Stoichiometry
Exercises
A good site with a variety of practice problems can be found here. Choose "Reactions" and then "Stoichiometry" from the menu on the left side when you get there. From Dr. Sergei Smirnov's site at New Mexico State University.
Stoichiometry Exercises
1. The combustion of butane occurs according to the following reaction:
2 C4H10 (g) + 13 O2 (g) ––�?gt; 8 CO2 (g) + 10 H2O (g)
If we start with 25.0 moles of butane, how many moles of water will form? How many moles of O2 will be consumed?
2. Calcium carbonate decomposes at high temperature, forming calcium oxide and carbon dioxide according to the following reaction:
CaCO3 (s) 
CaO (s) + CO2 (g)
If 10.0 kg of CaCO3 is decomposed by this reaction, how many kilograms of CaO will form? How many kilograms of CO2 will form?
3. Magnesium hydroxide decomposes on heating, forming magnesium oxide and water:
Mg(OH)2 (s) MgO (s) + H2O (g)
When a sample of Mg(OH)2 was heated in a crucible over a Bunsen burner flame, the weight of the crucible and its contents went from 43.78 g to 42.56 g. How many grams of Mg(OH)2 were initially present?
4. In the reaction of hydrogen sulfide with sodium hydroxide,
H2S (g) + 2 NaOH (aq) ––�?gt; Na2S (aq) + 2 H2O (l)
how many grams of NaOH are needed to react with 6.75 g of H2S ?
5. A student prepared bromobenzene in the Organic lab by reacting 10.0 g of benzene, C6H6, with bromine in the presence of a ferric bromide catalyst according to the reaction
C6H6 (l) + Br2 (l) –––––�?gt; C6H5Br (l) + HBr (g)
FeBr3
If the student obtained 12.9 g of bromobenzene, what was the percent yield?
Percent yield of bromobenzene = actual yield in grams / calculated yield in grams X 100.
6. The complete oxidation of potassium ferrocyanide with potassium permanganate in acidic aqueous solution occurs according to the following reaction:
10 K4Fe(CN)6 (aq) + 218 H2SO4 (aq) + 122 KMnO4 (aq) ––�?gt;
122 MnSO4 (aq) + 5 Fe2(SO4)3 (aq) + 81 K2SO4 (aq) + 60 HNO3 (aq) + 60 CO2 (g) + 188 H2O (l)
How many grams of sulfuric acid and potassium permanganate are required to oxidize 25.0 g of potassium ferrocyanide by this reaction? How many grams of carbon dioxide will form?
7. The reaction of magnesium metal with sulfur is carried out according to the following reaction:
8 Mg (s) + S8 (s) 8 MgS (s)
If 5.00 g of magnesium is heated with 10.0 g of sulfur, how many grams of MgS will form? How many grams of the excess reactant remain?
8. Aluminum oxide is reacted with 6.00 M hydrochloric acid. How many milliliters of 6.00 M HCl are needed to completely react with 15.0 g of Al2O3 according to the reaction
Al2O3 (s) + 6 HCl (aq) ––�?gt; 2 AlCl3 (aq) + 3 H2O (l)
9. A student reacted a 1.50 g sample containing an unknown amount of zinc metal with excess hydrochloric acid:
Zn (s) + 2 HCl (aq) ––�?gt; ZnCl2 (aq) + H2 (g)
The volume of hydrogen gas obtained was 244 mL at a temperature of 23.0°C and 1.00 atm. What is the percent by weight of zinc in the sample?
10. Crude iron is obtained in a blast furnace by reaction of iron ore, one component of which is magnetite, Fe3O4, with carbon monoxide:
Fe3O4 (s) + 4 CO (g) 3 Fe (l) + 4 CO2 (g)
The carbon monoxide, in turn, is prepared in part by the combustion of coke, which is 85 to 90% percent by weight carbon, according to the reaction
2 C (s) + O2 (g) ––�?gt; 2 CO (g)
How many kilograms of Fe3O4 are required to prepare 1.00 kg of crude iron? How many kilograms of coke, assumed to be 90% by weight carbon, are needed?
Answers
1. 125 mol of H2O formed, 162.5 mol of O2 consumed
2. 5.60 kg of CaO formed, 4.40 kg of CO2 formed
3. 3.95 g of Mg(OH)2
4. 15.8 g of NaOH
5. 64.2%
6. 145 g of H2SO4, 131 g of KMnO4, 17.9 g of CO2
7. 11.6 g of MgS formed, 3.40 g of sulfur remains as unreacted excess
8. 147 mL of 6.00 M HCl
9. Percent zinc by weight = 43.8%
10. 1.38 kg of Fe3O4, 0.319 kg of coke
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