# Percent Yield - Unsure of Which Molar Mass to Use

• jumbogala
In summary, the 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.
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.

jumbogala said:
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?

Borek
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?

jumbogala said:
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.

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

## 1. What is percent yield?

Percent yield is a measure of the efficiency of a chemical reaction, indicating how much of the desired product is actually produced compared to the theoretical maximum amount that could be produced.

## 2. How is percent yield calculated?

Percent yield is calculated by dividing the actual yield (the amount of product obtained in the reaction) by the theoretical yield (the maximum amount of product that could be obtained based on the reactants used) and multiplying by 100.

## 3. What is the importance of using the correct molar mass in percent yield calculations?

The molar mass is necessary for converting between mass and moles, which is crucial in calculating percent yield. Using the incorrect molar mass can result in inaccurate calculations and misleading percent yield values.

## 4. What should I do if I am unsure which molar mass to use in a percent yield calculation?

If you are unsure which molar mass to use, double check the chemical formula of the reactants and products and make sure you are using the correct units. You can also consult a reliable source, such as a chemistry textbook or your instructor, for the correct molar mass.

## 5. Can I still calculate percent yield without knowing the molar mass?

No, the molar mass is a necessary component in percent yield calculations. Without it, you will not be able to accurately convert between mass and moles, which is crucial in determining the percent yield of a reaction.

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