Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.To order this book direct from the publisher, visit the Penguin USA website or call 1-800-253-6476.In our example, we would say that ice is the limiting reactant.
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The first stoichiometry calculation will be performed using "1.75 mol O2" as our starting point, and the second will be performed using "2.75 mol H" is the smaller of these two answers, it is the amount of water that we can actually make.
The limiting reactant is hydrogen because it is the reactant that limits the amount of water that can be formed since there is less of it than oxygen.
Now that you're a pro at simple stoichiometry problems, let's try a more complex one.
Using the recipe for ice water (1 glass of water 4 ice cubes = 1 glass of ice water), determine how much ice water we can make if we have 10 glasses of water and 20 ice cubes.
Again, if we're given a problem where we know the quantities of both reactants, all we need to do is figure out how much product will be formed from each.
The smaller of these quantities will be the amount we can actually form.Next, to find the amount of excess reactant, we must calculate how much of the non-limiting reactant (oxygen) actually did react with the limiting reactant (ammonia). 1.70 g is the amount of ammonia that reacted, not what is left over.To find the amount of excess reactant remaining, subtract the amount that reacted from the amount in the original sample. That's NH3 and it's a gas, that's why the g is in parentheses. That's also a gas, it's also called nitric oxide, not to be confused with nitrous oxide. If we multiply this times one and a half, 1.5, now we have three hydrogens on this side. How many oxygens do we have on the right-hand side? And then making sure that you understand the ratios.And we combine that with some oxygen, molecular oxygen, it's also a gas. I'll write nitrous oxide, it's N2O, this is laughing gas. 1.5 times 2, we have three hydrogens on the right-hand side. We have one oxygen here, and we have one oxygen in this molecule, but we have one and a half of the whole molecule. You understand this is a 4:5 ratio, there's always going to be less ammonia than oxygen.The other reactants are partially consumed where the remaining amount is considered "in excess".This example problem demonstrates a method to determine the limiting reactant of a chemical reaction.The water is called the excess reactant because we had more of it than was needed.We can use this method in stoichiometry calculations.Hopefully, you didn't have too much trouble figuring out that we can make only five glasses of ice water.Let's go through this calculation carefully to see what we did (it'll be clear why we need to do this in a second).