Stoichiometry
I. Definition; Stoichiometry Map
_________________________ - calculation of amounts of substances
involved in chemical reactions using information obtained
from
b_________________ c________________ r______________
We will work with four types of Stoichiometry Problems:
Data Given
Quantity needed
MOLES MOLES
MOLES GRAMS
GRAMS MOLES
GRAMS GRAMS
Example of a Stoichiometry Problem: 2 A + 2 B ---> 3 C
Given 20.0 g of A and excess B, how many grams of C can be produced?
or
2 NaN3 (s) -->
2 Na (s) +
3 N2 (g) + heat
What mass of sodium azide is needed to inflate an airbag?
** ________________ and __________________ are conserved in every
chemical reaction, but _________________ are not necessarily conserved. **
True or False
1. Subscripts can be changed to balance an
equation.
T or F
2. The number of reactant moles must be the same as the product
moles in a balanced
equation.
T or F
3. The coefficients in the balanced equation relate the masses
of the
substances.
T or F
4. Changing the coefficients for a substance changes the molar
mass.
T or F
5. The molar mass for hydrogen, oxygen, chlorine, etc. (BrINClHOF)
is the same as the atomic mass in
g/mol.
T or F
2 H2 + O2 ----> 2 H2O
ATOMS: 4 atoms H + 2 atoms O ----> 4 atoms H and 2 atoms O
MOLECULES: 2 molecules H2 + 1 molecule O2 ----> 2 molecules H2O
MOLES:
2 MOLES H2 + 1 MOLE O2 ----> 2 MOLES H2O
II. Molar ratios
2 H2 (g) +
O2 (g) +
energy
--> 2 H2O (l)
_____________________ _________________ -
a conversion factor derived from the coefficients of a balanced equation
interpreted in terms of moles
What is the molar ratio between H2 and O2?
or
What is the molar ratio between H2 and H2O?
or
What is the molar ratio between O2 and H2O?
or
Practice Problems: Molar Ratios
For each following equation, 1) balance the equation and 2) give the appropriate molar ratios:
N2
+ H2 ---->
NH3
What is the molar ratio: a) between nitrogen
and hydrogen b) between ammonia and
hydrogen c) for hydrogen TO
ammonia
d) for nitrogen to ammonia
Answer:
1)
2a)
b)
c)
d)
a) molar ratio for N2 to H2
b) molar ratio for NH3 to H2
More Practice Problems: Molar Ratios
For each of the following equations, 1) balance the equation and 2)
give the appropriate molar ratios
SO2 +
O2 ---->
SO3
a) molar ratios between oxygen and sulfur trioxide
b) molar ratio for O2 to SO3
c) molar ratio for O2 to SO2
PCl3
+ Cl2 ---->
PCl5
a) molar ratio for PCl3 to Cl2
b) molar ratio for PCl3 to PCl5
4 NH3 + 3 O2 ----> 2 N2 + 6 H2O
a) molar ratio for NH3 to N2
b) molar ratio for H2O to O2
Fe2O3 + 3 CO ----> 2 Fe + 3 CO2
a) molar ratio for CO to CO2
b) molar ratio for Fe to CO
III. MOLE - MOLE Problems
Example:
N2 + 3 H2 ---> 2 NH3
Problem 1) If 2.00 moles of N2 reacts with sufficient H2, how many moles of NH3 will be produced?
Molar Ratio:
Solution:
Problem 2) If 6.00 mol of H2 reacted with sufficient nitrogen. How many moles of ammonia would be produced?
Molar Ratio:
Solution:
Problem 3) We want to produce 2.75 moles of ammonia. How many moles of nitrogen would be required?
Molar Ratio:
Solution:
Practice Problems: MOLE - MOLE
2 H2 + O2 ---> 2 H2O
1) How many moles of water are produced when 5.00 moles of oxygen are used?
2) If 3.00 moles of water are produced, how many moles of oxygen must be consumed?
3) How many moles of hydrogen gas must be used, given the data in problem 2?
IV. MOLE - MASS and MASS - MOLE Problems
Example: 2 KClO3 ---> 2 KCl + 3 O2
Problem #1: 1.50 moles of Potassium chlorate decomposes. How many grams of O2 will be produced?
Information needed:
Molar ratio: Molar Mass:
Solution:
Problem #2: If 80.0 grams of O2 was produced, how many moles of KClO3 decomposed?
Information needed:
Molar ratio: Molar Mass:
Solution:
Problem #3: We want to produce 2.75 mol of KCl. How many grams of KClO3 would be required?
Solution:
Practice Problems: MOLE - MASS and MASS - MOLE
2 H2 + O2 ---> 2 H2O
1) How many grams of H2O are produced when 2.50 moles of oxygen are used?
2) If 30.0 grams of H2O are produced, how many moles of oxygen must be
consumed?
3) How many grams of hydrogen gas must be used, given the data in problem two?
V. MASS - MASS Problems
Example #1: How many grams of chlorine can be liberated from the decomposition of 64.0 g of Gold (III) chloride?
1) Write the balanced equation.
2) Find molar masses of AuCl3 and Cl2
molar mass AuCl3 = __________ g/mol
molar mass Cl2 = ___________ g/mol
3) Set-up the problem
4) Solve
__________ g Cl2
Example #2: Calculate the mass of AgCl that can be prepared from 200.0 grams of AlCl3 and sufficient AgNO3.
Solution:
200.0 g AlCl3 x x x =
____________ g AgCl
Example #3: Calculate the mass of Lead (II) iodide produced by reacting 30.0 g Potassium iodide with excess Lead (II) nitrate.
_____________ + ____________ ---> ____________ + ____________
Solution:
Example #4: How many grams of Gold (III) choride can be made from 100.0 grams of chlorine and sufficient gold.
Solution:
Example #5: How many grams of sodium are required to react completely with 75.0 grams of chlorine to produce Sodium chloride.
Solution:
VI. Limiting Reagent Problems
Example #1: Sodium chloride can be prepared by the reaction of sodium metal with chlorine gas.
2 Na (s) + Cl2 (g) ---> 2 NaCl (s)
Suppose 6.70 mol Na reacts with 3.20 mol Cl2,
a) What is the limiting reagent?
b) How many moles of NaCl are produced?
Solution:
Example #2: 2 Cu (s) + S (s) ---> Cu2S (s)
a) What is the limiting reagent when 80.0 g Cu reacts with 25.0 g S?
b) What is the maximum number of grams of Cu2S that can be formed?
Example #3: Hydrogen gas can be produced in the laboratory by the reaction of magnesium metal with hydrochloric acid.
a) Identify the limiting reagent when 6.00 g HCl reacts with 5.00 g Mg.
b) How many grams of hydrogen can be produced when 6.00 g HCl is added to 5.00 g Mg?
Example #4: Acetylene will burn in the presence of oxygen. How many grams of water can be produced by the reaction of 2.40 mol C2H2 with 7.4 mol O2?
VI. Percent Yield Problems
____________________ - maximum amount of product that could be formed from given amounts of reactants
____________________ - the amount of product that actually forms
- the ratio of the actual yield to the theoretical yield expressed as a percent
Percent yield = x 100 %
Example #1: Calcium carbonate is decomposed by heating.
a) What is the theoretical yield of calcium oxide if 24.8 g calcium carbonate is heated?
b) What is the percent yield if 13.1 g CaO is produced?
Example #2: When 84.8 g of iron (III) oxide reacts with an excess of carbon monoxide, 54.3 g of iron is produced. What is the percent yield of this reaction?
Example #3: If 50.0 g of silicon dioxide is heated with an excess of
carbon, 27.9 g of silicon carbide is produced. What is the percent yield of this reaction?
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