Hope my answer is still relevant. 120.47 is the molecular weight of Mg(SO)4. Xc is calculated by (MW of Mg(SO)4)/(MW of Mg(SO)4*7H2O). Imagine Epsom salt as a solution containing pure Mg(SO)4 and some water. The 100 in the calculation is actually a mistake, its suppose to be 7*18, but the answer will still be 0.488 (maybe just typographical error)
How did you get Xc, where did you get 120.47?
Hope my answer is still relevant. 120.47 is the molecular weight of Mg(SO)4. Xc is calculated by (MW of Mg(SO)4)/(MW of Mg(SO)4*7H2O). Imagine Epsom salt as a solution containing pure Mg(SO)4 and some water. The 100 in the calculation is actually a mistake, its suppose to be 7*18, but the answer will still be 0.488 (maybe just typographical error)