Question

In a `10*g` mass of an iron oxide there are `4*g` of iron. How many grams of oxygen are there?

Answer 1

If I have `£10-00` and I give you `£4-00`, then how many pounds do I have left?

Explanation:

You have formed `10*g` of an oxide of iron; this could `FeO`, or `Fe_2O_3` or a mixed valence oxide. The point is that the constituent atoms are the metal and oxygen. If the iron content is `4*g` then surely there must be a `6*g` mass of oxygen.