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Pure copper may be produced by the reaction of copper (I) sulfide with oxygen gas:

Cu2S (s) + O2 (g) --> 2Cu (s) + SO2 (g)

If 3-kg of copper (I) sulfide reacts with excess oxygen, what mass of copper metal may be produced??

If 3-kg of copper (I) sulfide reacts with excess oxygen, 3.9 kg of copper metal may be produced.

Let's consider the following balanced equation for the obtaining of pure copper.

Cu₂S(s) + O₂(g) ⇒ 2 Cu(s) + SO₂(g)

The molar mass of Cu₂S is 95.61 g/mol. The moles corresponding to 3 kg (3000 g) of Cu₂S are:

The molar ratio of Cu₂S to Cu is 1:2. The moles of Cu obtained from 31 moles of Cu₂S are:

The molar mass of Cu is 63.55 g/mol. The mass corresponding to 62 moles of Cu is:

If 3-kg of copper (I) sulfide reacts with excess oxygen, 3.9 kg of copper metal may be produced.

You can learn more about stoichiometry here: link

it can be observed without changing the identity of the object.

it depends on what substance you're talking about as well as many other factors. in quantum mechanics, the observation of particles affects the observers ability to measure certain properties. for example: you can know the velocity of a particle, but you can't know it's position; or you can know it's position, but not it's velocity. it's like trying to find out where a grain of sand is by throwing other grains of sand at it. once you find out where it is, it's been moved. it's also like trying to find an answer to the question of whether or not a tree falling in a forest makes a sound if nobody is there to hear it. paradoxically, the only way to find out is to be there and hear it.