Percentage concentration can be expressed in units of what?
The unit you are looking for is the mole. The mole is used in chemistry to express the amount of reactants producing an amount of products. The mole can be used to express amounts of elements involved in a reaction, or amounts of compounds involved in a reaction. For example, consider the for mation of water from hydrogen and oxygen:
2H2 + O2 ---> 2H2O
What this means is 2 moles of hydrogen combined with 1 mole of oxygen will produce 2 moles of water. This can be used to express percentages as well. The "yields" sign (the arrow) is considered the same as the "equals" sign in math, so the mass on the reactants side of the equation will equal the mass on the products side of the equation. The Law of Conservation of Mass is demonstrated in this reaction.