Hw Help E15

[Luke Schwuchow 14A]

Hello! I was working on problem E.15 "The molar mass of the metal hydroxide M(OH)2 is 74.10g/mol. What is the molar mass of the sulfide of this metal?"
When solving the problem, I found the molar mass of the hydrogen and oxygen, then subtracted it (34.014) from the total molar mass (74.10). From there, I was left with 39.94g. When I search Chemistry Community prior to this, I noticed that people selected Calcium as the mystery metal because it is the element closest to this molar mass on the periodic table. However, I'm confused as how they selected this to be the element closest to this because both potassium and argon have molar masses that are closer to the value of 39.94. If someone could clear this up for me I would really appreciate it.

[Tomas Burgess 3F]

Hi, I think you just made a math mistake because 74.01 - 34.014 = 40.086. The molar mass of Calcium is 40.08, so this matches up perfectly and Calcium is definitely the mystery metal. Also, since the compound is M(OH)2, and each OH- ion has a 1- charge, we know the mystery metal has to have an ionic charge of 2+, and calcium is the only metal with an ionic charge of 2+ and a molar mass close to 40. Hope this helped!

[Ellen Brock 2I]

I understand the process of how you did your math. But how are we supposed to know we have to find a new mystery metal? I figured we would substitute S into the equation, how do you know that we need to take out the M to find the mass for Ca?