Question

What mass of phosphorus is contained in a `15.5*g` mass of `P_2O_5`?

Answer 1

`"Mass of phosphorus"~=7*g`.

Explanation:

We need (i) to assess the molar quantity of `P_4O_10` (which of course is the same as `P_2O_5`, and which formula we can use without loss of generality).

`"Moles of"` `P_4O_10` `=` `(15.5*g)/(283.89*g*mol^-1)=0.0546*mol`.

Now in one mole of `P_4O_10`, CLEARLY there are 4 moles of phosphorus. So (ii) we need to multiply the given molar quantity by `4xx30.9737*g*mol^-1`, where `30.9737*g*mol^-1` is the atomic mass of phosphorus.

`"Mass of phosphorus"` `=` `0.0546*molxx4xx30.9737*g*mol^-1`

`~=7*g`.