(1) The molecule of ferrous succinate contains 1 iron atom, 4 carbon atoms, 4 hydrogen atoms and 4 oxygen atoms, and *** 13 atoms; So:13;

(2) The mass ratio of carbon to oxygen in ferrous succinate is (12× 4): (16× 4) = 3: 4; So fill in: 3: 4;

(3) The molecular weight of ferrous succinate is 56+12× 4+1× 4+16× 4 =172. Therefore, fill in:172;

(4) According to the meaning of the question, each tablet contains ferrous succinate 100mg=0. 1g, and two tablets are taken every day, in which the mass of iron element is 0.1g× 2× 56172×100% ≈ 0.