[maccha]

1 mol of PCl5 reacts with water to produce 2 mol of HCl and another product. What is the other product?

I know it has to be something with 1 P, 1 O and 3 Cl but what I don't understand is how you know which order to write these elements in.. the answer key says POCl3, but how do you know that it's this rather than PCl3O?

The next part of the question then tells you that 2 mol PCl5 are reacted with NH3 to form 3 mol HCl and 1 mol "x". It says "analysis of P reveals that it is a chloride salt consisting of one Cl- and a complex cation containing nitrogen, phosphorus and chlorine.. so obviously there has to be 2 P , 1 N and 7 Cl and the answer is P2NCl7.. but again, how do you know the elements are in this order? And how does an anion like Cl- just "combine into" the compound like that?

Any help would be greatly appreciated, I'm so confused about this.

[Borek]

1 mol of PCl5 reacts with water to produce 2 mol of HCl and another product. What is the other product?

I know it has to be something with 1 P, 1 O and 3 Cl but what I don't understand is how you know which order to write these elements in.. the answer key says POCl3, but how do you know that it's this rather than PCl3O?

PCl₅ + H₂O = 2HCl + x

Assuming there are no other byproducts, and you are given all information about the reaction, x must have such composition that the reaction is balanced. But I agree that question can be a little bit confusing.

The next part of the question then tells you that 2 mol PCl5 are reacted with NH3 to form 3 mol HCl and 1 mol "x". It says "analysis of P reveals that it is a chloride salt consisting of one Cl- and a complex cation containing nitrogen, phosphorus and chlorine.. so obviously there has to be 2 P , 1 N and 7 Cl and the answer is P2NCl7.. but again, how do you know the elements are in this order? And how does an anion like Cl- just "combine into" the compound like that?

Any help would be greatly appreciated, I'm so confused about this.

From the description it is an ionic salt, not sure what do you mean by "combine into" - like "how does Cl^- combine into NaCl"?

As for the order - there are rules defined in IUPAC Red Book.