Atoms and molecules are very, very tiny. We cannot count the individual particles of a molecule. Hence, there are measures such as atomic mass, atomic weight, molecular count to estimate the number of atoms or molecules. *John Dalton* and *Thomson* first determined atomic weights between 1803 and 1805. Atomic weight was originally defined relative to that of the lightest element hydrogen taken as 1.00. Dalton took 1/16th of the mass of an atom of oxygen as the unit of atomic mass.

Nowadays, the carbon-12 isotope is considered as the standard atomic mass unit as a reference for measuring atomic masses.**One atomic mass unit** can be defined as the mass unit that is equal to 1/12th of the mass of 1 atom of C-12. The unit of atomic mass or **amu** can be defined as the mass of 1 H-atom. Because the relative weights of hydrogen, carbon and nitrogen are 1.01, 12.01 and 14.01, we can also say that the weight of hydrogen is 1.01 amu per atom, the weight of carbon is 12.01 amu per atom and the weight of nitrogen is 14.01 amu per atom. Another way to represent atomic and molecular mass is "Mole concept". Let’s discuss about mole concept and its application.

Nowadays, the carbon-12 isotope is considered as the standard atomic mass unit as a reference for measuring atomic masses.

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1 mole of a substance = 1 gram molecular mass = GMW

= 1 gram molecule = 6.02 x 10

= Atomicity x 6.023 x 1023 atoms

= Atomicity x 1 gm atom

= 22.4 liters of gaseous substance at S.T.P. (GMW)

Use the below widget to calculate the mole fraction.

The mole, in chemistry, is a measure of the amount of substance. It is defined according to the number of particles that the substance contains. In particular, we use the following definition.

**A mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12, where the carbon-12 atoms are unbound, at rest and in their ground state. The number of atoms in 0.012 kilogram of carbon is known as Avogadro number, and is determined empirically. The currently accepted value is 6.0221415 x 10**^{23} mol^{-1}.

**Explanation**

Mole of oxygen = 16 + 16 = 32 grams of Oxygen.

= Weight of 6.02 x 10^{23} molecules of Oxygen

= Weight of 2 x 6.02 x 10^{23} atoms of Oxygen

= 2 x 1 gram atom of Oxygen

= Weight of 22.4 liters of Oxygen at S.T.P

**6.023 x 10**^{23} is called the Avogadro numberThe number of particles in a mole is called Avogadro's constant or Avogadro's number. This number may be used to measure any kind of particle, such as atoms, ions or molecules. The number of atoms in 12 grams of Carbon – 12 is approximately equal to 6.023 x 10^{23}.

This is the number of particles per mole of Carbon -12 and it gives us our definition of Avogadro's number or Avogadro's constant.

So, 12.0 grams of Carbon contains 6.023 x 10^{23} atoms, therefore, 16.0 grams of oxygen will also contain 6.023 x 10^{23} atoms of oxygen. Similarly, one gram – atom of any other element will also contain 6.023 x 10^{23} atoms of that element and one molecule of any substance will contain 6.023 x 10^{23} molecules of that compound.

**Molar mass and moles**

Molar mass of an element is numerically equal to the element's atomic mass and has units grams per mole. Thus molar masses are conversion factors for moles and mass of a compound or an element.

Mass of one mole of particles is called**Molar mass**. In the case of atoms molar mass is equal to the atomic mass and in the case of molecules, molar mass is equal to the gram molar mass. So, Mass of 6.023 x 10^{23} molecules of any substance (element or compound) is equal to its gram molecular mass or one gram molecule.

Mole concept can be applied even to an ionic compound. One mole of an ionic compound represents one mole (6.023 x 10^{23}) formula units of that compound.

1 mole of atoms or ions = 6.023 x 10^{23} atoms or ions.

1 mole of electrons = 6.023 x 10^{23} electrons

**Moles and chemical equations**:

A chemical equation is an extremely useful way of summarizing a lot of information. The equation should show the amount of each substance involved in the reaction and its state. Here is an example:

2H_{2}(g) + O_{2}(g) → H_{2}O(l)

2 moles of hydrogen molecules react with 1 mole of oxygen molecules to form 2 moles of water, all in gaseous state.

Mole of oxygen = 16 + 16 = 32 grams of Oxygen.

= Weight of 6.02 x 10

= Weight of 2 x 6.02 x 10

= 2 x 1 gram atom of Oxygen

= Weight of 22.4 liters of Oxygen at S.T.P

This is the number of particles per mole of Carbon -12 and it gives us our definition of Avogadro's number or Avogadro's constant.

So, 12.0 grams of Carbon contains 6.023 x 10

Molar mass of an element is numerically equal to the element's atomic mass and has units grams per mole. Thus molar masses are conversion factors for moles and mass of a compound or an element.

Mass of one mole of particles is called

Mole concept can be applied even to an ionic compound. One mole of an ionic compound represents one mole (6.023 x 10

1 mole of atoms or ions = 6.023 x 10

1 mole of electrons = 6.023 x 10

A chemical equation is an extremely useful way of summarizing a lot of information. The equation should show the amount of each substance involved in the reaction and its state. Here is an example:

2H

2 moles of hydrogen molecules react with 1 mole of oxygen molecules to form 2 moles of water, all in gaseous state.

Some Points about Mole Concept and Stoichiometry:

- Calculation of the mass of an atom of an element

= Gram atomic mass / Avogadro number - Calculation of the mass of one molecule of a substance:

= Gram molecular mass / Avogadro number - Calculation of the number of atoms in a given mass of an element:

= Mass of element in grams / gram atomic mass x N_{0} - Number of molecules in a given mass of the substance

= Mass of substance in grams / gram molecular mass x N_{0} - Calculation of the number of molecules present in V liters of gas at S.T.P

= Volume of the gas in liters / 22.4 liters x N_{0}

The formula for water says that 1 mole of water contains 2 moles of hydrogen and 1 mole of oxygen atoms. Similarly, the formula for ammonia which is NH

1 mole of nitrogen atoms combines with 3 moles of hydrogen atoms to form ammonia.

Once we have established the number of moles of each element present in a covalent substance like water or ammonia, we can work out its molar mass.

M(H

= 2 x 1 g/mol + 1 x 16 grams/mol

= 18 grams / mole

(Oxygen has an atomic mass of 16 and Hydrogen has an atomic mass of 1)

Therefore, say for example, we need to find the mass of 1 mole of water.

Mass of 1 mole of water = 18 grams/mol x 1 mol = 18 grams

Number of moles of magnesium

= $\frac{Mass\ in\ grams\ of\ Magnesium} {Atomic\ mass\ of\ Mg}$

= $\frac{6 g}{24 g/ mol}$

= 0.25 mole of Mg.

Molecular mass of CaCO3 = 100

Number of moles of Calcium Carbonate

=$\frac{ Mass\ of\ Calcium\ carbonate\ in\ grams}{Molar\ mass\ of\ calcium\ carbonate}$

= $\frac{25 g}{100 g. mol^{-1}}$ = 0.25 mole of calcium carbonate.

1. Molecular mass of CO_{2} = 44 grams/mole.

Moles of CO_{2}

=$\frac{ Mass\ of\ Carbon\ dioxide }{ molar\ mass\ of\ carbon\ dioxide}$

= $\frac{11 grams}{44 grams .mol^{-1}}$

= 0.25 moles of carbon dioxide.

2. 6.02 x 1023 molecules of Carbon dioxide = 1 mole of CO_{2}

3.01 x 1022 molecules of CO_{2}

=$\frac{ 1 }{ 6.02 \times 1023 \times 3.01\times 1022 }$moles

= 0.05 moles of Carbon dioxide.

1 mole of Nitrogen = 2 x 1gram atom of N_{2} = 2 x 14 = 28 grams.

(ATOMIC MASS OF N IS 14)

Therefore, half a mole of Nitrogen = 28 x ½ = 14 grams.

Gram molecular mass of CaCO_{3} = 100 g

Now, one mole of CaCO_{3} contains = 3 moles of Oxygen atoms.

Or, 100 grams of CaCO_{3} contains = 3 x 6.02 x 10^{23} oxygen atoms.

Therefore,

Oxygen atom in 300 grams of CaCO_{3}

= $\frac{3 \times 6.02 \times 10^{23}}{100\times 300 }$= 54.207 x 10^{23} oxygen atoms.

More topics in Mole Chemistry | |

Molar Mass | Molar Volume |

Molar Concentration | |

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