Textbook E15

[juliaddc]

The problem is:
"The molar mass of the metal hydroxide M(OH)2 is 74.10g.mol-1. What is the molar mass of the sulfide of this metal?"
My first question is about the molar mass given, where is the 74.10 coming from, because the molar mass of (OH)2 = 34.02. After looking at the textbook solutions, I know I'm supposed to subtract 34.02 from 74.10, I just don't understand why that is, or why I have to find the mass of a sulfide?

Thank you!

[Chem_Mod]

It appears that in order to find the mass of the sulfide of the metal that is in M(OH)2, we need to find out what the identity of the metal is first. So we are simply given the molar mass of M(OH)2 (74.10 g/mol) and we find that the molar mass of (OH)2 is 34.02 g/mol, so we use these values to find the molar mass of M (which ends up being calcium after calculating 74.10 - 34.02 = 40.08 g/mol, and this molar mass is the mass of calcium). We then see that calcium is in group 2, which means it will form Ca2+ to get to a more stable e- configuration (the configuration of the nearest noble gas argon), and we see that we need to make a sulfide out of the metal, so we think sulfur; sulfur is in group 16 (2 away from the last group) so it will form S2- to get to a more stable e- configuration (the configuration of nearest noble gas argon). Combine the two, we get Ca2+ + S2- which will get us CaS. Adding together the molar masses of calcium and sulfur from the periodic table, we can now find what the question asked for in the first place (What is the molar mass of the sulfide of this metal?)

Hope this helps!