[Lamborghini]

I am very confused on this question on my study guide. I would appreciate any help anyone can give!

If you mix 9 moles of copper and 8 moles of nitric acid and produce 200 grams of copper (ii) nitrate, what is your percent yield?

[mikasaur]

Hey lamborghini and welcome to the forum!

Before we can help you, though, you must show your attempts at solving the problem. It's one of our rules you can read about here.

One of the best ways of answering any chemistry question is to start with a balanced equation, and this question is no different...

[Lamborghini]

I got as far as balancing it, then I get confused on moles/g and the percent yield
Cu + 4HNO₃ ---> Cu(No₃)₂ + NO₂ + H₂O

[mikasaur]

Good. That's a balanced equation.

This problem is going to test your skills with limiting reagents and theoretical yields. Do you know about those concepts?

If you have 9 moles of copper and 8 moles of nitric acid, do you think you'll use up all of both those substances? Or will one be in excess (you have too much of it)?

[Lamborghini]

I believe there will be more Cu because there is 1 more mole of it.

[mikasaur]

Hmmm...

True there is more Cu than HNO₃, but that's not the complete story. If I had a cake recipe that uses 1 cup of sugar and 4 cups of flour, and I had in my cabinet 8 cups of sugar and 8 cups of flour, which ingredient is going to limit how much cake I can make?

When you answer that, understand why. I have the same amount of each ingredient -- 8 cups -- so why is one of them limiting?

Then apply that to our balanced equation. Which substance is limiting and why?

Can you think of where to proceed from there?

[Arkcon]

Good.  So, using just the nitric acid moles, how many moles of copper nitrate should you get?

How many grams copper nitrate is that?

How much is that different from 200 grams?

You may need to use your class notes, or textbook index to find some formulas for % yield.

You'll also want to try similar problems nearby in your textbook.   You're not likely to get the exact same problem on tomorrows exam.

[sjb]

Erm. Cu + 4HNO₃ ---> Cu(NO₃)₂ + NO₂ + H₂O isn't balanced...

[mikasaur]

Erm. Cu + 4HNO₃ ---> Cu(NO₃)₂ + NO₂ + H₂O isn't balanced...

Ack, that's what I get for reading through a post too quickly!

Good catch. Lamborghini, Arkcon is correct in that you may need to use your class notes and textbook to find how to think through these problems. It is unlikely that you'll get the exact same problem in a test and so learning how to do a problem -- rather than just finding the answer for a specific problem -- is key.