Ch. 4 Stoichiometry
Ch. 4 Stoichiometry
Ch. 4 Stoichiometry
Topic 5
Stoichiometry I
is the study of the lows of chemical combination by weight and volume
“Chemical Formula”
Chemical formula is a set of chemical symbols to present a chemical substance.
Ex. (NaCl , H2O)
♣ How to write a chemical formula?
Magnesium chloride Calcium oxide
Mg Cl Ca O
Valency 2 1 2 2
MgCl2 CaO
N.B. Simplify.
Valency is the number of electrons gained or shared by one atom during a chemical
reaction.
N.B. Radical is a group of different atoms, linked together, carries +ve or -ve
charge, behaves as one atom during chemical reactions.
2) Compounds containing radicals with enough oxygen:
(OH–) hydroxide, (NO3–) nitrate, (SO42–) sulfate, (CO32–) carbonate, ( HCO3–)
hydrogen carbonate (PO43–) phosphate, (S2O32)Thiosulfate
IGCSE Grade (10)
3) Compounds containing radicals with less oxygen:
(NO2–) nitrite, (SO32–) sulfite
Remember (NH4+) ammonium is a positive radical, without oxygen
Aluminum hydroxide Sodium sulfate
Al OH Na SO4
3 1 1 2
Al(OH)3 Na2SO4
• Molecular formula: represents number and type of atoms in one molecule ex. C2 H6
• Empirical formula: shows the simplest whole number ratio of the different atoms
present in a substance ex. CH3.
• A chemical equation: shows the reactants, products and their ratio involved in a
chemical reaction.
* Word equation * Balanced symbolic
• Relative molecular mass (Mr): is the sum of the relative atomic masses of all atoms
present in one molecule.
One mole of the substance has a mass equal to the relative formula mass in
gram.
Part of the definition of relative atomic mass is ‘the average mass of naturally occurring
atoms of an element. Some relative atomic masses are not whole numbers.
Element Y has only two different types of atoms present in an element Y is shown
35
Y: 37Y = 3:1. Calculate the relative atomic mass of element Y to one decimal place.
The answer:
IGCSE Grade (10)
Stoichiometry II
Mole: is the amount of substance that contains 6x1023 particles.
Mole – Mass:
Mass
Number of moles (n) = -------------------------------------
Molar mass [Ar or Mr]
1. Calculate the number of moles for the following :
a) 2.4 grams of magnesium.
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b) 13 grams of Zinc.
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IGCSE Grade (10)
c) 0.25 mole of aluminium hydroxide.
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Number of particles
Number of moles (n) =-------------------------------------
6x1023
3. Calculate the number of particles for the following:
b) Number of molecules.
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IGCSE Grade (10)
5. For 4 grams of NaCl, find:
a) Number of moles.
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b) Number of molecules.
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Mole – Volume:
Volume
Number of moles (n) =-------------------------------------
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n=CxV
8. Calculate the concentration in mol/dm3 for the following:
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12. Calculate the volume of 1 mol/dm3 solution of H2SO4 required to react with
6g of Mg, and the volume of H2 produced at r.t.p.
Mg + H2SO4 MgSO4 + H2
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IGCSE Grade (10)
N.B. For reactions involving only gases, the volume ratio equals the mole ratio
in the balanced equation.
13. Calculate the volume of methane needed to react with 70 dm3 of oxygen.
CH4 + 2 O2 CO2 + H2O
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14. Find the volume of oxygen needed to react completely with 15 g of C2H6.
C2H6 + 7/2 O2 2 CO2 + 3H2O
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IGCSE Grade (10)
15. What volume of carbon dioxide at r.t.p. will be formed when 50 g of calcium
carbonate react with an excess of hydrochloric acid as shown below?
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16. When pure zinc reacted with dilute sulfuric acid, 2.4 dm3 of hydrogen gas
were collected at r.t.p. Calculate the mass of zinc?
Zn + H2SO4 ZnSO4 + H2
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IGCSE Grade (10)
3
17. What volume of 0.4 mol/dm Hydrochloric acid is needed to react completely
with 0.24 g of magnesium?
Mg + 2 HCl MgCl2 + H2
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18. Calculate the mass of silver formed when 5.52 g of silver carbonate are
heated, and find the volume of gases produced?
2 Ag2CO3 4 Ag + 2 CO2 + O2
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IGCSE Grade (10)
19. Find the volume of ammonia gas formed when 1.605 g of ammonium
chloride is heated?
NH4Cl(s) NH3 (g) + HCl (g)
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20. In an experiment, 25.0 cm3 of aqueous sodium hydroxide, 0.4 mol/dm3 was
neutralized by 20.0 cm3 of aqueous oxalic acid, H2C2O
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(b) Use your answer in (a) to find out the number of moles of H2C2O4 in 20 cm3 of
solution.
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IGCSE Grade (10)
3 3
(c) Calculate the concentration of the 0.04 mol/dm NaOH (aq) in g/dm
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21. How many moles of Cu are required for the production of 15 moles of water
for the following:
[Cu(H2O)5] 2+ Cu2+ + 5H2O
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IGCSE Grade (10)
Percentage purity:
Calculated mass of reactant
% purity = ---------------------------------------- X 100
Given mass of reactant
22. When 10 g of impure zinc reacted with dilute H2SO4, 2.4 dm3 of hydrogen
gas ware collected at r.t.p. Calculate the percentage purity of zinc?
Zn + H2SO4 ZnSO4 + H2
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23. 2 g of an alloy (Cu and Al) reacted with HCl, 2.4 dm3 of H2 evolved.
Calculate the % purity of Al in the alloy?
2Al + 6 HCl 2 AlCl3 + 3 H2
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IGCSE Grade (10)
Percentage Yield :
Actual mass
% Yield = --------------------------- X 100
Calculated mass
24. Heating 12.4 g of copper (II) carbonate produced only 7 g of copper (II)
oxide. What is the % yield of copper oxide?
CuCO3 CuO + CO2
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IGCSE Grade (10)
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27.Calculate the percentage composition of H2O in CuSO4. 5H2O & the percentage
composition of oxygen.
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IGCSE Grade (10)
(a) How many moles of hydrogen were formed?
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29. Calculate the mass of FeS produced from the reaction between 28g of iron
with 20g of sulfur
Fe + S FeS
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30. Calculate the mass of CO2 produced from the reaction between 2 dm3 of
C2H4 with 0.5 dm3 of O2 at r.t.p.
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IGCSE Grade (10)
Empirical & molecular formulae:
C6H6 = (CH) n n= 6
31. A hydrocarbon contains 80 % carbon. Find the empirical formula and the
molecular formula, if the Mr of the compound is 30.
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IGCSE Grade (10)
32. A carbohydrate has 40% of its mass carbon, 6.66% hydrogen. Find the
empirical and molecular formulae, given that its Mr is 180.
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33. Calculate the empirical formula of the formed compound from the reaction of
0.24g Mg with 0.16g oxygen.
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34. Compound contains 0.12g carbon & 0.02 g hydrogen. Calculate the empirical
formula of this compound; calculate the molecular formula of the compound if
its formula weight is 56g.
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IGCSE Grade (10)
35.Calculate the empirical formula of an organic compound containing 92.3%
carbon & 7.7% hydrogen by mass. If the molecular weight of the organic
compound is 78, what is its molecular formula?
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36. 4.2g of cerium reacted with oxygen to form 5.16g of an oxide of cerium.
Complete the following to determine the formula of this oxide.
Number of moles of cerium atoms used ………………………………………….
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IGCSE Grade (10)
(II) What is the molecular formula of the acid?
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38. An excess of hydrochloric acid was added to 1.23g of barium carbonate. The
volume of carbon dioxide collected at r.t.p. was 0.120dm3. The impurities did
not react with the acid. Calculate the percentage purity of the barium
carbonate.
40. 2g (an excess) of iron is added to 50 cm3 of 0.5 M sulfuric acid. When the
reaction is over, the reaction mixture is filtered. The mass of the unreached iron
is found to be 0.6g. (Fe = 56)
a. What mass of iron took part in the reaction?
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IGCSE Grade (10)
c. How many moles of sulfur acid reacted?
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d. Write the equation for the reaction, and deduce the charge on the iron ion that formed.
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e. What volume of hydrogen (calculated at rtp) bubbled off during the reaction?
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41. When calcium carbonate is heated strongly, this chemical change occurs:
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c. (i) What mass of calcium oxide is obtained from the thermal decomposition of 50g
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(ii) What mass of carbon dioxide will be given off at the same time?
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