Question

a) Define atomic mass unit
b) Distinguish between molecular mass and molar mass
c) Give an example of (i) diatomic, (ii) triatomic molecules of compounds.

Answer 1

Hint: Size of atoms is so minuscule that we can’t just weigh them with any standard equipment. So, how do we do it? Here the atomic mass unit comes into play. It relatively compares the masses to make a better sense.

Complete step by step solution:

a) An atom of Oxygen-16 weighs \[2.66\,\times \,{{10}^{-23}}\]g. While having the masses measured in grams is not wrong, it would be far more practical to have a general system that allows us to more effectively compare relative atomic masses. So collectively it was decided on using the Carbon-12 nuclide as the reference standard by which all other masses would be compared.
Atomic Mass unit (amu) is defined as a mass equal to one twelfth the mass of an atom of carbon-12. For example: One atom of helium-4 has a mass of 4.0026 amu
b)

Basis	Molecular mass	Molar mass
Definition	Molecular mass is the mass of a molecule of a substance. It is also known as Molecular weight.	Molar mass is the mass of 1 mole of a substance or Avogadro’s number of the molecules of that substance.
Calculation	It is calculated as the ratio of the mass of one molecule of the substance to the mass of one Carbon-12 atom.	It is calculated as the ratio of the mass of a substance to the amount of the substance.
Mathematical representation	\[MolecularMass\,=\,\dfrac{Mass\,of\,one\,molecule\,of\,substance}{Mass\,of\,one\,C-12\,atom}\,\times \,\dfrac{1}{12}\]	\[MolarMass\,=\,\dfrac{Mass\,of\,the\,substance}{Amount\,of\,the\,substance}\,\]
Unit	Atomic mass unit (amu)	Kg/Mole or g/mole

c) Diatomic molecule - A molecule made up of two atoms.
Example: CO or Carbon Monoxide (Carbon and Oxygen), NO or Nitric Oxide (Nitrogen and Oxygen), \[{{F}_{2}}\] or Fluorine (2 Fluorine), \[{{H}_{2}}\] or Hydrogen (2 Hydrogen)
Triatomic molecule - A molecule made up of three atoms.
Example: \[MgC{{l}_{2}}\] or Magnesium Chloride (1 Magnesium and 2 Chloride), \[{{H}_{2}}O\] or Water (2 hydrogen and 1 Oxygen), HCN or Hydrogen cyanide (1 Hydrogen 1 Carbon 1 Nitrogen), \[{{O}_{3}}\] or Ozone (3 Oxygen)

Note: A molecule can also have even more than 3 covalently bonded atoms. These molecules are commonly termed as polyatomic molecules or radicals.