E9 part b

[Chelsea Greene 1L]

When finding the formula unit of a compound for epsom salt, do I first find the number of moles present and then use Avagadro's number to find the number of formula units? And if so do I use MgSO4 and 7 H2O or just MgSO4 and 1 H20 (since the formula unit is based on the empirical formula).

[Charlotte McDonough 2B]

Yeah you just have to use avogadros number to get formula units

[Chem_Mod]

In the case of formula units of crystalline materials you would use the molar mass and avogadro's number as you have suggested. However you would use 7H2O for your calculation not 1 H2O. The way to think of this is that each unit is a MgSO4 that is linked with 7H2O molecules. If you use only 1 H2O you would be discounting the other six H2O molecules linked to each MgSO4 molecule.

[Sammy Liu]

Your process is correct, and you would have to use MgSO4 * 7 H2O. This is because every MgSO4 forms a structure with 7 H2O molecules.