1
STOICHIOMETRY / Balancing Equations
VI. STOICHIOMETRY
A. Balancing Equations
1. Reactions between elements
Example(1):
H2 + Cl2 HCl
Example(2):
H2 + O2 H2O
Example(3):
N2 + F2 NF3
Example(4):
Li + O2 Li2O
2. Reactions between elements and compounds
Example(5):
Na + H2O NaOH + H2
Example(6):
FeO + O2 Fe2O3
Example(7):
SO2 + O2 SO3
3. Reactions between compounds containing polyatomic ions
Example(8):
Fe2(SO4)3 + Ca(NO3)2 Fe(NO3)3 + Ca SO4
Example(9):
Al2(SO4)3 + Na3PO4 AlPO4 + Na2SO4
4. Combustion reactions
a) When an organic compound is completely combusted, the products produced are the same products that you would get if the individual elements were combusted.
carbon burns to give CO2
hydrogen burns to give H2O
sulfur burns to give SO2
Example(10): What are the products when C3H8S2 is combusted?
b) To balance a combustion reaction:
1st balance all atoms except oxygen
2nd count the oxygen atoms on the right
3rd balance the O2 on the left
Example(11):
C5H12 + O2
Example(12):
C3H8S2 + O2
Example(13):
C6H12O6 + O2
Example(14):
C4H10 + O2
1
STOICHIOMETRY / Stoichiometric Calculations
B. Stoichiometric Calculations
The textbook does stoichiometry using the conversion factor method. Recall the three steps:
1st write the relation between the quantities
2nd write the TWO conversion factors from the relation
3rd given quantity conversion factor = answer
1. Mole - mole calculations
Example(1): How many moles of H2O are produced by the complete reaction of 8 moles of H3PO4 with excess Mg(OH)2?
H3PO4 + Mg(OH)2 Mg3(PO4)2 + H2O
Example(2): How many moles of Mg(OH)2 are used up by the 8 moles of H3PO4?
H3PO4 + Mg(OH)2 Mg3(PO4)2 + H2O
Example(3): How many moles of H3PO4 are required to make 15 moles of Mg3(PO4)2?
H3PO4 + Mg(OH)2 Mg3(PO4)2 + H2O
2. Gram-mole calculations
Example(4): How many grams of CO2 are produced by the complete combustion of 0.25 moles of C6H12O6 ?
Example(5): If 2.16 grams of Al are reacted completely with H2SO4, how many moles of H2 will be produced?
Al + H2SO4 Al2(SO4)3 + H2
3. Gram-gram calculations
Example(6): How many grams of CO2 are produced by the complete combustion of 3200 grams (about 1 gallon) of octane (C8H18)?
C8H18 + O2 CO2 + H2O
Example(7): How many grams of O2 are used up in the reaction?
4. Limiting Reagent Calculations
If the reactants are not mixed in the correct stoichiometric ratio, one reactant will run out before the other. The reactant which runs out first is called the limiting reagent. The amount of any product produced is determined by the amount of the limiting reagent.
Example(8): How many moles of H2O are produced by the reaction of 3.5 moles of HCl with 3.0 moles of NaOH?
HCL + NaOH NaCl + H2O
Example(9): How many grams of Al2O3 are produced by the reaction of 20.0 grams of Al with 15.0 grams of oxygen?
Al + O2 Al2O3
1
STOICHIOMETRY / Molarity and Titrations
C. Molarity and Titrations
1. Molarity (M): the number of moles of solute per liter of solution.
M =
2. Molarity problems
a) Finding molarity of a solution
Example(1): What is the molarity of a Ba(NO3)2 solution, if 7.0 moles Ba(NO3)2 are dissolved in enough water to make 2.0 liters of solution?
Example(2): If 3.0 moles of NaCl dissolved in enough water to make 250 mL of solution , what is the molarity of the solution?
Example(3): 500 mL of an aqueous solution of CaCl2 contain 222 grams of CaCl2, what is the molarity of the solution?
b) Finding number of moles and grams of solute
Example(4): How many moles of H2SO4 are in 40.0 mL of a 3.0 M solution?
Example(5): How many grams of NaOH are in 150 mL of a 0.20 M solution?
Example(6): How many grams of NaOH are required to make 150 mL of a 0.20 M solution?
c) Finding volume
Example(7): What volume of a 12 M acetic acid contains 3.0 moles of acetic acid?
Example(8): How many milliliters of a 2.0 M NaOH solution would you use if you needed 0.30 moles of NaOH for an experiment?
3. Titrations
a) Finding the molarity of the NaOH (titrant)
Example(9): If 16.00 mL of an unknown NaOH solution are required to neutralize 0.04000 moles of HCl, what is the molarity of the NaOH solution?
Example(10): If 12.43 mL of NaOH are used to titrate 1.486 g of KHP (GMW = 204.2 g/mol), what is the molarity of the NaOH solution?
b) Finding the molar mass of an unknown acid
Example(11): If 20.00 mL of 1.500 M NaOH are required to titrate 2.000 g of an unknown acid, what is the molar mass of the acid?
Example(12): 16.52 mL of 0.2004 M NaOH are used to neutralize 0.6322 g of an unknown acid. Find the molar mass of the acid.
c) Finding the Molarity of an unknown solution
Example(13): If 20.00 mL of 3.000 M of NaOH are needed to neutralize 10.00 mL of HNO3, what is the molarity of the HNO3.
4. Dilution
Example(14): If 50 mL of a 6 M NaOH solution are diluted to a new volume of 100 mL, what is the new molarity?
Example(15): To what new volume must 200 mL of a 10 M solution be diluted to obtain a 2.5 M solution?
Example(16): How much water must be added to 200 mL of a 10 M solution to obtain a 2.5 M solution?
Example(17): If 500 mL of water are added to 100 mL of a 12 M solution, what will be its new molarity?
Example(18): How would you make 200 mL of a 0.4 M HCl solution starting with a 10 M stock solution?