## E15

Kimberly Koo 2I
Posts: 99
Joined: Sat Aug 17, 2019 12:17 am

### E15

Question E15 says that the molar mass of metal hydroxide M(OH)2 is 74 g.mol^-1 and then asks for the molar mass of the chloride of this metal. I did not see any chloride in the molecular formula so then I checked the answer key and they solved for the molar mass of another different substance, calcium sulfide. Does anyone know which substance's molar mass the book question was asking for or how to solve it?

Althea Zhao 1B
Posts: 101
Joined: Mon Jun 17, 2019 7:24 am

### Re: E15

You probably read the second half of E16, which asks for chloride. E15 asks: "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?"

Jenna Ortiguerra 4G
Posts: 50
Joined: Sat Aug 17, 2019 12:18 am

### Re: E15

1) First, you would have to calculate the molar mass of the (OH)2.
O=2(15.9994)=31.9988
H=2(1.00795)= 2.01588
31.9988+2.01588= 34.01468, which is ~34.02 (seen in the solution manual).

2) Subtract the given molar mass of the metal hydroxide and the molar mass we calculated above.
74.10 g*mol^-1 - 34.02 g*mol^-1 = 40.08 g*mol^-1

3) 40.08 g*mol^-1 is VERY CLOSE to the mass of calcium, which is your mystery element.

4) The formula for calcium sulfide is CaS. Therefore, to find the molar mass, add the masses of calcium (what we got in step 2) and sulfur.
40.08 g*mol^-1 + 32.065 g*mol^-1 = 72.15 g*mol^-1

I hope this helped!

ramiro_romero
Posts: 90
Joined: Sat Sep 07, 2019 12:16 am

### Re: E15

I own the 6th edition and I didn't see chloride mentioned anywhere in the problem. However, this is how I answered.

First, I subtracted the molar mass of (OH) from the molar mass of the metal hydroxide: (74.1) g/mol metal hydroxide - (32) g/mol (OH)= (42.1) g/mol metal

Next, I added the molar mass of sulfur to the molar mass of the metal: (42.1) g/mol metal + (32/07) g/mol sulfur= (74.17) g/mol metal sulfide