[methic]

Hey everybody. First time posting here.

I had a chem problem that I just can't figure out where I went wrong. Hoping someone could tell me where I messed up. Here's the question:

Common brass is a copper and zinc alloy containing 37.0% zinc by mass and having a density of 8.48g/cm³ . A fitting composed of common brass has a total volume of 130.0cm³. How many atoms of copper does the fitting contain?

So, here's what I did:

I got the mass of the sample by multiplying density by volume:

8.48 x 130.0 = 1102.4g

Then I got the percentage of copper by subtracting the percentage of zinc from 1:

1.0 - .37 = .63

Then I multiplied that by the total mass to get the mass of the copper alone

1102.4g x .63 = 694.512g

Then I converted that to moles by dividing by the molar mass of Cu

694.512g / 63.546 = 10.929 moles

Then I multiplied that by Avogadro's number to get the atoms:

10.929 moles x 6.022 x 10²³ = 6.5816 x 10²⁴atoms

I did this problem multiple times, and each time it got marked wrong. The correct answer it gave was 3.87×10²⁴ atoms; but I have no idea how it came up with that. There was a second part of this question which asked the number of atoms in the zinc portion, and I got that part correct on the first try. Did I do this part wrong?

[demoninatutu]

Hi Methic,

Well, if you've made a mistake then I've made it too, your reasoning looks fine to me. It certainly wouldn't be wrong to bring this up with whoever set the problem (i.e. politely ask for a detailed work-through).

Whatever happens, please let us know.

[opti384]

It seems to me that there are no errors.

[FreeTheBee]

Looks fine indeed. It seems like the other answer is calculated using the zinc percentage.

1102.4*0.37/63.546*6.02e23 = 3.86e24

[demoninatutu]

Ah, that's it FreeTheBee!

[methic]

Looks fine indeed. It seems like the other answer is calculated using the zinc percentage.

1102.4*0.37/63.546*6.02e23 = 3.86e24

Hmmm. I'll talk to the professor today and see what's up. The assignment was given through a chemistry homework website and was done all online. Hope it was an error on their part. At least my reasoning seems to be good.

[demoninatutu]

It happens, I usually find one or two errors in textbooks. The most common cause is that the question starts out as 'Find the no. of atoms of Zn...' but gets changed later because an editor thinks it is too easy/difficult and, in the rush to get things published, someone forgets to check the answer was changed accordingly.

I'm actually quite in awe of the relatively few mistakes that copy editors let through in a publication the size of a textbook. When you come to write your dissertation/thesis, you'll find it's remarkably difficult to find them all.