SCH3U
Name:
Stoichiometry (Mass to Moles) – Part 1
2 Mg(s)
+
O2(g)
 2 MgO(s)
Note that the Law of Conservation of Mass is always obeyed.
Consider:
4NH3 + 5O2  6H2O + 4NO
In words, this tells us that for every _______ moles of NH3, _______ moles of O2 are required
Is 4 g NH3 / 5 g O2 a conversion factor?
Stoichiometry questions (1) -Factor-Label Method
Consider: 4NH3 + 5O2  6H2O + 4NO
1. How many moles of H2O are produced if 0.176 mol of O2 are used?
2. How many moles of NO are produced in the reaction if 17.00 mol of H2O are also produced?
3. How many grams of H2O are produced if 1.9 mol of NH3 are combined with excess oxygen?
4. How many grams of O2 are required to produce 0.3 mol of H2O?
5. How many grams of NO is produced if 12 g of O2 is combined with excess ammonia?
H1
SCH3U
Name:
Moving along the stoichiometry path
We always use the same type of information to make the jumps between steps:
grams (x)  moles (x)  moles (y)  grams (y)
Example:
Consider:
4NH3 + 5O2  6H2O + 4NO
a) How many moles of H2O can be made using 0.50 mol NH3?
b) What mass of NH3 is needed to make 1.50 mol NO?
c) How many grams of NO can be made from 120.0 g of NH3?
The steps to solving stoichiometric problems are as follows- CHART METHOD
1.
2.
3.
4.
5.
6.
7.
8.
Write the chemical equation
Balance the chemical equation
Write the molar ratio for the equation
Write all given masses of substances in the equation
Write the molar masses for all substances
Find the number of moles of each substance
Find the new molar ratio
Solve for the unknown
Example:
Given 4NH3 + 5O2  6H2O + 4NO
4 NH3
How many grams of NO can be made from 120.0 g of NH3?
5 O2
6 H2O
4 NO
Molar Ratio(MR)
Mass (m)
Molar Mass (MM)
Moles (n)
H2
SCH3U
Name:
Stoichiometry Questions:
Follow the rules for significant digits. Show all calculations.
1. 2 C4H10 + 13 O2 -> 8 CO2 + 10 H2O
a) What mass of O2 will react with 400.0 g C4H10? [1431 g]
b) How many moles of water are formed in a)? [44.7 mol]
2. 3 HCl + Al(OH)3 -> 3 H2O + AlCl3
How many grams of aluminum hydroxide will react with 5.30 moles of HCl? [ 737 g]
3. Ca(ClO3)2 -> CaCl2 + 3 O2
What mass of O2 results from the decomposition of 1.00 kg of calcium chlorate? [32 kg]
4. The reaction of Ca with water can be predicted using the activity series. What mass of water is needed to completely
react with 2.35 g of Ca? [
5. Fe2O3 + 3CO -> 2Fe + 3CO2.
a) How many moles of carbon monoxide are required to react with 163.0 g of iron (III) oxide? [ 3.06 mol CO]
b) How many grams of CO2 are produced from a reaction that also produces 23.9 grams of Fe? [ 9.44 g CO2]
6. 3Cu + 8HNO3  3Cu(NO3)2 + 4H2O + 2NO
a) How many moles of copper (II) nitrate can be prepared from 17.0 moles of Cu? [ 17.0 mol]
b) How many grams of copper (II) nitrate can be prepared using 3.8 moles of HNO3? [ 227.5 g]
c) What mass of water results from the reaction of 8.50 kg of copper metal? [ 5.532 kg]
7. According to the following equation how many moles of lithium hydroxide are required to react with 20.0 moles of
CO2?
CO2 + 2LiOH  Li2CO3 + H2O
[ 40 mol]
8. How many moles of ammonia, NH3 are produced when 6.00 moles of hydrogen gas react with excess nitrogen gas?
3H2 + N2  2NH3
[ 4 mol]
9. What mass, in grams, of glucose is produced when 3.00 moles of water react with carbon dioxide?
6CO2 + 6H2O  C6H12O6 + 6O2
[ 90.1 mol]
10. What mass in grams of magnesium oxide is produced when 2.00 moles of magnesium react with oxygen?
[80.6 g]
11. The reaction below is run using 824.0 grams of NH3 and excess oxygen, how many moles of NO are formed? How
many moles of H2O are formed?
4NH3 + 5O2  4NO + 6H2O
[48.38 g NO; 72.58 g H2O]
12. If mercury (II) oxide decomposes, how many grams of mercury (II) oxide are needed to produce 125 grams of
oxygen? [1692g]
13. How many grams of SnF2 are produced from the reaction of 30.0 grams of HF with Sn?
Sn + 2HF  SnF2 + H2
[1.92 g]
14. What mass of aluminum is produced by the decomposition of 5000.0 grams of Al2O3? [2646.3 g= 2.6463 kg]
15. How many molecules of hydrogen chloride gas will be produced when 33.5 grams of chlorine combine with excess
hydrogen? [5.69 x1029 molecules]
H3
SCH3U
Name:
Stoichiometry- Limiting Reagents
A balanced chemical equation indicates the number of ________________of each REACTANT that will react and the
number of ________________of each PRODUCT that will be produced in the reaction
Even if one reactant is present in excess only the amount ________________to react, dictated by the molar ratio, will
actually react.
The amount reacting will be determined by the reactant that is in the ________________amount (_________________).
Problems of this type may be recognized by the fact that information will be given about at ________________two
reactants.
Q - How many moles of NO are produced if __ mol NH3 are burned in __ mol O2?
4 mol NH3, 5 mol O2
4 mol NH3, 20 mol O2
8 mol NH3, 20 mol O2
In limiting reagent questions we use the limiting reagent as the “given quantity” and ________________ the reagent
that is in excess.
Example: How many grams of NO are produced if 4 moles NH3 are burned in 20 mol O2?
Solving Limiting Reagents: grams to moles
Q - How many g NO are produced if 20 g NH3 is burned in 30 g O2?
1. First we need to calculate the number of moles of each reactant
NH3
O2
Molar Ratio
What we Have
Limiting Factor Test
The _______________value indicates the Limiting Reactant
Determine the reactant in excess and the excess amount if 20 g NH3 is burned in 30 g O2?
Reactant
NH3
O2
Molar Ratio
What we have
Limiting Factor Test
What we need
Excess
Stoichiometry (given = limiting)
H4
SCH3U
Name:
1) Express all chemical quantities as moles
2) Determine the limiting reagent via a chart
3) Perform the stoichiometry using the limiting reagent as the “given” quantity
Example: How many g NO are produced if 20 g NH3 is burned in 30 g O2?
Given:
4NH3
+
5O2

4NH3
5O2
Molar Ratio (MR)
Mass (m)
6H2O
6H2O
+
4NO
4NO
Molar Mass (MM)
Moles (n)
Practice Questions:
Follow the rules for significant digits. Show all calculations.
1. 2Al + 6HCl  2AlCl3 + 3H : If 25.0 g aluminum was added to 90.0 g HCl, what mass of H2 will be produced? [2.49 g H2]
2. N2 + 3H2  2NH3 : If you have 20.0 g of N2 and 5.0 g of H2, which is the limiting reagent? [ N2]
3. 4Al + 3O2  2 Al2O3 : What mass of aluminum oxide is formed when 10.0 g of Al is burned in 20.0 g of O2? [42.5g Al2O3]
4. C3H8 + 5O2  3CO2 + 4H2O: When C3H8 burns in oxygen, CO2 and H2O are produced. If 15.0 g of C3H8 reacts with
60.0 g of O2, how much CO2 is produced? [44.9 g CO2]
5. MgCl2 + 2AgNO3  Mg(NO3)2 + 2AgCl: If 25 g magnesium chloride was added to 68.0 g silver nitrate, what mass of
AgCl will be produced? [57 g AgCl]
6. Silicon dioxide (quartz) is usually quite uncreative but reacts readily with hydrogen fluoride according to the
following equation. SiO2 (s) + 4HF (g)  SiF4 (g) + 2H2O (l): If 2 moles of HF are exposed to 4.50 moles of
SiO2, which is the limiting reactant? [HF]
7. If 20.50 grams of chlorine is reacted with 20.50 grams of sodium, which reactant is in excess? [Na]
8. Using what you learned from problem #7, how much sodium chloride would be produced in 20.5 grams of chlorine
is reacted with 20.50 grams of sodium? [33.8 g NaCl]
9. Magnesium sulfate reacts with potassium. How much magnesium would be produced if 35.0 grams of magnesium
sulfate is exposed to 45.0 grams of potassium? [7.07 g Mg]
10. What is the maximum number of grams of PH3 that can be formed when 6.20g of solid phosphorus reacts with 6.0g
of hydrogen gas to form PH3 gas? [6.8 g PH3]
11. Solid copper is formed when solid aluminum reacts with aqueous copper (II) sulfate in a single-replacement
reaction. How many grams of copper can be obtained when 29.0g of Al react with 156.0g of copper (II) sulfate?
[62.1g Cu]
12. If you begin with 1250g of nitrogen gas and 225.0g of hydrogen gas in the reaction that forms ammonia gas (NH3),
how much ammonia will be formed? What is the limiting reagent? How much of the reagent is left when the
maximum amount of ammonia is formed? [1265 g NH3, 210 g N2]
13. 550.0g of aqueous lithium hydroxide reacts with 550.g of aqueous aluminum nitrate in a double replacement
reaction. How many grams of solid aluminum hydroxide are produced? What is the limiting reactant? [ Al(NO3)3 –
the limiting reactant, 201 g of Al(OH)3]
H5
SCH3U
Name:
PERCENTAGE YIELD (P. 336-338)
Yield:
Theoretical yield:
Actual yield:
Percent Yield =
Q: Give 4 possible reasons why the actual yield in a chemical reaction often falls short of the theoretical yield.
Sample problem:
What is the % yield of H2O if 138 g H2O is produced from 16 g H2 and excess O2? [143 g; 96.7%]
Step 1: Write the balanced chemical equation:
Step 2: Determine actual and theoretical yield. Actual is given, theoretical is calculated:
Step 3: Calculate % yield:
Practice problem: What is the % yield of NH3 if 40.5 g NH3 is produced from 20.0 mol H2 and excess N2? [227 g, 17.8%]
H6
SCH3U
Name:
PERCENTAGE YIELD (P. 336-338)
Practice problem:
When 5.00 g of KClO3 is heated it decomposes according to the equation: 2KClO3  2KCl + 3O2
a) Calculate the theoretical yield of oxygen. [1.96 g]
b) Give the % yield if 1.78 g of O2 is produced. [90.9%]
c) How much O2 would be produced if the percentage yield was 78.5%? [1.537 g O2]
Challenging question:
2H2 + O2  2H2O
What is the % yield of H2O if 58.0 g H2O are produced by combining 60.0 g O2 and 7.00 g H2?
Hint: determine limiting reagent first. [62.4 g; 92.9%]
More Percent Yield Questions:
1. The electrolysis of water forms H2 and O2. What is the % yield of O2 if 12.3 g of O2 is produced from the
decomposition of 14.0 g H2O? [12.43 g; 98.9%]
2. 107 g of oxygen is produced by heating 300 grams of potassium chlorate. Calculate % yield. [117.5 g; 91.1%]
3. What is the % yield of ferrous sulphide if 3.00 moles of Fe reacts with excess sulfur to produce 220 grams of ferrous
sulphide? Fe + S  FeS [263.7 g; 83.4%]
4. Iron pyrites (FeS2) reacts with oxygen according to the following equation:
4FeS2 + 11O2  2Fe2O3 + 8SO2
If 300 g of iron pyrites is burned in 200 g of O2, 143 grams of ferric oxide is produced. What is the percent yield of
ferric oxide? [181.48 g Fe2O3; 78.8%]
5. 70 grams of manganese dioxide is mixed with 3.5 moles of hydrochloric acid. How many grams of Cl2 will be
produced from this reaction if the % yield for the process is 42%? [57.08 g Cl2; (42% x 57.08 g Cl2)/(100%) = 24 g Cl2]
MnO2 + 4HCI  MnCl2 + 2H2O + Cl2
H7