Define valency. What is the importance of valency of an element or ions?

Define molecular mass and formula unit mass.

Calculate the formula unit mass of ${\mathrm{CaCl}}_2$. (Atomic masses: $\mathrm{Ca}=40 \mathrm{u}, \mathrm{Cl}=35.5 \mathrm{u}$)

An element $\mathrm{M}$ has a valency of $2$. What will be its simplest formula for oxide of the element?

An element $\mathrm{M}$ has a valency of $2$. What will be its simplest formula for bromide of the element?

If the valency of nitrogen is $3$ and that of hydrogen is $1$ , write down the formula for ammonia.

An element has  $\mathrm{X}$ has a valency of $4$, while another element $\mathrm{Y}$ has a valency of $1$. What will be the formula of the compound formed between $\mathrm{X}$ and $\mathrm{Y}$?

An element $\mathrm{A}$ of valency $3$ combines with another element $\mathrm{B}$ of valency$2$. What will be the formula of the compound formed?