Bell Ringer
• How many moles of Sodium Sulfite, Na2SO3,
are contained in 235.5 grams?
• FW of Na2SO3 = 126.06 g/mole
• 235.5 g Na2SO3
•
X
1 mole Na2SO3 = 235.5 moles Na2SO3
126.06 g Na2SO3
126.06
• = 1.86815802 moles Na2SO3 = 1.868 moles Na2SO3
Mass Problems
(Stoichiometry)
Mass Relationships
within
Chemical Reaction
Learning Target
• I CAN solve a mass-mass relationship
problem (stoichiometry) using dimensional
analysis.
What is Stoichiometry?
• Simply put, STOICHIOMETRY is the study of
the RELATIONSHIP between REACTANTS and
PRODUCTS in a chemical reaction.
– In other words, if there is X AMOUNT OF
REACTANTS, what MASS OF PRODUCT can be
produced?
• Recall that a CHEMICAL EQUATION represents
what occurs on the SUBMICROSCOPIC LEVEL
in a chemical reaction.
• An EQUATION is the chemist’s “RECIPE” so to
speak.
• In STOICHIOMETRY, the chemist studies the
AMOUNT of REACTANTS/PRODUCTS
REQUIRED/PRODUCED in a CHEMICAL
REACTION.
• Generally, a STOICHIOMETRIC CALCULATION
has THREE PARTS:
• 1. CONVERT the STARTING QUANTITY of
substance [REACTANTS OR PRODUCTS] to
MOLES:
– A. If the reactant/product is an ELEMENT, find the
ATOMIC MASS on the periodic table; remember
the unit is g/mole.
– B. If the reactant/product is a COMPOUND,
calculate the FORMULA WEIGHT of the
compound, also in g/mole.( Formula Weight Calculator )
• 2. Establish the MOLE RATIO from the
BALANCED EQUATION:
• The mole ratio is the RATIO OF THE
COEFFICIENTS of the two substances in
question.
– Example 2 H2 + O2  2 H2O
– What is the MOLE RATIO between Hydrogen and
Water?
–
2 mol H2 : 2 mol H2O
• 3. Convert MOLES OF FINAL SUBSTANCE
[REACTANTS/PRODUCTS] to GRAMS of FINAL
SUBSTANCE.
– You will need to calculate the FORMULA WEIGHT
OF THE FINAL SUBSTANCE.
• 4. Round the FINAL ANSWER to SIGNIFICANT
FIGURES.
EXAMPLE PROBLEM
• Table Salt, Sodium Chloride, is formed by the
reaction of Chlorine gas with Sodium metal
according to the equation:
•
2 Na + Cl2  2 NaCl
• How many grams of Sodium Chloride could be
formed from 45.0 grams of Sodium?
Steps
• 1. If necessary, BALANCE THE EQUATION.
• 2. LABEL the amount(s) GIVEN and the
amount(s) to BE FOUND.
•
2 Na + Cl2  2 NaCl
45.0 g
?g
• 3. Set up a DIMENSIONAL ANALYSIS:
• Starting Amount
•
•
Mole Ratio (coeffiecients from the equation)
1 Mole
FW
Final
Product of
Top Line
______________________ =_____ = _____
Product of
FW
1 Mole
X
Bottom Line
Start
•
•
FINAL
ANSWER
Grams Moles
MoleGrams
• IF SET UP CORRECTLY ALL UNIT CANCEL
EXCEPT THE LAST ONE!
• 4 Begin with the AMOUNT GIVEN and work
toward the AMOUNT PRODUCED.
• 45.0 g Na 1 mole Na 2 mol NaCl
58.44 g =
•
X
22.989 g Na 2 mol Na 1 mol NaCl
• 5. MULTIPLY all values ACROSS THE TOP, then all
values ACROSS THE BOTTOM, then DIVIDE the TOP
by BOTTOM.
•
•
= 5,259.60 g = 114.39 g = 114 g NaCl
45.978
Practice Problem
• If Calcium Hydroxide reacts with Hydrogen
Chloride according to the following equation,
how many grams of Calcium Chloride would
be formed if 75.0 grams of Calcium Hydroxide
are used in the reaction?
•
Ca(OH)2 + 2 HCl
 CaCl2 + 2 H2O