## Fundamental E. 15 Question...

SMIYAZAKI_1B
Posts: 63
Joined: Sat Aug 24, 2019 12:15 am

### Fundamental E. 15 Question...

Question regarding Fundamental E 15:
The question on the textbook states... 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?
I am very unsure of what the textbook is trying to state. Does the "metal" part refer to the sulfide...? Or am I not interpreting the problem right at all? I can solve that metal part of this molecule is 40.08 g/mol but...

Sion Hwang 4D
Posts: 51
Joined: Wed Sep 18, 2019 12:21 am

### Re: Fundamental E. 15 Question...

Anything with "sulfide" means that there are negatively charged sulfur ions in the compound (S ^ -2).
Since the unknown metal has two hydroxide ions associated with it in the problem, we know that the metal must have a positive 2 charge. Hence, only 1 sulfur ion would need to be attached to it to zero out the charge.
Hence, the sulfide of the metal would be M(S).

Hence, to solve the problem, you take the molar mass of the metal hydroxide (74.10g/mol), and subtract the molar mass of hydroxide x 2 (since there are two hydroxide ions).
The molar mass of the metal is thus 40.08 g/mol.
Then, you simply add the molar mass of sulfur to this mass (as there is only 1 sulfur ion associated).

40.08 g/mol + 32.07 g/mol = 72.15 g/mol.

Hence, 72.15 g/mol is the molar mass of the metal sulfide.

Sanjana K - 2F
Posts: 102
Joined: Sat Sep 07, 2019 12:17 am
Been upvoted: 1 time

### Re: Fundamental E. 15 Question...

One thing to keep in mind is that when you're trying to find the molecular formula of the sulfide, you shouldn't just add an "S" to the metal. Based on the molar mass of the metal, you should try and identify it so that you can figure out its charge. If you know it's charge, you can then figure out how many sulfurs you actually need to make the molecule neutral.

KaitlynBali_4B
Posts: 51
Joined: Fri Aug 02, 2019 12:16 am

### Re: Fundamental E. 15 Question...

Yes, I agree with Sion Hwang 4D 's solution. To clarify, the M is referring to an element on the periodic table. The element is solved to be Calcium, a metal in group two on the periodic table. Knowing that Sulfur has a -2 charge was not necessary to solve this problem (although knowing this would have helped). This problem is only focusing on the concepts of molar masses.

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