Stoichiometry and mole concept

1. STOICHIOMETRY AND MOLE CONCEPT
1. Use of molar mass to predict reacting masses
A balanced chemical equation is a way of describing the relative quantities of reactants
and products that are involved in a reaction. The coefficients may be read to indicate
the relative numbers of atoms, ions, or molecules involved in the reaction.
The following equation, for magnesium metal burning in oxygen, can be read "two
atoms of magnesium combine with one molecule of oxygen to form two 'molecules'
(actually ions Mg 2+ and O2-) of magnesium oxide."
2Mg(s) + O2(g) → 2MgO(s)
A better way to read it is "two moles of magnesium metal combine with one
mole of oxygen gas to form two moles of magnesium oxide".
From this, the relative masses of the reactants and products can be predicted:
2Mg(s) + O2(g) → 2MgO(s)
2×24.3g 32g 2×40g
If the relative masses of reactants and products in a reaction can be predicted from a
balanced equation and knowledge of molar masses, then the actual mass of any
reactant or product can be calculated by the use of ratios.
Method 1
Example one:
What mass of sodium carbonate will be obtained if 3.36 g of pure sodium hydrogen
carbonate is heated? (The other products of the reaction are carbon dioxide and water.)
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2. Method 2
Example two:
What mass of carbon will be converted to carbon monoxide in reducing 1000 g of
iron(III) oxide to iron metal? What masses of iron and carbon monoxide should be
formed?
Note that with three "unknowns" in this problem, three algbraic symbols, x, y, z, are
used. The values of x, y, and z are calculated by simple ratio:
Mass of carbon converted = x = 226 g
Mass of iron formed = y = 700 g
Mass of carbon monoxide formed = z = 526 g
The total mass of reactants should equal the total mass of products:
1000 g + 226 g = 700 g + 526 g
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3. Example three:
What mass of lead can be extracted by heating 120 g of solid lead sulfide in air, forming
lead oxide and sulfur dioxide, and then heating the lead oxide with carbon, to form
metallic lead and carbon monoxide?
This problem can be solved as above, by writing the equations and carrying out all ratio
calculations. An alternative method uses percentage composition: the problem can be
summed up as "how much lead can be separated from 120 g of lead sulfide?"
86.6% of 120 g = 104 g = mass of lead that can be extracted from 120 g of lead sulfide.
2. Reacting volumes
For 1 mole of any gas at room temperature is 24dm3
1dm3 = 1000cm3
Example
Calculate the volume of oxygen at r.t.p necessary to burn 1.4 g of butane.
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4. 3 Mole and concentration of solution
Most of the practical work and information in questions gives volumes in cm3. You will
have to change from cm3 into dm3 (1dm3 = 1000cm3)
Example
Calculate the volume of sodium hydroxide, concentration 0.16mol/dm3, needed to
neutralize 20 cm3 of sulphuric acid, concentration 0.2 mol/dm3.
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5. 4 Moles % yield and % purity
Percentage yield
Excess of magnesium carbonate was added 25cm3 of sulphuric acid, concentration 2.0
mol/dm3. The untreated magnesium carbonate was removed by filtration. The solution
of magnesium sulphate was evaporated to give 6.7 g of hydrated magnesium sulphate
crystals. Calculate the percentage yield.
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6. Percentage purity
Example
7.0 g of impure calcium carbonate was heated and 2.42g of carbon dioxide is was
collected. Calculate the percentage purity of the calcium carbonate.
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7. Exercises:
1. What mass of copper can be extracted from 5.0 g of copper(II) sulfate by dissolving
the copper sulfate in water and adding zinc metal? (The other product is zinc sulfate).
Zn + CuSO4 + H2O → ZnSO4 + Cu
2. What mass of potassium iodide is needed to react exactly with 8.0 g of lead nitrate, to
form lead iodide? (The other product is potassium nitrate).
Pb(NO3)2 + 2KI → PbI2 + 2KNO3
3. When calcium carbonate is heated strongly, it forms calcium oxide and carbon
dioxide.What mass of calcium carbonate is needed to make 50.0 g of calcium oxide?
CaCO3 → CaO +CO2
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8. 4. Sodium carbonate reacts with hydrochloric acid to form sodium chloride, water, and
carbon dioxide. Some hydrochloric acid was added to some sodium carbonate: 6.0 g of
sodium chloride were formed. What mass of carbon dioxide was produced?
Na2CO3 + 2HCl → 2NaCl + H2O + CO2
5. What mass of lead oxide would need to be reacted with nitric acid to produce 10.0g
of lead nitrate?
PbO + 2HNO3→ 2Pb (NO3)2 + H2O
6.Ammonium nitrate (NH4NO3), an important fertilizer, produces N2O gas and H2O when
it decomposes. Determine the mass of water produced from the decomposition of 25.0
g of solid Ammonium nitrate.
NH4NO3 →N2O + 2H2O (ans: 11.2 g H2O)
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9. 7. Calculate the mass of magnesium oxide formed when 3.0 g of magnesium reacts
with excess oxygen.
2Mg + O2→ 2MgO
8. What volume of carbon dioxide is produced by the reaction when 1.4 g of butane is
completely burnt in oxygen?
C4H8 + 6O2→ 4CO2 + 4H2O
9. What is the volume of oxygen needed to react with 20cm3 of ethane?
2C2H6 + 7O2→ 4CO2 + 3SO3
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10. 10. Calculate the volume of sulphur trioxide formed when 20.0 g of iron (III) sulphate is
heated.
Fe2(SO4)3→ Fe2O3 + 3SO3
11. When aluminum was reacted with an excess of hydrochloric acid, 0.72dm3 of
hydrogen at r.t.p was formed. Calculate the mass of aluminum used.
2Al + 6HCl → 2AlCl3 + 3H2
12.25 cm3 of sodium carbonate 0.10 mol/dm3 was neutralized by, was neutralized by
31.0 cm3 of hydrochloric acid. Calculate the concentration of the acid in mol/dm3.
Na2CO3 + 2HCl →2NaCl + CO2 + H2O
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11. 13. 12.0 g of Ethanoic acid reacted with an excess of ethanol to form 7.2 g of ethyl
ethanoate. Calculate the percentage yield.
CH3COOH + C2H5OH→CH3COOC2H5+ H2O
14. An excess of nickel oxide reacted with 50.0 cm3 hydrochloric acid, concentration 1.6
mol/dm3, to form 6.1 g of hydrated nickel chloride crystals. Calculate the percentage
yield.
NiO + 2HCl → NiCl2 + H2O
NiCl2 + 6H2O → NiCl2.6H2O
15. 9.30 g of impure NaHCO3 was heated 2.24 g of carbon dioxide was formed.
Calculate the percentage purity of the sodium hydrogen carbonate.
2NaHCO3→ Na2CO3 + H2O
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12. 16.Solid lithium hydroxide is used in space vehicles to remove exhaled carbon dioxide
from the living environment by forming solid lithium carbonate and liquid water. What
mass of gaseous carbon dioxide can be absorbed by 1.00 kg of lithium hydroxide?
2LiOH + CO2 → Li2CO3 + H2O
17. A solution of lead (II) nitrate is mixed with a solution of potassium bromide.
a)Write balanced molecular, complete ionic, and net ionic equations for this reaction.
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b) How many moles of lead (II) bromide could be produced from 6 moles of potassium
bromide?
c) How many moles of lead (II) bromide could be produced from 0.846 moles of
potassium bromide?
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