Which of the following contains the maximum number of molecules?

Write the chemical formulae of the compounds formed by the following ions:
$(a)$ $Mg^{2+}$ and $S^{2-}$
$(b)$ $Cu^{2+}$ and $OH^-$
$(c)$ $Al^{3+}$ and $Br^-$
Name the compounds formed in each case.

As per the law of definite proportion,carbon and oxygen combine in the ratio of $3:8$. Compute the mass of oxygen gas that would be required to react completely with $15\,g$ of carbon.

$A$ 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?

You are provided with a fine white-coloured powder which is either sugar or salt. How would you identify it without tasting?

$A$ gas jar contains $1.7 \, g$ of ammonia gas. Calculate the following:
$(i)$ Molar mass of ammonia.
$(ii)$ How many moles of ammonia are present in the gas jar?
$(iii)$ How many molecules of ammonia are present in the sample?