It is the limiting reactant. Now use the moles of the limiting reactant to calculate the mass of the product. Remember to use the molar ratio between the limiting reactant and the product.

In flasks 1 and 2, a small amount of Mg is used and therefore the metal is the limiting reagent. In flask 3, the reagents are added in a stoichiometric ratio. In flask 4, excess Mg is added and HCl ...

Then knowing he needed 4 mA to power each LED for backlighting he used some 1978 vintage current limiting diodes designed to pass 2mA each regardless of voltage source, within limits of course.