How do you find the limiting reagent?If you understand this method, learning how the find the limiting reagent is not too bad - you always get two options of to which is a limiting reagent, so a monkey has a 50% chance of getting each question correct!!!
Lets use an example problem to help explain.
2. In a balanced equation, what would be the limiting reagent if 25.3 grams of C7H16 reacted with 128 grams of O2?
C7H16 + 1102 --> 4C2H5O2
*always completely ignore the right side of the equation for these problems*
So now, you must find the GFW (molar mass) of both C7H16 and O2.
To find the GFW of each compound, you must multiply the number of atoms in each element by the elements atomic mass and add it to the product(s) of other elements in the compound
Finding the GFW of C7H16 = 7 x 12 (atomic mass of carbon is 12 and carbon in this compound contains 7 atoms) + 16 x 1 = 100
The GFW of C7H16 is 100 g/mol
After you find the GFW of each compound, you must calculate the number of moles that the compound contains.
To do this, you use the following equation: # moles = grams/GFW
so, the # moles of C7H16 = 25.3/100
# moles of C7H16 = .253
Now, go back to finding the GFW of O2, which is much simpler
GFW of O2 = 2 x 16 = 32 g/mol
*O2 is a very common compound in these problems (in fact, they are included in almost every problem that requires you to find the limiting reagent). So you should just memorize that 32 is its GFW*
# moles of O2 = 32/128
# moles of O2 = 4
So, now you have .253 moles of C7H16 and 4 moles of O2. Now, the best way to find the limiting reagent is by multiplying the number of moles for each compound by how many other elements there are in it. So, you must look at the left-side of the equation and you must use the first number of each compound to tell how many elements are in it - so in C7H16, there is only one element, while in 11O2, there are 11 elements.
Multiply .253 x 11 = 2.783
Multiply 4 x 1 = 4
So, the smaller of these numbers is the limiting reagent, so we come to a conclusion that C7H16 is the limiting reagent.
If you are ready to practice these problems without help, go to our worksheet of practice problems on finding limiting reagents |
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