## E15

Britney Tran IJ
Posts: 60
Joined: Fri Aug 30, 2019 12:18 am

### E15

The molar mass of the metal hydroxide M(OH)2 is 74.10 g?mol21. What is the molar mass of the sulfide of this metal?

-can someone explain the steps to solving this? I went over the answer but I'm still a little confused.

Courtney Situ 2B
Posts: 78
Joined: Wed Sep 30, 2020 10:03 pm

### Re: E15

Hi there!
So first off, you want to find the molar mass of the metal M. To do this, you use the given molar mass of M(OH)2 (74.10g/mol) and subtract the molar masses of the substances other than M (the hydroxides OH). Note that there are two hydroxides, and the molar mass of one hydroxide is 17.02 g/mol. I wrote it below:

molar mass of M(OH)2 - molar mass of 2 OH's = molar mass of M
74.10 g/mol - 2(17.02 g/mol) = 40.08 g/mol

That 40.08 g/mol is the molar mass of M. If you look at the periodic table, this matches up with Ca, so the mystery metal is Ca! The sulfide of calcium is calcium sulfide, or CaS. You can add the individual molar masses of Ca and S to find the molar mass of CaS, and that's your answer!
Hope this helps!

Kelly Yun 2I
Posts: 81
Joined: Wed Sep 30, 2020 10:03 pm

### Re: E15

Hi!

Just to add on, Calcium as a sulfide would be CaS because a Calcium ion has a positive charge of 2 (Ca2+) and Sulfide ion has a negative charge of 2 (S2-) :)

Ellison Gonzales 1H
Posts: 64
Joined: Wed Sep 30, 2020 10:00 pm

### Re: E15

This helps a lot thank you, but I was wondering how you knew to isolate M or what concept is used to know to subtract (OH)2

EmilyGillen_1A
Posts: 73
Joined: Wed Sep 30, 2020 9:49 pm
Been upvoted: 1 time

### Re: E15

Ellison Gonzales 3F wrote:This helps a lot thank you, but I was wondering how you knew to isolate M or what concept is used to know to subtract (OH)2

For me, I know we needed to isolate 'M' because when I looked at the periodic table, there was no element called 'M.' I saw they had given the molar mass, so I figured with the information they gave us we needed to solve for a mystery mass ('M') because we know the mass of (OH) from the periodic table and the total molar mass (74.1 grams), so the only thing missing is the mass of 'M'.

Using molar mass of (OH)x2 [x2 because there are two (OH) in the equation] and the given molar mass, you solve for the one variable left, 'M'.

(OHx2)= 34 grams
Total molar mass = 74.1 grams
mass 'M' = X grams (variable) Therefore: X+ 34 = 70.1 grams
X = 40.1 grams
(Then calcium comes into play because it has a molar mass of 40.1 grams)
Does that make a little more sense?

Ellison Gonzales 1H
Posts: 64
Joined: Wed Sep 30, 2020 10:00 pm

### Re: E15

Yes, thank you, Emily! Your reasoning helps me better understand this question.

EmilyGillen_1A
Posts: 73
Joined: Wed Sep 30, 2020 9:49 pm
Been upvoted: 1 time

### Re: E15

Ellison Gonzales 3F wrote:Yes, thank you, Emily! Your reasoning helps me better understand this question.

I am glad that helps!! That is kind of a general principle you can use to solve for unknowns, so understanding the concept should help a lot with future problems too :)