Question

The specific heat of a metal was found to be 0.38 J/g*°C. When 0.5007 g of this metal was heated in a stream of chlorine gas, it produced 1.0586 g of a metal chloride, MClx.

a. Determine the formula of the metal chloride and the accurate atomic weight of the metal.
b. What is the color of the chloride?

Answer 1

I don't believe this problem can be solved without using the 0.38 J/g*C as a starting point. I looked that up in a table and found the two closest elements were Zn and Cu. Basically, then, we decide which is the right one.
........M + Cl2 ==> MCl2
....0.5007 + x .....1.0586
so g Cl2 must be 1.0586-0.5007 = 0.5579 g

mols unknown = 0.5007/atomic mass Zn
mols Cl = 0.5579/35.45 = ?
Find the ratio; it is about 1 to 1.84 which isn't quite 1 to 2. Try Cu the same way and you get mols Cu 1 to mols Cl 1.99 so I would choose Cu as the unknown metal and the formula of MClx as CuCl2. That color is green

Now for the more exact atomic mass of Cu.
In the formula 1 mol M forms 1 mol MCl2.
mols CuCl2 = grams CuCl2/molar mass CuCl2.
mols Cl2 used = 0.5579/2*35.45 (but you need to look up atomic mass Cl and do this more accurately). Finally, mols CuCl2 = g CuCl2/molar mass CuCl2. You know mols and g, solve for molar mass CuCl2. That's approx 134. Then subtract 2*atomic mass Cl to find accurate atomic mass Cu.

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