Monday
December 22, 2014

Homework Help: Chemistry

Posted by Laura on Thursday, August 25, 2005 at 10:59am.

A 2.612 g sample of a copper oxide, when heated in a stream of H2 gas, yields .592 g H20. What is the formula of the copper oxide?

Again, I need help with this problem. Answer = CuO


Ok nvm.. i got it.


Set up the reaction as best we can surmise at the moment.


CxOy + H2 ---> H2O + Cu

It is not important for this problem to worry about the Cu on the right side. I put it there for completeness. The key point is that all of the oxygen from the oxide ends up in the water. That means the mole quantity of oxygen in the oxide equals the mole quantity of oxygen in the water.

What then is the mole quantity of water?

The mass of water is 0.592 grams. The molar mass of water is approximately 18 g/mol.

0.592 g H2O/18 g/mol = 0.033 mol H2O
(The g units cancel out and the mol unit ends up on top.)

There is one mole of oxygen for every mole of water

0.033 mol H2O × 1 O/H2O = 0.033 mol O.

There is that mole quantity of O in the oxide. It takes of some of the mass of the oxide. What is the mass of 0.033 mol O? The molar mass of O is 16 g/mol.

0.033 mol O × 16 g/mol = 0.53 g O
(The mol units cancel out.)

That mass of oxygen is part of the 2.612 g mass of oxide. Mass of copper in the oxide is the difference between the total mass and the mass of the oxygen.

2.612 g mass of copper oxide - 0.53 g O = 2.08 g copper.

How many moles of copper is that? The molar mass of copper is approximately 63.5 g/mol.

2.08 g Cu/ 63.5 g/mol = 0.033 mol Cu.

CxOy = C(0.033)O(0.033)

However, we like whole numbers. If both subscripts are divide by 0.033 we get

C(1)O(1) or CuO.

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