The relative mass of each element can be found from the periodic table (the larger number). For instance, P2O5, P = 31.0 O = 16.0, thus the formula mass (Mr) is 2(31) + 5(16) = 142 amu (atomic mass unit). I used a not really specific periodic table. Maybe your teacher is referring to open Schoology (a website where teacher can post files or announcements to students in his or her class). Sorry if i got something mistaken.
Answer:
Explanation:
The molecular formula of the monohydrate formed = 
The molecular mass of the monohydrate formed = 
So, Mass = 24 + 14 + 6 × 1 + 5 × 16 = 155 g
Mass of phosphorus = 31 g
Thus,
MgCl₂ ---> Mg(2+) + 2Cl(-)
0,01-x.............x.............2x
MgCl₂ ---> Mg(2+) + 2Cl(-)
0,01-x.............x.............2x
0...................0,01.......0,02
The molality of chloride ions is 0,02M.
