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### Metal Hydroxides and sulfides

Posted: **Tue Oct 02, 2018 3:55 pm**

by **ian_haliburton_1f**

In the practice problem set of Fundamentals E, there is a question (E15) that requires knowledge about metal hydroxides and sulfides. It seems that if you are given a metal hydroxide’s formula, you can derive a corresponding sulfide through the hydroxide’s molar mass. I am not sure how and why works, so I would appreciate some insight into the structure of these substances and their relationship to one another.

### Re: Metal Hydroxides and sulfides

Posted: **Tue Oct 02, 2018 4:11 pm**

by **Jeffrey Xiao 4A**

For that problem, just find the molar mass of (OH)2 which is ~34.014 g/mol, then subtract that number from 74.10 g/mol (the total molar mass of the metal hydroxide) to find the molar mass of the metal. Then using a periodic table, find which element has the molar mass found previously which should be calcium. Finally, then find the molar mass of calcium sulfide. Calcium sulfide has one atom of calcium and one atom of sulfur as calcium has +2 charge and sulfur has +2 charge, so molar mass of CaS is the answer found by adding molar mass of Calcium to molar mass of Sulfur (40.078 + 32.06)g/mol.

### Re: Metal Hydroxides and sulfides

Posted: **Tue Oct 02, 2018 4:12 pm**

by **Manya Bali 4E**

You can find out what metal it is without any complicated concepts!

The problem gives you the molecular formula M(OH)2. We need to find what element M is.

We know that the molar mass of the entire compound is 74.10 g/mol. We don't know what the molar mass of M (the unknown metal) is but we do know the molar masses of oxygen (16.00 g/mol) and hydrogen (1.01 g/mol).

This means that the molar mass of (OH)2 must be 2(16.00)(1.01) or 34.02 g/mol.

Now let's subtract the known molar masses: 74.10 - 34.02 = 40.08 g/mol. This is the molar mass of the unknown metal. We can figure out what the unknown metal is by looking at the periodic table and seeing which element has an atomic weight of 40.08.

Hope this helps!