Calculate the molecular mass of potassium dichromate $\left({\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\right)$. (Atomic masses $\mathrm{K}=39 \mathrm{u}, \mathrm{Cr}=52 \mathrm{u}$)

Calculate the molecular mass of potassium ferrocyanide, ${\mathrm{K}}_4\left[\mathrm{Fe}\right(\mathrm{CN}{)}_6]$

(Atomic masses :$\mathrm{K}=39.0 \mathrm{u}, \mathrm{Fe}=56.0 \mathrm{u}, \mathrm{N}=14.0 \mathrm{u}$)

Calculate the percentage of water of crystallisation in washing soda, whose formula is ${\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}$.

What is meant by monoatomic ions? Give two examples.

What is meant by polyatomic ions? Give two examples.

Give three examples of bivalent cations.