Which of the following species is electrically neutral and why?
$K^{+}, Cl^{-}, S^{2-}, P^{3-}, K$

Calculate the number of atoms in $120 \, g$ of calcium and $120 \, g$ of iron. Which one has more number of atoms and what is the difference? (Given: atomic mass of calcium $= 40 \, u$ and iron $= 56 \, u$)

$(a)$ 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 $H = 1 \ u, N = 14 \ u, O = 16 \ u, C = 12 \ u$)

$A$ sample of ethane $(C_2H_6)$ gas has the same mass as $1.5 \times 10^{20}$ molecules of methane $(CH_4)$. How many $C_2H_6$ molecules does the sample of gas contain?

$(a)$ State the law of conservation of mass.
$(b)$ What mass of silver nitrate will react with $5.85 \, g$ of sodium chloride to produce $14.35 \, g$ of silver chloride and $8.5 \, g$ of sodium nitrate,if the law of conservation of mass is true?

What is the difference between $2H$ and $H_2$?