Atoms And Molecules

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The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound.

Formula unit mass is calculated in the same manner as the molecular mass.

The difference between formula unit and molecular mass.

Formula unit mass calculated for those substances whose constituent particles are ions. For example, sodium chloride.

NaCl. Its formula unit mass can be calculated as–

Na = 23u, Cl = 35.5 u

1 × 23 + 1 × 35.5 = 58.5 u

The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound.

Formula unit mass is calculated in the same manner as the molecular mass.

The difference between formula unit and molecular mass.

Formula unit mass calculated for those substances whose constituent particles are ions. For example, sodium chloride.

NaCl. Its formula unit mass can be calculated as–

Na = 23u, Cl = 35.5 u

1 × 23 + 1 × 35.5 = 58.5 u

One mole is defined as the amount of substance of a system which contains as many entities like, atoms, molecules and ions as there are atoms in 12 grams of carbon 12".

The number of atoms present in 12 grams of carbon 12 which is equal to 6.023X 10²³. This is also known as Avogadro's constant.

1 mole of water =6.022 ×10²³  molecules of water

In case of gases, a mole is defined as that amount of the gas which has a volume of 22.4 litres at STP.

1 mole of Oxygen gas = 22.4 litres of oxygen at STP

one mole of carbon dioxide gas = 22.4 litres of carbon dioxide at STP.

A mole is a measuring unit in chemistry.

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.  It is, therefore, the relative mass of molecule expressed in atomic mass units (u)

For example, 

The molecular mass of HNO₃ = the atomic mass of H + the atomic mass of N+ 3 × the atomic mass of O
= 1 + 14 + 48 = 63 u