Question

Calculate the no. of atoms in:
a) 0.5 mol atom of nitrogen
b) 0.2 mol molecules of hydrogen
c) 3.2 g of sulphur (Z for sulphur=32)

Answer 1

Hint: Avogadro’s number is the number of molecules present in 1 mole which is equal to $6.023 \times {{10}^{23}}$ entities. It is denoted by the symbol ${{N}_{A}}$ . This value is directly used to find the number of atoms in certain different types of molecules which have different representational units.

Complete step by step answer:
- Everything present in this universe is made up of atoms. The mass of an atom cannot be measured truly due to its very small size. So a relative term is used for it called relative atomic mass. It is based on the mass of 1 carbon- 12 atom. It was adopted internationally in 1961 and according to it, relative atomic is given by
\[Relative\text{ }atomic\text{ }mass\text{ }=\text{ }\dfrac{mass\text{ of 1 atom of element}}{\dfrac{1}{12}\text{x mass of 1 C-12 atom}}\]
- Mole is the amount of substance that contains the atoms, molecules or other particles in an entity equal to the atoms present in 12 g of C-12 isotope. It defines the basis of physical chemistry and so its number is called Avogadro's number which is equal to 6.023x${{10}^{23}}$entities. It is denoted by the symbol ${{N}_{A}}$ .
- Some standard terms used in mole concept are
1 gram atom = 1 mole atom
1 gram molecule = 1 mole molecule
1 gram ion = 1 mole ion
- In the first part, we need to find a number of 0.5 mol atoms of nitrogen. We know that the number of 1 mole of atoms for any substance is equal to avogadro’s number.
So, 0.5 moles will be equal to 0.5 ${{N}_{A}}$.
- In the second part, we need to find the number of 0.2 mol molecules of hydrogen. As we have seen above, 1 gram molecule = 1 mole molecule. As we know the molecular formula for hydrogen is ${{H}_{2}}$ . So, the number of atoms in 0.2 mol hydrogen will be $0.2 \times 2 \times {{N}_{A}}$
So, the answer for this part will be 0.4 ${{N}_{A}}$.
- In the third part, we need to find a number of 3.2 g of sulphur (Z for sulphur = 32). We know, for 32 g sulphur, the number of atoms is ${{N}_{A}}$.
So, for 3.2 g, the number of atoms will be $\dfrac{3.2x{{N}_{A}}}{32}=0.1 {{N}_{A}}$

Note: Mole is not only related to mass but also to volume as any matter that has a certain mass will occupy a certain volume. 1 mole is the amount of molecules present in 22.4 litres of a gas calculated at standard temperature and pressure.