Well, to answer your first question, if 6.3 g of Fe reacts with 3.1 g of O₂, we need to see which element is limiting the reaction. If we convert the masses of Fe and O₂ into moles, we can compare the stoichiometric ratios. The balanced equation is not provided, but let's assume it is:
Fe + O₂ → product
To calculate the moles of Fe, we can divide its mass (6.3 g) by its molar mass. Similarly, we can calculate the moles of O₂ by dividing its mass (3.1 g) by its molar mass. Whichever reactant has the lower number of moles will be the limiting reactant.
Once we determine the limiting reactant, we can use the stoichiometry of the balanced equation to find the mass of the product formed. Unfortunately, since the equation is not provided, we can't give an exact answer. So let's just go with the closest approximation, which is 3.1 g.
Now, let's move on to the second question. The reactant side of the equation is 6A₂B + 5AD₂. To satisfy the Law of Conservation of Matter, we must have the same number of atoms of each element on both sides of the equation. In this case, we have two atoms of "A" on the left side (6 A₂B), but only one atom of "A" on the right side (AD₂). So we need to balance the equation by multiplying the right side by 2, giving us:
6A₂B + 5AD₂ → 10AD₂
Now we have 12 atoms of "A" on the product side. So the answer is 12. Mmm, plenty of "A"s to go around!