Calculate the number of molecules in $90$ grams of water.

Give two examples to show that the law of conservation of mass applies to physical changes also.

$(a)$ What is the law of constant proportions? Explain with the help of an example.
$(b)$ Which postulate of Dalton's atomic theory is in agreement with this law?

$A$ silver ornament of mass $m \, g$ is polished with gold equivalent to $1\%$ of the mass of silver. Compute the ratio of the number of atoms of gold and silver in the ornament.

The atomic numbers of three elements $A, B$ and $C$ are $9, 10$ and $13$ respectively. Which of them will form a cation?

Compute the difference in masses of $10^{3} \, \text{mol}$ each of magnesium atoms and magnesium ions. (Mass of an electron $= 9.1 \times 10^{-31} \, \text{kg}$)