Percent Yield - Unsure of Which Molar Mass to Use

[jumbogala]

Homework Statement

A percent yield lab says to add 5.00 g of CuCl₂•2H2O(s) to 50 mL of dissolved water. Then, add 1.0 g of steel wool (100% iron) to the beaker.

I am trying to find out how many moles of CuCl₂ react here. I am confused about the dihydrate.

Homework Equations

The reaction given in the lab is

Fe(s) + CuCl₂(aq) → FeCl₂(aq) + Cu(s)

The Attempt at a Solution

To find out how many moles of CuCl₂•2H₂O(s) I used, do I calcuate the molar mass including the dihydrate? Or do I use just the molar mass of CuCl₂?

I think I am supposed to include the dihydrate in the molar mass calculation, but I'm confused because the dihydrate doesn't appear in the reaction. It doesn't react with anything. So wouldn't I need to figure out how many moles of CuCl₂ there were, not how many moles of CuCl₂•2H₂O(s)?

Also, I know the CuCl₂•2H₂O(s) is the excess reactant, but to show that, I need to find out how many moles of it I have first.

 

[Bystander]

do I calcuate the molar mass including the dihydrate?

You're preparing the solution from the dihydrate. Does the balance know you're only interested in CuCl₂?

 

[jumbogala]

When you measure out 5.00 g onto the balance, the dihydrate is included. The number of moles of reactant sitting on the balance would be calculated using the molar mass of CuCl₂ • 2H₂O.

That much I know. But when you add the CuCl2•2H2O(s) to a beaker of water, what happens to the dihydrate? Does it stay attached to the CuCl₂? I guess it doesn't matter... 1 mol of CuCl2•2H2O(s) would still correspond to 1 mol of CuCl2 right?

 

[Borek]

1 mol of CuCl2•2H2O(s) would still correspond to 1 mol of CuCl2 right?

Yes.

Hydration water can be important when you are interested in the amount of water present. Say, you prepare a solution by using 1 g of dihydrate and 10 g of water and you need to know the exact, final concentration. But in typical situations we add water to the mark - so the final amount doesn't depend on the presence of hydration water at all.

 

[jumbogala]

Okay, that makes sense. In this case, the amount of water doesn't matter anyway. Thanks!