When 7.00 g of hydrogen react with 70.0 g of nitrogen, hydrogen is considered the limiting reactant because:
7.5 mol of hydrogen would be needed to consume the available nitrogen.
7.5 mol of nitrogen would be needed to consume the available hydrogen.
hydrogen would produce 7.5 mol more ammonia than nitrogen.
nitrogen would produce 7.5 mol more ammonia than hydrogen.

7.5 mol of hydrogen would be needed to consume the available nitrogen.

Explanation:

When hydrogen reacts with nitrogen, ammonia is formed as shown below;

3H₂ (g) + N₂ (g) → 2NH₃ (g)

As seen from the equation, every 3 moles of H₂ react with a mole of N₂ to form 2 moles of NH₃.

The limiting factor in a chemical reaction is the reactant that gets depleted first.

Because the molar mass of nitrogen gas is approximately 28g/mol, 70g of nitrogen gas would be 2.5 moles.

The reaction ratio of nitrogen to hydrogen in the reaction is 1 : 3. The reaction would require 2.5 * 3 (7.5) moles of hydrogen for a complete reaction.

However since there are only 7g on hydrogen, (Remember 1 mole of H₂ is approximately 2g), the available moles of H₂ is 7 / 2 = 3.5

3.5 moles fall short of the 7.5 moles of H₂ required for a complete reaction. H₂ gets depleted first before N₂.  The reaction would require 4 more moles of H₂.