$(a)$ Define polyatomic ions. Write an example.
$(b)$ Calculate the formula unit mass of $CaCO_{3}$. (Atomic mass of $C = 12 \ u, Ca = 40 \ u, O = 16 \ u$)
$(c)$ Calculate the molecular mass of the following:
$(i)$ $HNO_{3}$
$(ii)$ $CH_{3}COOH$
(Atomic mass of $H = 1 \ u, N = 14 \ u, O = 16 \ u, C = 12 \ u$)

(N/A) group of atoms carrying a charge is known as a polyatomic ion.
Example: $NH_{4}^{+}, PO_{4}^{3-}, SO_{4}^{2-}$.
$(b)$ Formula unit mass of $CaCO_{3} = (1 \times \text{Atomic mass of } Ca) + (1 \times \text{Atomic mass of } C) + (3 \times \text{Atomic mass of } O)$
$= 40 \ u + 12 \ u + (3 \times 16 \ u) = 40 + 12 + 48 = 100 \ u$.
$(c)$ $(i)$ Molecular mass of $HNO_{3} = (1 \times \text{Atomic mass of } H) + (1 \times \text{Atomic mass of } N) + (3 \times \text{Atomic mass of } O)$
$= 1 \ u + 14 \ u + (3 \times 16 \ u) = 1 + 14 + 48 = 63 \ u$.
$(ii)$ Molecular mass of $CH_{3}COOH = (2 \times \text{Atomic mass of } C) + (4 \times \text{Atomic mass of } H) + (2 \times \text{Atomic mass of } O)$
$= (2 \times 12 \ u) + (4 \times 1 \ u) + (2 \times 16 \ u) = 24 + 4 + 32 = 60 \ u$.