# Determining the copper oxide used

## 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?

## The Attempt at a Solution

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

## Answers and Replies

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cepheid
Staff Emeritus
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## 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?

## 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.

Borek
Mentor
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,