## fundamentals e15

305115396
Posts: 30
Joined: Fri Sep 28, 2018 12:16 am

### fundamentals e15

The molar mass of the metal hydroxide M(OH)2 is 74.10 g/mol. What is the molar mass of the sulfide of this metal?

could someone explain this step by step? The solutions manual is confusing because it says that the answer should be the molar mass of calcium sulfide but I don't get how calcium sulfide come up.

megansardina2G
Posts: 61
Joined: Fri Sep 28, 2018 12:23 am
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### Re: fundamentals e15

For this problem, you are looking for the molar mass of the sulfide of the metal in the compound M(OH)2. The M is unknown, so the first step is to solve for M, or find the metal in the compound.

Since the molar mass of this compound is given, you can deduce the metal's molar mass by subtracting the mass of the other elements:
O has a molar mass of 16.0 and hydrogen has a molar mass of 1.01, so the molar mass of (OH)2 is 34.02 (2*16.0 + 2*1.01 = 34.02)
74.10-34.02 = 40.08 g/mol

If you look on the periodic table, 40.08 matches the molar mass for Calcium, so that is the mystery metal.

Since the problem asks to find the molar mass of the sulfide of the metal, you can deduce that the answer will be the molar mass of a compound of calcium and sulfur. Sulfur has a molar mass of 32.07, so 40.08 + 32.07 = 72.15 g/mol, which is the answer.

Hope this helped!

Gina Maglanoc_1H
Posts: 3
Joined: Fri Sep 28, 2018 12:17 am

### Re: fundamentals e15

In this scenario, I am still unsure what "sulfide of the metal" refers to. Is the question insinuating that there is a compound we need to identify and not just one mystery metal since the word "sulfide" is included but Sulfur is not present in the given metal hydroxide?

Sang Hyoun Hong 3G
Posts: 31
Joined: Fri Sep 28, 2018 12:27 am

### Re: fundamentals e15

You found that the metal, M, is calcium. So the sulfide of the metal would be CaS. Sulfide is the sulfur ion with the metal. In order to find the molar mass of CaS, you add the molar mass of Ca with S. You should get an answer of 72.14gmol^-1

Sean Reyes 1J
Posts: 67
Joined: Fri Sep 28, 2018 12:24 am

### Re: fundamentals e15

The metal hydroxide, shown as M(OH)2, has a molar mass of 74.10 g/mol.

Hydroxide alone has a charge of -1, which would imply that the metal has a charge of +2 since there are 2 hydroxides per metal.
Sulfur when ionized has a charge of +2, which would mean that the salt would look like: MS.

Using the molar mass of the metal hydroxide, you can find how much the metal alone weighs. This is done by subtracting the mass of the two hydroxides from the molar mass:
74.10 g/mol - (mass of both the hydroxides) = molar mass of M

Adding the molar mass of the metal to the molar mass of sulfur (32.06 g/mol) would give the molar mass of the metal sulfide.

I hope that this helps you a little bit!