# Stoichiometric calculation of composition of a compound

I have a question from a high school review package. I don't need the answer, but one thing that is bugging me is that, I feel as if I do not understand the question at all. I don't know what it wants me to find.

The question is:

Copper oxide is a black powder. It can be decomposed by heating it with an excess of charcoal, a form of carbon.

The charcoal reacts with the copper oxide to produce copper and carbon dioxide. Any excess charcoal that was used can be separated from the copper by adding water. The charcoal will float on the water while the more dense copper will sink to the bottom. The charcoal can then be skimmed off.

Mass of copper produced = $$\pu{2.76 g}$$

Mass of copper oxide used = $$\pu{3.45 g}$$

a) Use this data to determine the simplest formula of copper oxide.

b) Predict the valence/charge of the copper in the copper oxide formed.

I basically got: $$\frac{2.76}{63.546} = \pu{0.043 mol}$$

The balanced chemical equation that I deduced was $$\ce{2CuO + C -> 2Cu + CO2}$$

However, I am not sure that my understanding of the question is correct. Is there a mistake in my understanding.

Note: I was able to solve the question.

• Why do you think your understanding is wrong? feels as if the question is asking for two things: the copper oxide formula and the valency of copper in the oxide. – Safdar Jul 25 '20 at 7:47
• @Safdar would the copper oxide formula be the balanced equation? I couldn't get how the formula for copper oxide could be simplified.. for the valency I think I get it. – Paul Jo Jul 25 '20 at 7:49
• They are asking you to find the value of x in $\ce{Cu_xO}$. Simple as that. – Safdar Jul 25 '20 at 7:50
• @Safdar OOHHH, thank you so much! I was thinking of xCuO but that didn't make sense, but this works perfectly. Thanks again! – Paul Jo Jul 25 '20 at 7:51
• Safdar, A minor correction. they are asking to determing Cu_xO_y. We cannot assume that there is only one oxygen, although your suggestion is right. – M. Farooq Jul 25 '20 at 7:58

This is an example of a classical method of determining chemical formulas by elemental analysis.

You are not supposed to know the chemical formula of copper oxide neither they are asking you to write a balanced equation. All they are telling you is that

(i) Mass of pure oxide of copper = 3.45 g

(ii) Mass of copper obtained after reduction (we do not need to know the chemical equation) = 2.76 g

Use mass balance, how many grams of oxygen atoms must be there in that oxide?

Now you know the masses of Copper and Oxygen.

Two options

(a) Use mol and find out molar ratios between the two elements

(b) Alternatively, if you do not need to invoke the mole concept ... old school way. Just for trying

Think about the fact that 16 (exactly 16) is the mass unit of 1 oxygen atom. What mass of copper atoms would be associated with 16 units of O if copper's atomic mass is 63.54 units. Use mass balance and some arithmetic.

• Thank you so much for the help! :) So I got 3.45-2.76 = 0.69 g as the mass of oxygen and 2.76 g as the mass of copper, and dividing both by their molar mass got me 0.043 moles. Would the formula for copper oxide by CuO this way then, or is there something I've missed?? – Paul Jo Jul 25 '20 at 8:03
• Yes, right. Deduce the simplets ratios, which leads to 1:1 – M. Farooq Jul 25 '20 at 14:18