Question #2bf92

1 Answer
Apr 29, 2016


Hydrogen gas, #"H"_2#


The balanced chemical equation that describes a particular reaction essentially tells you how many molecules of each reactant are needed in order for the reaction to take place.

More specifically, the coefficients that follow each chemical species that takes part in the reaction tells you how many molecules are involved in that reaction.

In your case, the balanced chemical equation that describes the synthesis of water looks like this

#color(red)(2)"H"_ (2(g)) + "O"_ (2(g)) -> 2"H"_ 2"O"_ ((l))#

This tells you that #color(red)(2)# molecules of hydrogen gas will react with #1# molecule of oxygen gas, and form #2# molecules of water.

Now, because molecules are so very, very small, we usually work with very large collections of molecules called moles.

A mole is simply a collection of #6.022 * 10^(23)# molecules of a substance #-># this is known as Avogadro's number.

This means that the above reaction can be said to consume #color(red)(2)# moles of hydrogen gas for every #1# mole of oxygen gas and produce #2# moles of water.

You know that your container contains #8# moles of hydrogen gas and #8# moles of oxygen gas.

In order to find the limiting reagent, you must determine which reactant will be completely consumed before every molecule of the other reactant gets the chance to take part in the reaction.

Since the reaction consumes twice as many moles of hydrogen gas than it does of oxygen gas, and since you have equal numbers of moles of each present in the mixture, you can say that hydrogen gas will be your limiting reagent.

More specifically, the reaction will consume all the moles of hydrogen gas and only

#8 color(red)(cancel(color(black)("moles H"_2))) * "1 mole O"_2/(color(red)(2)color(red)(cancel(color(black)("moles H"_2)))) = "4 moles O"_2#

The remaining #4# moles of oxygen gas will not take part in the reaction because there are no more moles of hydrogen gas left.

Therefore, hydrogen gas, #"H"_2#, will be your limiting reagent.