[toadesque]

Ok I'm not asking for you guys to do my homework but I really need help getting pointed in the right direction. Like explain to me what I need to do in order to solve each question.

Here are some questions I need help with.

a. A 10.4 g sample of pure copper was dissolved in nitric acid and diluted to exactly 500 mL with distilled water.

b. A 50.0 mL portion of this copper (II) solution was reacted with an impure sample of zinc metal that weighed 1.07 g. (Zinc was the limiting reactant in the reaction)

The copper metal that was collected after the reaction with zinc weighed 0.836 g. The reactions for each step are given below.

Cu + 4HNO3 => Cu(NO3)2 + 2NO2 + 2H2O
Zn + Cu (2+) = Zn (2+) + Cu

These are the questions

a. What volume of 6.0 M nitric acid was required to react with the pure copper in "a"?

b. What is the molarity of the copper (II) solution produced in "a"?

c. Determine the percentage of the zinc in the sample from "b".

D. How many moles of nitrogen dioxide gas were produced in the reaction of copper metal with nitric acid in "a"?

e. What volume would the nitrogen dioxide gas produced in "a" occupy at 800 torr and 20 degrees celsius?

f. What would be the pressure of the nitrogen dioxide gas produced in "a" if it was confined to a 3.50 L flask at 20 degrees celsius?


Again, I'm not asking you guys to do all these for me. I'm just really lost. When I did the problems in the book, they didn't look anything like this. For example

In the book, it will say: "Calculate the molarity of a solution made by dissolving 3.50 mol of NaCl in enough water to produce 2.00 L of solution."

Obviously that is ridiculously easy, but that's how it is in my book. But on this practice test (which is very similar to the actual test), the questions look completely different. And they also build off each other so if you can't answer one question, you may not be able to answer the next 3 questions. And then you're screwed.

[enahs]

Lets start with the first two and move from there.

A) How many mols of pure copper do you have? Using your balanced equation, how many mols of nitric acid are required to react with that many mols of copper metal? If you have a stock solution of nitric acid that is 6M, how many mL's gives you that many mols?

B) This is really easy, as in your example of what you think easy is. How many mols of copper do you have in how much water?

[toadesque]

Lets start with the first two and move from there.

A) How many mols of pure copper do you have? Using your balanced equation, how many mols of nitric acid are required to react with that many mols of copper metal? If you have a stock solution of nitric acid that is 6M, how many mL's gives you that many mols?

Ok well

moles of copper = .176
4 moles of nitric acid?

if the formula is M = moles/liters....Is the M 6? The moles .176? Not quite sure what you meant by use the balanced equation..

B) This is really easy, as in your example of what you think easy is. How many mols of copper do you have in how much water?

Ok would it just be .176 moles Cu / .5 L H2O = .352 M Cu

[enahs]

M = mols
       liter

M * Volume (in liters) =   mols    *   liters = mols
                                      liters

So you know how many mols of copper you have. You know that for every mol of copper you have, you needed 4 mols of nitric acid.  So you now have how many mols of nitric acid you need; given the definition of molarity, how many liters of the 6M solution do you need to get that number of mols?

[toadesque]

Ok I got .66 L? Is that correct?

[enahs]

Excellent mistake. As I really like to point out: Math is a tool, do not forget about the science!
So you are saying you took 0.66L (660 mL) and diluted it to exactly 500mL?

So you answer the question you asked me, is that correct?

In every math step, include your units! Make sure they cancel and give you the units you need in the end!
Units represent the science! 

[toadesque]

lol oops I just realized I got the weight wrong for Copper in the first place. I was looking at cobalt by accident.

So it was .1637 actually.

Ok so you do

.1637 x 4 moles of HNO3 which = .6547 mol HNO3

Then what you said

6.0 M HNO3= .6547 mol HNO3 / L

so L = .109 HNO3

I think that's right. Please tell me it is LOL

Am I ready to go on to the next ones now? haha

[toadesque]

Ok I did part

e. What volume would the nitrogen dioxide gas produced in "a" occupy at 800 torr and 20 degrees celsius?

 I got 7.48 L. Someone care to check that please!

I also did this one

f. What would be the pressure of the nitrogen dioxide gas produced in "a" if it was confined to a 3.50 L flask at 20 degrees celsius?

I got P = 2.3

[Borek]

Probably 2xOK, although being consistent with significant numbers won't hurt.

[toadesque]

Hm I still don't get this one

c. Determine the percentage of the zinc in the sample from "b".

[enahs]

Hm I still don't get this one

c. Determine the percentage of the zinc in the sample from "b".

So, you collected pure Copper. How many mols of pure copper did you collect?
Based on your balanced equation, how many mols of zinc metal were required to get this amount of copper? What is the mass of that number of mols of zinc?
So then what percent of zinc was in the starting compound?