Unit 5 Notes Part 3: Stoichiometry
5.10


Stoichiometry
Stoichiometry – ____________________________________________________________________
Interpreting Chemical Equations
o The ___________________in a chemical reaction tell the __________of the number of moles of
each compound. This is called the_____________________. ** Equations must be BALANCED!!!!
Example: N2(g) + 3H2(g) --> 2NH3(g)
In words, ___________of nitrogen reacts with ___________of hydrogen to form or yield
__________ of ammonia. The mole ratio is ___ to ____ to ____.
N2(g)
N2(g) + 3H2(g) --> 2NH3(g)
H2(g)
NH3(g)
1. moles
2. mass
3. volume
5.11
Mole to Mole Stoichiometry Conversions
Use the mole ratios. The given unit will be ___________and the answer will be in_____________.
N2(g) + 3H2(g) --> 2NH3(g)
Examples:
How many moles of ammonia are produced when 0.60 moles of nitrogen react with hydrogen?
Given Quantity
Mole Ratio of Substances
Desired Quantity
(coefficients)
mol N2
mol NH3
mol NH3
mol N2
How many moles of nitrogen are required to make 7.24 moles of ammonia?
Given Quantity
Mole Ratio of Substances
(coefficients)
Desired Quantity
How many moles of hydrogen are required to react with 185.2 moles of nitrogen?
Given Quantity
Mole Ratio of Substances
(coefficients)
Desired Quantity
Practice:
____ Al + ____O2 --> ____ Al2O3 (reaction type = _______________)
1. How many moles of aluminum are needed to form 2.3 moles of aluminum oxide?
2. How many moles of oxygen are required to react completely with 0.84 moles of aluminum?
5.12 Mass to Mass Stoichiometry Conversions
Use dimensional analysis to convert from mass units of one substance to mass units of another substance in the
chemical equation.
start with
given
x
convert given to moles x
(use_______)
use the mole ratio
to switch to the
other substance
x
convert to
desired units
(use______)
Examples:
N2(g) + 3H2(g) --> 2NH3(g)
Calculate the mass of ammonia produced when 5.4 g of hydrogen reacts with nitrogen.
How much nitrogen is required to react with 356.2 g of hydrogen?
Practice:
____C2H2 + ____O2 --> ____CO2 + ____ H2O
How many grams of oxygen are required to burn 13.0 grams of ethyne (C 2H2)?
How many grams of CO2 are produce when 42.0 grams of ethyne react with oxygen?
5.13
Other Stoichiometry Conversions
May involve mass-volume, volume-volume, mole - mass, particles-mass or density, just to name a few! Use
dimensional analysis to convert between units.
No matter what units you're asked to convert between, use these steps:
start with given --> convert given unit to moles ---> mole ratio --> convert moles to desired unit
REMEMBER!
1 mole = molar mass grams (different for each element/compound)
1 mole = 22.4 liters at STP
1 mole = 6.02 x 1023 particles (atoms, ions, molecules, formula units)
Examples:
1. How many liters of oxygen are produced when 29.2 g of water is decomposed? The balanced equation is:
2H2O --> 2H2 + O2
2. How many molecules of hydrogen are produced by the reaction of tin with 10 grams of hydrofluoric acid? The
UNbalanced equation is Sn + HF --> SnF2 + H2
Practice:
1. How many moles of aluminum chloride are produced when 1980 grams of chlorine react with aluminum? Write
your own balanced equation before you start the stoichiometry problem!
2. What volume of chlorine is required to react with 65.3 grams of aluminum? Use your balanced equation from
#1.
5.14

Limiting Reactants
Chemical equations resemble recipes in that they take specific amounts, or ratios, of reagents (starting
materials) in order to produce the product(s). When there is not enough of one reagent to "use up" all of
the other reagent, it is called the ______________________because it _____________the amount of
product you can get from the reaction. The reagent that has some "left over" is called the ___________
because there was too much of it to react with the limiting reactant completely.
**How to spot a limiting reactant problem:
In a limiting reactant problem you will be given 2 starting units - one for each of the reactants. You have
to choose which one to use to find out how much of the products will be formed. Therefore, you will have 2
dimensional analysis problems to figure out the answer to the question – whichever one gives you the lower
answer is your limiting reactant!
Example:
2Al + 3CuCl2 --> 3Cu + 2AlCl3
Calculate the number of grams of copper produced if 5.0 g of aluminum reacts with 5.0 g of copper (II) chloride.
Ex: Calculate the number of grams of aluminum chloride produced if 15.0 g of aluminum reacts with 25.0 g of
copper (II) chloride.
5.15

Percent Yield
Percent Yield is a way of determining “how effective” your experiment was.
%Yield = (Actual/Theoretical) x 100
OR
(I made/I should have made) x 100
Ex: In the formation of sodium chloride from sodium metal and chlorine gas, 34.6 g of NaCl are produced. It was
calculated that 50.0 g of NaCl should have been produced. What is the percent yield of the reaction?
Ex: A lab technician estimates that 45.0 g of product should be made from a reaction. If the technician carries
out the reaction with a 87% yield, how much product was made?
Ex: 2Al + 3CuCl2 --> 3Cu + 2AlCl3
Consider the above equation. When 3.5 g of Al react with excess CuCl2, 9.0 g of Cu result. What is the percent
yield of the reaction?
Ex: 2Li + Na2S  Li2S + 2Na
A student carried out the above reaction in a lab. She started the reaction with 1.4 g of lithium and 2.9 g of
sodium sulfide. If she calculated a 78% yield, how much sodium metal did she recover during the lab?