



Stoichiometry
Tutorial
Finding and Using Molar Ratios




The coefficients in a balanced chemical
equation can be used to determine the relative number of molecules, formula
units, or moles of a compound involved in a chemical reaction.
Example:
N_{2} (g) + 3 H_{2}
(g) > 2 NH_{3} (g)
1 molecule of nitrogen (N_{2})
reacts with 3 molecules of hydrogen (H_{2}) to form 2 molecules
of ammonia (NH_{3})
OR
1 mole of nitrogen (N_{2}) reacts
with 3 moles of hydrogen (H_{2}) to form 2 moles of ammonia
(NH_{3})
The coefficients in a balanced equation
can be used to write a molar ratio. Molar
ratios are conversion factors that can be used to relate:

moles of product formed from a certain
number of moles of reactant

moles of reactant needed to form a certain
number of moles of a product.

the number of moles of a particular reactant
needed to completely react with a certain number of moles of a second
reactant.
For the following reaction:
4 NH_{3} (g) + 5 O_{2}
(g) > 4 NO (g) + 6 H_{2}O (g)
the following molar ratios can be written:



















4 moles NH_{3} 
5 moles O_{2} 


4 moles NH_{3} 
4 moles NO 

4 moles NH_{3} 
6 moles H_{2}O 






















5 moles O_{2} 
6 moles H_{2}O 

4 moles NO 
6 moles H_{2}O 




The inverses of each of these molar ratios
can also be written.
Example:
What is the molar ratio between Li and N_{2} in the following
reaction:
6 Li (s) + N_{2} (g) > 2
Li_{3}N (s)




















Answer: 

























OR 








































Using molar ratios in calculations:
Molar ratios are used as conversion factors
to relate the number of moles of one substance with the number of moles
of another substance. The following example illustrates the steps used
in any moletomole conversion problem.
Example:
Using the equation given in the previous example, use the following steps
to determine the number of moles of Li_{3}N that can be prepared
using 0.24 mol Li.




















Write down the units
you are looking for, including the formula of the compound, and an
equals sign:

Write the number of moles
you were given in the problem
mol Li_{3}N 
= 
0.24 mol Li 




Put a multiplication
sign after the number of moles and draw a line:
mol Li_{3}N 
= 
0.24 mol Li 
x 
____________ 






Above and below the line,
write the molar ratio. Since you
want to get rid of the units you started with (in this case "mol
Li), put the units "mol" and the formula of the compound
you started with on the bottom. Put the units "mol" and
the formula of the compound you're looking for on the top.
mol Li_{3}N 
= 
0.24 mol Li 
x 
___mol
Li_{3}N 




mol Li 

Use the coefficients
from the balanced chemical equation to complete the molar ratio.
Place the coefficient in front of the formula that you started with
on the bottom and the coefficient in front of the formula that you
are looking for on the top.
6
Li (s) + N_{2} (g) > 2
Li_{3}N (s)
mol Li_{3}N 
= 
0.24 mol Li 
x 
2
mol Li_{3}N 




6
mol Li 

Cancel out the units
and do the math.
mol Li_{3}N 
= 
0.24 mol Li 
x 
2
mol Li_{3}N 
= 
0.080 mol Li_{3}N 




6 mol
Li 





Practice Problems:
Use the following equation
to answer the questions below:
2 CH_{3}OH
(l) + 3 O_{2} (g) > 2 CO_{2} (g) + 4 H_{2}O
(l)

How many moles of water
will be produced from the combustion of 0.27 moles of CH_{3}OH?

How many moles of O_{2}
are needed to burn 2.56 moles of CH_{3}OH?

How many moles of CO_{2}
are produced from the combustion of 5.25 moles of CH_{3}OH?

How many moles of water
are produced when 3.25 moles of CO_{2} are formed?



























