How to Find Limiting Reagents
Everyday Examples:
What is a limiting reagent?
Limiting Reagent - a chemical that runs out first.
Moles – the mole is the measure of the amount of chemical substance.
The abbreviation for Moles is mol.
Molar mass [(Gram Formula Weights)(GFW)] -- How much each element weighs.
e.g. Carbon (C) has a GFW of 12.
The unit for GFW is g/mol or grams per mole.
Here is a link to a great online Periodic Table of Elements to help you find the GFW.
http://www.webelements.com/
Look at each element's mass (the number under the symbol for the element). Round the element's weight to the nearest whole number. (except for Cl -- keep at 35.5g/mol)
How to find GFW of a compound:
Add the weights of the elements together:
Compound: CaCO3
Ca x 1, C x 1, O x 3
Ca: 40 x 1 = 40
C: 12 x 1 = 12
O: 16 x 3 = 48
40 + 12 + 48 = 100 g/mol
Equations you will need:
# of moles = # of grams (g) / GFW
3 Worked Example Problems:
For the balanced equation shown below, what would be the limiting reagent if 43.5 grams of C2H5NSCl were reacted with 81.4 grams of O2?
C2H5NSCl+5O2=>2CO2+2H2O+NO+SO3+HCl
*trick: multiply the coefficient in the equation in front of the opposite compound, by the number of moles of the compound you are solving for:
C2H5NSCl: 43.5 grams / 110.5 grams per mole = 0.4 moles (5) = 2 moles
O2: 81.4 grams / 32 grams per mole = 2.5 moles (1) = 2.5 moles
2 moles < 2.5 moles
C2H5NSCl is the limiting reagent.
For the balanced equation shown below, what would be the limiting reagent if 69.2 grams of N2 were reacted with 344 grams of H2O?
N2+4H2O=>2H2O2+N2H4
N2: 69.2 grams / 28 grams per mole = 2.5 moles (4) = 10 moles
H2O: 344 grams / 18 grams per mole = 21.5 moles (1) = 21.5 moles
10 moles < 21.5 moles
N2 is the limiting reagent.
For the balanced equation shown below, what would be the limiting reagent if 86.7 grams of C4H10S were reacted with 451 grams of O2?
C4H10S+8O2=>4CO2+5H2O+SO3
C4H10S: 86.7 grams / 90 grams per mole = 0.96 moles (8) = 7.68 moles
O2: 451 grams / 32 grams per mole = 14 moles (1) = 14 moles
7.68 moles < 14 moles
C4H10S is the limiting reagent.
Ready to move onto the quiz?
What is a limiting reagent?
- Let's say you own a sushi restaurant. You have hungry customers who want high-quality sushi. So, you buy equal amounts of rice and seaweed to roll the sushi. It takes more rice than seaweed to make each roll, so eventually the rice will run out first, which makes it the limiting reagent, because you have enough seaweed to make more rolls, but not enough rice. Then, if you buy more rice, and not seaweed, the seaweed will run out first, because you have enough rice to make more rolls, but not enough seaweed.
- Again, you have hungry customers to serve. This time you are a fisherman supplying a sushi restaurant. You have equal amounts of tuna and salmon, but tuna is in higher demand at the sushi restaurant. Your tuna supply will run out first, so it is your limiting reagent.
Limiting Reagent - a chemical that runs out first.
Moles – the mole is the measure of the amount of chemical substance.
The abbreviation for Moles is mol.
Molar mass [(Gram Formula Weights)(GFW)] -- How much each element weighs.
e.g. Carbon (C) has a GFW of 12.
The unit for GFW is g/mol or grams per mole.
Here is a link to a great online Periodic Table of Elements to help you find the GFW.
http://www.webelements.com/
Look at each element's mass (the number under the symbol for the element). Round the element's weight to the nearest whole number. (except for Cl -- keep at 35.5g/mol)
How to find GFW of a compound:
Add the weights of the elements together:
Compound: CaCO3
Ca x 1, C x 1, O x 3
Ca: 40 x 1 = 40
C: 12 x 1 = 12
O: 16 x 3 = 48
40 + 12 + 48 = 100 g/mol
Equations you will need:
# of moles = # of grams (g) / GFW
3 Worked Example Problems:
For the balanced equation shown below, what would be the limiting reagent if 43.5 grams of C2H5NSCl were reacted with 81.4 grams of O2?
C2H5NSCl+5O2=>2CO2+2H2O+NO+SO3+HCl
*trick: multiply the coefficient in the equation in front of the opposite compound, by the number of moles of the compound you are solving for:
C2H5NSCl: 43.5 grams / 110.5 grams per mole = 0.4 moles (5) = 2 moles
O2: 81.4 grams / 32 grams per mole = 2.5 moles (1) = 2.5 moles
2 moles < 2.5 moles
C2H5NSCl is the limiting reagent.
For the balanced equation shown below, what would be the limiting reagent if 69.2 grams of N2 were reacted with 344 grams of H2O?
N2+4H2O=>2H2O2+N2H4
N2: 69.2 grams / 28 grams per mole = 2.5 moles (4) = 10 moles
H2O: 344 grams / 18 grams per mole = 21.5 moles (1) = 21.5 moles
10 moles < 21.5 moles
N2 is the limiting reagent.
For the balanced equation shown below, what would be the limiting reagent if 86.7 grams of C4H10S were reacted with 451 grams of O2?
C4H10S+8O2=>4CO2+5H2O+SO3
C4H10S: 86.7 grams / 90 grams per mole = 0.96 moles (8) = 7.68 moles
O2: 451 grams / 32 grams per mole = 14 moles (1) = 14 moles
7.68 moles < 14 moles
C4H10S is the limiting reagent.
Ready to move onto the quiz?