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 NaN_{3 (s) }>
2 Na _{(s) }+
3 N_{2 (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 H_{2 }+ O_{2} > 2 H_{2}O
ATOMS: 4 atoms H + 2 atoms O > 4 atoms H and 2 atoms O
MOLECULES: 2 molecules H_{2} + 1 molecule O_{2} > 2 molecules H_{2}O
MOLES:
2 MOLES H_{2} + 1 MOLE O_{2} > 2 MOLES H_{2}O
II. Molar ratios
2 H_{2 (g) }+
O_{2 (g) }+
energy
> 2 H_{2}O _{(l)}
_{_____________________ _________________ }
a conversion factor derived from the coefficients of a balanced equation
interpreted in terms of moles
What is the molar ratio between H_{2 }and O_{2}?
or
What is the molar ratio between H_{2} and H_{2}O?
or
What is the molar ratio between O_{2} and H_{2}O?
or
Practice Problems: Molar Ratios
For each following equation, 1) balance the equation and 2) give the appropriate molar ratios:
N_{2 }
+ H_{2} >
NH_{3}
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 N_{2} to H_{2}
b) molar ratio for NH_{3} to H_{2}
More Practice Problems: Molar Ratios
For each of the following equations, 1) balance the equation and 2)
give the appropriate molar ratios
SO_{2 } +
O_{2} >
SO_{3}
a) molar ratios between oxygen and sulfur trioxide
b) molar ratio for O_{2} to SO_{3}
c) molar ratio for O_{2} to SO_{2}
PCl_{3 }
+ Cl_{2} >
PCl_{5}
a) molar ratio for PCl_{3} to Cl_{2}
b) molar ratio for PCl_{3} to PCl_{5}
4 NH_{3} + 3 O_{2} > 2 N_{2} + 6 H_{2}O
a) molar ratio for NH_{3} to N_{2}
b) molar ratio for H_{2}O to O_{2}
Fe_{2}O_{3} + 3 CO > 2 Fe + 3 CO_{2}
a) molar ratio for CO to CO_{2}
b) molar ratio for Fe to CO
III. MOLE  MOLE Problems
Example:
N_{2} + 3 H_{2} > 2 NH_{3}
Problem 1) If 2.00 moles of N_{2} reacts with sufficient H_{2}, how many moles of NH_{3} will be produced?
Molar Ratio:
Solution:
Problem 2) If 6.00 mol of H_{2} 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 H_{2} + O_{2} > 2 H_{2}O
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 KClO_{3} > 2 KCl + 3 O_{2}
Problem #1: 1.50 moles of Potassium chlorate decomposes. How many grams of O_{2} will be produced?
Information needed:
Molar ratio: Molar Mass:
Solution:
Problem #2: If 80.0 grams of O_{2} was produced, how many moles of KClO_{3} decomposed?
Information needed:
Molar ratio: Molar Mass:
Solution:
Problem #3: We want to produce 2.75 mol of KCl. How many grams of KClO_{3} would be required?
Solution:
Practice Problems: MOLE  MASS and MASS  MOLE
2 H_{2} + O_{2} > 2 H_{2}O
1) How many grams of H_{2}O are produced when 2.50 moles of oxygen are used?
2) If 30.0 grams of H_{2}O 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 AuCl_{3} and Cl_{2}
molar mass AuCl_{3} = __________ g/mol
molar mass Cl_{2} = ___________ g/mol
3) Setup the problem
4) Solve
__________ g Cl_{2 }
Example #2: Calculate the mass of AgCl that can be prepared from 200.0 grams of AlCl_{3} and sufficient AgNO_{3}.
Solution:
200.0 g AlCl_{3} 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) + Cl_{2} (g) > 2 NaCl (s)
Suppose 6.70 mol Na reacts with 3.20 mol Cl_{2},
a) What is the limiting reagent?
b) How many moles of NaCl are produced?
Solution:
Example #2: 2 Cu (s) + S (s) > Cu_{2}S (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 Cu_{2}S 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 C_{2}H_{2} with 7.4 mol O_{2}?
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?
