E.15

[Cole Reynolds 4F]

The molar mass of the metal hydroxide M(OH)2 is 74.10 g.mol-1. What is the molar mass of the sulfide of this metal?
I don't see an S in M(OH)2 so I'm not exactly sure what sulfide they are talking about in this problem. Could someone walk me through it?

[BryantChung_4B]

You're given one thing: the molar mass of M(OH)2. We can subtract molar mass of (OH)2 from this to find the mass of M, the unknown metal.
Next, the sulfide is simply the molecule after (OH)2 is replaced by a sulfur ion, becoming MS. You can just add the known mass of M to the mass of sulfur to find the total molar mass of the sulfide.

[Adrianh72]

How do we know that (OH)2 is replaced by a sulfur ion? Is it solely based on the fact that sulfide is mentioned in the question?

[Leyna Dang 2H]

I needed clarification on this too. Why does finding "sulfide" indicate that we have to subtract (OH)2? Is sulfide just used as a broader term for nonmetal??

[Eileen Si 1G]

Even though they don't explicitly include S in M(OH)2, the sulfide they are talking about in this problem is just given information that is needed to calculate the final molar mass of the metal.
For example, the molar mass of (OH)2 is 34.02 g.mol-1.
By subtracting the molar mass of (OH)2 is 34.02 g.mol-1 from the molar mass of the metal hydroxide M(OH)2 (74.10 g.mol-1), we get 40.08 g.mol-1.
Then, by looking at a periodic table, we can figure out that the only element with a molar mass of 40.08 g.mol-1 is calcium.
Because the problem already gave us the information that sulfide is included in this metal, we can add the molar mass of sulfur (32.07 g.mol-1) to the molar mass of calcium (40.08 g.mol-1) to calculate the final molar mass of the sulfide of this metal which is 72.15 g.mol-1.