# Homework Help: Finding the concentration in a solution

1. Dec 1, 2011

### morgan8222

I am doing a lab to find the concentration of copper in a penny. I am confused on how to find the concentration of copper in each solution I prepared. One solution for example:

2 ml of 0.080 M Cu(NO3)2
add 2 ml of concentrated ammonium hydroxide
dilute solution to 50 ml

Can someone help me out?

2. Dec 1, 2011

### sjb-2812

What is the definition of concentration?

3. Dec 1, 2011

### CrimpJiggler

I'm confused. You have to find the concentration of copper in a penny. What has this got to do with adding 2 mL of NH4OH to Cu(NO3)2? Anyhow, you want to know how to find the concentration of copper in that 54 mL solution you prepared. Firstly, figure out how much copper is in 2 mL of 0.08M copper nitrate solution. M means moles per liter. This means there are 0.08 moles of copper nitrate in a liter of that solution. You only have 2 mL so how many moles are in that?

4. Dec 1, 2011

### sjb-2812

What 54 ml solution?

5. Dec 1, 2011

### Staff: Mentor

Concentration of copper, or of tetraamminediaquacopper(II)? Or total copper in any form?

6. Dec 1, 2011

### morgan8222

It is a total of 50 ml. Water is added to the solution until it is 50 ml.
0.080 M Cu(NO3)2 = 0.00016 moles in 2 ml

7. Dec 1, 2011

### morgan8222

"Total concentration of copper" in this solution I prepared.

8. Dec 2, 2011

### sjb-2812

Is there any other source of copper?

9. Dec 2, 2011

### Staff: Mentor

Then it is simple dilution. Number of moles of copper doesn't change, it is constant - just the volume changes.

Note: it looks messy, as these are two cross posted threads merged.

10. Dec 21, 2011

### chikis

Concentration can be defined as the measure of amount of substance dissolved in known volume of a given solution. It is expressed in (amount in grammes of the substance per cubic centimete of solution when the mass concentration is concerned) gcm-3. It can also be expressed as in (amount in mole of substance per cubic decimeter of solution if the molar concentration is required ) moldm-3.
I can infer from this defination that it is not easy to determine the concentration of a substance say copper in a penny unless the copper is disolved in a given volume of a solution. Your views are welcome. Thank you.

11. Dec 21, 2011

### Staff: Mentor

Not bad, but not perfectly OK as well. Concentration can be also expressed as mass per mass (say 10% w/w NaCl solution means in each 100 g of solution there is 10 g of NaCl - whatever the volume is). It can be also expressed in terms of molar fraction, moles per mass of solvent (it is known as molality then) and so on.

See above. There are other ways of expressing concentration that will be perfectly suitable to copper in penny. Mass per mass percentage would be my choice.

12. Dec 21, 2011

### chikis

But like what I just said if any person want to determine the concentration of copper in a penny; at least that person have to dissolve that penny in a solution before it will be easy to find the concentration of copper in it. How about that?

Last edited: Dec 21, 2011
13. Dec 21, 2011

### Staff: Mentor

You mean for analysis? Yes - and no. If the coin has identical composition in all its volume, there are methods (I am not sure about English name - something like spark spectroscopy, spark atomic emission spectroscopy) that can be used to analyze composition just by locally heating the surface with electric arc. But the simplest approach is to dissolve the sample and use some wet method like titration, such an approach doesn't require any costly hardware.

14. Dec 21, 2011

### chikis

15. Dec 22, 2011

### Staff: Mentor

w/w = weight/weight or mass percentage

v/v = volume/volume or volume percentage

See concentration lectures for more details.