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Mass-Mass Problems
Knowing the molar relationships between the substances involved
allow us to calculate the mass relationships. This quantitative study of
chemical reactions is called stoichiometry. Mass-Mass problems
generally involve the following steps:
1. Changing the mass given to the number of moles with the formula;
mass given
# of moles = -----------------------molar mass
2. Determine the number of moles of the unknown by comparing the
molar ratio;
number of moles of given
number of moles of unknown
-------------------------------- = -------------------------------------coefficient of given coefficient of unknown
3. Change the number of moles of the unknown to mass with the
formula;
mass = # of moles x molar mass
Ex: 1. How many grams of water are produced when 7.00 grams of oxygen react with an
excess of hydrogen according to the reaction shown below?
Given
Unknown
2H2(g) + O2(g) ----> 2H2O(g)
7.00g
xg
1. Changing the mass given to the number of moles with the formula;
mass given = 7.00g
7.00g
# of moles = -----------------------32.0g/mole
# of moles of oxygen = 0.219 mole
2. Determine the number of moles of the unknown by comparing the molar ratio;
0.219
X
------- = ------1
2
Number of moles of water (unknown) = 0.438 mole
3. Change the number of moles of the unknown to mass with the formula;
# of moles of water = 0.438 mole
mass of water = 7.89 g
Ex: 2. How many grams of sulfuric acid are required to react completely with 15.0
grams of zinc in a single displacement reaction?
given
unknown
Zn(s) + H2SO4(aq) ----> ZnSO4(aq) + H2(g)
15.0g
Xg
1. Changing the mass given to the number of moles with the formula;
moles of zinc = 0.229 mole
2. Determine the number of moles of the unknown by comparing the molar ratio;
0.229
X
------- = ------1
1
Number of moles of sulfuric acid (unknown) = 0.229 mole
3. Change the number of moles of the unknown to mass with the formula;
# of moles of sulfuric acid = 0.229 mole
mass of sulfuric acid = 22.5 g
22.5 grams of sulfuric acid to completely react with 15.0 grams of zinc.
2. How many grams of sulfuric acid are required to react completely with 15.0
grams
of zinc in a single displacement reaction?
given
unknown
Zn(s) + H2SO4(aq) ----> ZnSO4(aq) + H2(g)
15.0g
Xg
1. Changing the mass given to the number of moles with the formula;
mass given = 15.00g
15.00g
# of moles = ------------------------ = 0.229 mole of Zn
65.4g/mole
2. Determine the number of moles of the unknown by comparing the molar ratio;
0.229
X
------- = ------1
1
Number of moles of sulfuric acid (unknown) = 0.229 mole
3. Change the number of moles of the unknown to mass with the formula;
# of moles of sulfuric acid = 0.229 mole
mass of sulfuric acid = 0.229 mole x 98.1 g/mole
mass of sulfuric acid = 22.5 g