In the reaction #Zn + 2HCl -> ZnCl_2 + H_2#, how many moles of hydrogen will be formed when 4 moles of #HCl# are consumed?
Your tool of choice here will be the mole ratio that exists between zinc metal,
#"Zn"_text((s]) + color(red)(2)"HCl"_text((aq]) -> "ZnCl"_text(2(aq]) + "H"_text(2(g]) uarr#
You're dealing with a single replacement reaction in which zinc displaces the hydrogen from hydrochloric acid. The products of the reaction are aqueous zinc chloride and hydrogen gas.
Now, as you can see from the balance chemical equation, a
This tells you that in order for the reaction to take place, you need to have twice as many moles of hydrochloric acid as you do of zinc metal.
At the same time, you have a
This means that the reaction will always produce half as many moles of hydrogen gas as you have moles of hydrochloric acid.
Since you know that
#4 color(red)(cancel(color(black)("moles HCl"))) * "1 mole H"_2/(color(red)(2)color(red)(cancel(color(black)("moles HCl")))) = color(green)("2 moles H"_2)#