Question

Calculate the formula unit masses of ${\text{ZnO, N}}{{\text{a}}_{\text{2}}}{\text{O, }}{{\text{K}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$(atomic masses of Zn=65u, Na=23u, K=39u, C=12u, and O=16u)

Answer 1

Hint: The formula unit mass of any substance is the sum or total of the atomic masses of all the atoms in the formula unit of a compound. With the help of this definition we will see complete step by step solutions.

Complete step by step solution:
Firstly we will take ${\text{ZnO}}$ and will find out its formula unit mass. Given mass of Zn is 65u and mass of O is 16u.

- In ${\text{ZnO}}$ one atom of Zn and one atom of O is present so, the total mass of the molecule will be the sum of individual masses of all the atoms. Therefore, the mass of ${\text{ZnO}}$will be 65u+16u=81u.

- Next we will calculate the formula unit mass of ${\text{N}}{{\text{a}}_{\text{2}}}{\text{O}}$. We know that mass of Na is 23u, mass of O is 16u, two atoms of Na is present therefore, the total formula unit mass of a given molecule will be $(2 \times 23) + 16$ = 62u.
- Same procedure we will follow to calculate the formula unit mass of ${{\text{K}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$. We know that 2 atoms of K are present, so the total mass of the whole molecule will be $(2 \times 39) + 12 + (3 \times 16)$ = 138u.

So, from above calculation we can say that formula unit mass of ${\text{ZnO}}$is 81u, formula unit mass of ${\text{N}}{{\text{a}}_{\text{2}}}{\text{O}}$ is 62u and formula unit mass of ${{\text{K}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$is 138u.

Note: Always remember that average masses of the atoms are used for calculation of formula mass here rather than the masses of the ions i.e. we are using weight of Zn and O atom in ZnO and not $Z{n^{2 + }}$ and ${O^{2 - }}$. However, the mass of electrons is negligibly small in comparison to the mass of atoms, so we can ignore their mass.