$A$ gold sample contains $90\%$ of gold and the rest copper. How many atoms of gold are present in one gram of this sample of gold?

$(a)$ What do the following formulae stand for:
$(i)$ $2O$
$(ii)$ $O_{2}$
$(iii)$ $O_{3}$
$(iv)$ $H_{2}O$
$(b)$ Give the chemical formulae of the following compounds:
$(i)$ Potassium carbonate
$(ii)$ Calcium chloride
$(c)$ Calculate the formula unit mass of $Al_{2}(SO_{4})_{3}$.
(Given atomic mass of $Al = 27 \, u, S = 32 \, u, O = 16 \, u$)

Define mole. Give its symbol. What is the advantage of this mole concept?

Who proposed the laws of chemical combination?

Calculate the molecular mass of the following compounds:
$(i)$ $NH_{4}NO_{3}$
$(ii)$ $H_{2}SO_{4}$
$(iii)$ $Na_{2}CO_{3}$
$(iv)$ $NaOH$
$(v)$ $HNO_{3}$
$(vi)$ $Al_{2}(SO_{4})_{3}$

$(i)$ Name the body which approves the nomenclature of elements and compounds.
$(ii)$ The symbol of sodium is written as $Na$ and not as $S$. Give reason.
$(iii)$ Name one element which forms diatomic and one which forms tetraatomic molecules.