What is the number of hydroxyl groups in pyrophosphoric acid?

Which of the following statements is correct?

Which form of phosphorus is thermodynamically most stable?

The reaction of $PCl_3$ with water produces ..........

Explain the nature of hydride compounds of group-$15$ elements. Explain the nature of oxide compounds of group-$15$ elements. Write a note on halide compounds of group-$15$ elements.

Ammonia is dried over