Consider the reaction:
2 N2(g) + 5 O2(g) + 2 H2O(g) → 4 HNO3(g)
If a reaction mixture contains 28 g of N2, 150 g of O2, and 36 g of H2O, what is the limiting reactant? (Try to do this problem in your head without any written calculations.)
The reaction requires certain amounts of each reactant to occur.
For each reaction, you need 2 "parts" N2, 5 parts O2, 2 parts H20.
We need 2 parts N2 and 2 parts H20, but there's only 28g of N2. So between just N2 and H2O, N2 is the limiting reactant.
Let's look at the O2 now. It's a bit more complicated since it's not comparing 2 parts to 2 parts, but instead 2 parts to 5 parts.
To get the 28 grams and 150 grams to "align", let's multiply the 28 grams by the ratio 5:2, since there are 5 parts O2 and 2 parts N2.
28 * (5 / 2) = 70.
Since 70 < 150,...