A 2.004-gram sample of calcium is completely reacted with oxygen to produce 2.804 g of calcium oxide. What is the mass percentage of oxygen in calcium oxide?

Question

I got 28.5 percent. Is that correct?

Answer 1

mass O in CaO = 2.804 - 2.004 = 0.8 g O

mass % O in CaO = 0.8 g O /2.804 g (x100%) = 28.59%

So, I agree with your answer.

You could also do this as follows:

2Ca + O2 ==> 2CaO

2.004 g Ca x 1 mol Ca/40 g = 0.05 moles Ca

Since CaO has a 1:1 mole ratio of Ca to O, there will also be 0.05 moles of O.

0.05 mol O x 16 g/mol = 0.80 g of O

0.80 g O / 2.804 g (x100) = 28.59%