Question #d8c0c

The key here is the balanced chemical equation that describes this synthesis reaction

#color(red)(2)"Na"_ ((s)) + "Cl"_ (2(g)) -> 2"NaCl"_ ((s))#

In any balanced chemical equation, the stoichiometric coefficients tell you the ratio that exists between the number of moles of each chemical species that takes part in the reaction.

In your case, the balanced chemical equation tells you that for every #color(red)(2)# moles of sodium metal that take part in the reaction, the reaction consumes #1# mole of chlorine gas and produces #2# moles of sodium chloride.

You can thus say that the two reactants must always be in a #color(red)(2):1# mole ratio in order for the reaction to take place. Keep in mind, this mole ratio must be respected regardless of the exact number of moles of each reactant.

#"moles of Na"/"moles of Cl"_2 = color(red)(2)/1 -># this ratio must always be true

You know that #4# moles of sodium metal are available for your reaction. In order for the reaction to consume all the moles of sodium metal available, it requires

#4 color(red)(cancel(color(black)("moles Na"))) * "1 mole Cl"_2/(color(red)(2)color(red)(cancel(color(black)("moles Na")))) = color(darkgreen)(ul(color(black)("2 moles Cl"_2)))#

Therefore, you can say that this reaction will consume #3# moles of sodium metal and #2# moles of chlorine gas.