grams and vice versa.

What is 1 mole called?

Avogadro’s number is the number of units in one mole of a substance, or 6.02214076 × 1023. This number is also called the Avogadro constant.

What is the symbol of a mole?

1 mole = 6.022 × 10 23. This number is also known as the Avogadro number and is denoted by NA, SI unit symbol is “mol”.

Why mole is important?

A mole is a very important unit of measurement that chemists use. A mole of something means you have 602,214,076,000,000,000,000,000 of that thing, like how having a dozen eggs means you have twelve eggs. Chemists have to measure using moles for very small things like atoms, molecules, or other particles.

How many atoms are in a mole?

The value of the mole is equal to the number of atoms in exactly 12 grams of pure carbon-12. 12.00 g C-12 = 1 mol C-12 atoms = 6.022 × 1023 atoms • The number of particles in 1 mole is called Avogadro’s Number (6.0221421 x 1023).

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