How many moles of oxygen atoms are present in one mole of ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7$.

Valencies or the charge numbers of some ions are given below: 
Aluminium ion  $3$
Potassium ion  $1$
Sulphate ion  $2$
Magnesium ion $2$
Nitride ion   $3$
Fluoride ion $1$

Using the above information, write down the chemical formulae of Aluminium nitride.

Valencies or the charge numbers of some Ions are given below: 
Aluminium ion  $3$
Potassium ion  $1$
Sulphate ion  $$$2$
Magnesium ion $2$
Nitride ion   $3$
Fluoride ion $1$

Using the above information, write down the chemical formulae of Magnesium nitride.

Valencies or the charge numbers of some Ions are given below: 
Aluminium ion  $3$
Potassium ion  $1$
Sulphate ion  $2$
Magnesium ion $2$
Nitride ion   $3$
Fluoride ion $1$

Using the above information, write down the chemical formulae of Aluminium sulphate.

Valencies or the charge numbers of some Ions are given below: 
Aluminium ion  $3$
Potassium ion  $1$
Sulphate ion  $2$
Magnesium ion $2$
Nitride ion   $3$
Fluoride ion $1$

Using the above information, write down the chemical formulae of Potassium fluoride.

Valencies or the charge numbers of some Ions are given below: 
Aluminium ion  $3$
Potassium ion  $1$
Sulphate ion $2$
Magnesium ion $2$
Nitride ion   $3$
Fluoride ion $1$

Using the above information, write down the chemical formulae of Magnesium fluoride.

Valencies or the charge numbers of some ions are given below: 
Aluminium ion  $3$
Potassium ion  $1$
Sulphate ion  $2$
Magnesium ion $2$
Nitride ion   $3$
Fluoride ion $1$

Using the above information, write down the chemical formulae of Potassium nitride.