Here is an open-ended example problem we were given. But no work was given, just answers.

GIVEN: A copper wire with a mass of 0.316g was dissolved & converted to CuO.

Calculate the number of moles of Cu present?

Calculate the number of moles of CuO that may be produced?

Calculate the weight of CuO that can be produced?

Do you know the meaning of moles?

One mole of a substance contains the 'Avogadro's constant' number of particles, and when it concerns atoms and molecules, one mole as the same mass in grams as the relative mass of the substance.

To help you understand, one mole of hydrogen atoms contains 6.02 x 10^23 atoms, and since it's relative atomic mass is 1, then, one mole of hydrogen atom has a mass of 1 g.

Another one, 1 mole of carbon atoms: 6.02 x 10^23 atoms; having mass 12 g.

Back to your question.

a) The relative mass of Cu 64.
So, one mole of Cu has a mass of 64g.

Now, use proportions:
64 g -> 1 mole
1 g -> 1/64 moles
0.316 g -> 1/64 * 0.316 moles = 4.9375 x 10^-3 moles

b) Now, you need to write a balanced equation, showing how Cu is converted to CuO;

2Cu + O_2 \rightarrow 2CuO

Since the mole ratio of Cu : CuO is 2:2, that is 1:1, that means you can get 4.9375 x 10^-3 moles of CuO

c) Now, the relative mass of CuO is (64 + 16) = 80.
So, one mole of CuO has a mass of 80g.

Use proportions again:
1 mole -> 80 g
4.9375 x10^-3 moles -> 80 * 4.9375 x 10^-3 = 0.395 g.

Here you are :)