Determining the copper oxide used

  • Thread starter Bashyboy
  • Start date
  • #1
1,421
5

Homework Statement


Oxides of copper include CuO and Cu2O. In a crucible, you heat 1.51 g of one of these copper oxides in the absence of air and obtain 1.21 g of pure Cu. Which oxide did you have?


Homework Equations





The Attempt at a Solution


I am really quite uncertain regarding how to even begin a problem like this.
 

Answers and Replies

  • #2
cepheid
Staff Emeritus
Science Advisor
Gold Member
5,192
38

Homework Statement


Oxides of copper include CuO and Cu2O. In a crucible, you heat 1.51 g of one of these copper oxides in the absence of air and obtain 1.21 g of pure Cu. Which oxide did you have?


Homework Equations





The Attempt at a Solution


I am really quite uncertain regarding how to even begin a problem like this.
Chemistry has been a while for me, but I think you do this using simple ratios. Looking up the "standard atomic weight" of Oxygen, you get approximately 16 (rounding up). For copper, it's approx. 64 (again, rounding up). Let's say you broke up a single bond in the compound CuO, giving you one copper atom and one oxygen atom. The fraction of the total mass made up by copper would then be

(atomic weight of Cu)/(atomic weight of Cu + atomic weight of O)

The trick is that this is also the fraction of the total mass of the whole sample that you would expect to be in copper (since you'd get one copper atom for every oxygen atom). In contrast, for Cu2O, you'd get 2 copper atoms for every one oxygen atom, and the fraction of the total mass made up by copper would then be


(2*atomic weight of Cu)/(2*atomic weight of Cu + atomic weight of O)

So, assuming that heating the sample in a vacuum simply breaks the chemical bonds and gives you back the individual elements (which is the part I'm unsure about because I'm no chemist), then whichever one of these equations best matches the measured ratio of copper mass to total compound mass (1.21/1.51) tells you which compound you started with.
 
  • #3
Borek
Mentor
28,543
2,979
Let's say you broke up a single bond in the compound CuO, giving you one copper atom and one oxygen atom.
It is not a single bond actually - there are two bonds between copper and oxygen in CuO. But apart from that your idea about mass ratios is perfectly valid,
 

Related Threads on Determining the copper oxide used

  • Last Post
Replies
3
Views
1K
Replies
1
Views
9K
Replies
2
Views
2K
Replies
1
Views
14K
Replies
4
Views
2K
  • Last Post
Replies
1
Views
3K
Replies
5
Views
4K
Replies
11
Views
7K
Replies
11
Views
11K
Top