the product of a chemical reaction is phosphoric acid (h3po4). which elements must be in the reactants and how many atoms of each

Answer 1

A word to the wise.
You START sentences with a capital letter. You note that h3po4 doesn't mean a thing although H3PO4 does.
The question wants you to recognize that you must start with at least 3 atoms of H, 1 atom of P, and 4 atoms of O to make 1 molecule of H3PO4. As to how many atoms of each you need; that is undefined since the problem doesn't tell you how much H3PO4 is to be produced.

Answer 2

could you explain that a little more, please

Answer 3

The formula of phosphoric acid, H3PO4, means each molecule of H3PO4 consists of 3 atoms of hydrogen, 1 atom of phosphorus, and 4 atoms of oxygen.