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Chemistry
WAEC / WASSCE 2014
Stoichiometry & Mole Concept
Which of the following organic compounds could be represented by the empirical formula CH2O?
A
Ethanal
B
Ethane
C
Ethanol
D
Ethanoic acid
medium Difficulty
Verified Correct Answer: Option D
Chemical Principles & Mechanism
No expanded working has been attached for this question yet. The verified answer is Option D.
Source: Myschool Past Questions Archive (Record #37886)
Chemistry
WAEC / WASSCE 2014
Matter & Separation Techniques
The production of biogas from the fermentation of waste materials is an example of
A
combustion
B
incineration
C
refining
D
recycling
medium Difficulty
Verified Correct Answer: Option D
Chemical Principles & Mechanism
No expanded working has been attached for this question yet. The verified answer is Option D.
Source: Myschool Past Questions Archive (Record #37887)
Chemistry
WAEC / WASSCE 2014
Matter & Separation Techniques
Solution
The purity of a solid sample can best be determined by its?
A
boiling point
B
melting point
C
conductivity
D
solubility
medium Difficulty
Verified Correct Answer: Option B
Chemical Principles & Mechanism
- The presence of impurities in a substance results in a lower melting point due to a process called melting point depression.
- That implies a pure solid has a SHARP melting point which is of the standard value when compared.
Source: Myschool Past Questions Archive (Record #37888)
Chemistry
WAEC / WASSCE 2014
Matter & Separation Techniques
Which of the following gases is used to retard the vaporization of the filament in a light bulb?
A
Argon
B
Helium
C
Hydrogen
D
Oxygen
medium Difficulty
Verified Correct Answer: Option A
Chemical Principles & Mechanism
No expanded working has been attached for this question yet. The verified answer is Option A.
Source: Myschool Past Questions Archive (Record #37889)
Chemistry
WAEC / WASSCE 2014
Matter & Separation Techniques
The most common process in the extraction of metals from their ore is
A
catalysis
B
electrolysis
C
oxidation
D
reduction
medium Difficulty
Verified Correct Answer: Option D
Chemical Principles & Mechanism
No expanded working has been attached for this question yet. The verified answer is Option D.
Source: Myschool Past Questions Archive (Record #37890)
Chemistry
WAEC / WASSCE 2014
Acids, Bases, and Salts
Solution
The oxide from electric power station is
A
carbon (IV) oxide
B
copper (ll) oxide
C
nitrogen (IV) oxide
D
sulphur (IV) oxide
medium Difficulty
Verified Correct Answer: Option C
Chemical Principles & Mechanism
Nitric oxide is an important component of the air pollution generated by automotive engines and thermal power-generating plants.
sulphur dioxide may also be produced in case of fuel containing sulphur, Nigeria crude is sulphur free,
Source: Myschool Past Questions Archive (Record #37895)
Chemistry
WAEC / WASSCE 2014
Matter & Separation Techniques
Solution
(a)(i) Define ionic bond.
(ii) What type of bond (s) exist (s) in: I. magnesium oxide; II. ammonium ion?
(b) Determine the oxidation number of sulphur in Na\(_2\)S\(_2\)O\(_2\).
(c) State Faraday's first law.
(d) Give one example each of: (i) acid salt; (ii) base salt.
(e) Name the type of energy change that occurs in each of the following processes
(i) I\(_{2(s)}\) \(\to\) I\(_{2(g)}\)
(ii) Cl\(_{(g)}\) + e\(^{-}\) \(\to\) Cl\(^{-_(g)}\)
(f) State the effect of each of the following aqueous solutions on litmus paper: (i) Na\(_2\)SO\(_{4(aq)}\) (ii) AlCl\(_{3(aq)}\)
(g) Define the term efflorescence.
(h) Give two uses of activated charcoal.
(i) State one use of each of the following processes in the chemical industry: (i) hydrogenation of vegetable oil; (ii) cracking; (iii) esterification.
(j) Calculate the amount of silver deposited in moles when 10920 coulombs of electricity is passed through a solution of a silver salt. [ Faraday constant-- 96500 C mol\(^{-1}\)]
medium Difficulty
Verified Correct Answer: Option
Chemical Principles & Mechanism
(a)(i) Ionic bond is the electrostatic force of attraction between oppositely charged ions/cations and anions.
OR
Ionic bond is defined as the type of bond that is formed from/ by/ through the transfer of electrons from one atom of an element (metal) to an atom of another (non - metal).
(ii) (I) – Ionic / electrovalent.
(II) – „Coordinate (Covalent) / dative.
(b) Oxidation number of sulphur in Na\(_2\)S\(_2\)O\(_2\);
2 x Na + 2 x S + 3 x 0 = 0
2 x 1 + 2s + 3(-2) = 0
2 + 2s – 6 = 0
2s = 4
s = +2
c) Farady's first law states that the mass of an element discharged during electrolysis is directly proportional to the of current passed.
(d)(i) NaHSO\(_4\), NaHCO\(_3\), KHSO\(_3\), etc. (ii) Mg(OH)Cl, Zn(OH)Cl, Zn(OH)NO\(_3\), etc.
e)(i) Sublimation energy. (ii) Electron affinity.
f) (i) Na\(_2\)SO\(_{4(aq)}\) No effect on litmus paper/Neutral to litmus.
(ii) AlCl\(_{3(aq)}\) —Turns blue litmus paper red/Acidic to litmus.
(g) Efflorescence: Loss of (all apart from) water of crystallization by a substance on exposure to the air/Atmosphere.
(h) Activated Charcoal: (i) To remove colouring matter. (ii) To absorb gases. (iii) To remove impurities from liquids. (iv) Whitening of teeth. (v) To remove scent/To decolourize.
(i)(i) Hydrogenaton of Vegetable Oil: Production of margarine/Hardening of oils.
(ii) Cracking: Increase the yield of petrol/Improves the quality of petrol to produce raw materials or petrochemicals.
(iii) Esterification: Production of akanoates. —as solvents for paints/gums/cellulose. —in perfumery. —in making cellulose acetate (for photographic films). —as flavouring agents.
j) Ag\(^{-}\) + eg\(^{-}\) \(\to\) Ag
96500c of electricity deposits I mole of Ag
10920c of electricity = \(\frac{10920 \times 1.0}{96500}\)
= 0.113moles
= 0.11 moles.
Source: Myschool Past Questions Archive (Record #51469)
Chemistry
WAEC / WASSCE 2014
Atomic Structure and Periodic Table
Solution
(a) Define each of the following terms:
(i) ion; (ii) isotopes.
(b)(i) Consider the:element \(_{12}\)Mg and \(_{3}\)Al
I. Write the electron configuration of each element. II. Explain briefly why the first ionization energy of \(_{12}\)Mg is greater than that of \(_{13}\)Al.
(ii) Write the formulae of three different oxides of period 3 elements that react with water.
(c)(i) What are allotropes?
(ii) Name the two crystalline allotropes of carbon.
(iii) Give one industrial use of each allotrope named in (c)(ii).
(d) On warming crystals of sodium chloride with concentrated tetraoxosulphate (VI) acid, a gas was evolved.
(i) List two physical properties of the gas produced.
(ii) Write a balanced equation for the reaction.
(e) A certain chip W used in a circuit of a microcomputer has a mass of 5.68mg. Calculate the amount of W in the microcomputer. [ W = 28 gmol\(^{-1}\) ].
medium Difficulty
Verified Correct Answer: Option
Chemical Principles & Mechanism
a)(i) Ion: (Electrically) charged atom or group of atoms formed from an atom by loss or gain of electron.
(ii) Isotopes: Atoms of the same element with the same number of protons but different number of neutrons/Mass number
(b)(i) I. \(_{(12)}\)Mg — 1s\(^2\)2s\(^2\)2p\(^6\)3s\(^2\)
\(_{(13)}\)Al — 1s\(^2\)2s\(^2\)2p\(^6\)3s\(^2\)3p\(^1\)
II. The 3p electron in Aluminium is slightly further away from the nucleus than the 3s electron of magnesium. The shielding / screening effect by the inner electrons is more in Al than in Mg. These two effects outweigh the increase in nuclear charge of Al, hence nuclear attraction for 3s is greater than the 3p.
OR
The 3s outermost orbital in Mg is fully filled and is therefore more stable than the 3p orbital in Al which is neither completely filled nor half-filled and, hence less stable. OR
3s orbital of Mg is close to the nucleus than the 3p orbital of Al, hence the nuclear attraction for the 3s is greater than the 3p.
(ii) Na\(_2\)O, MgO, P\(_4\)O\(_{(10)}\), SO\(_3\), CI\(_2\)O\(_7\), SO\(_2\), Na\(_2\)O\(_2\)
(c)(i) Allotropes are different forms of the same element in the same physical state.
(ii) Diamond and graphite.
(iii) Diamond —For making jewellery.
—For cutting hardly surface materials.
—Making abrassives
(ii) Graphite —As a lubricant.
—For making pencals.
—For making electrodes.
—Neutron moderator.
—Making crucibles.
d)(i) —Colourless.
—Has choking smell.
—Denser than air.
—Fumes in moist air.
—Soluble in water.
(ii) 2NaCl + H\(_2\)SO\(_4\) --> Na\(_2\)SO\(_4\) + 2HCl
OR
NaCl + H\(_2\)SO\(_4\) \(\to\) NaHSO\(_4\) + HCl
e) Mass of W = 5.68mg = 5.68 x 10\(^{-3}\)g
Moles of W = \(\frac{5.68 x 10^3}{28}\)
= 2.03 x 10\(^{-4}\) moles
Source: Myschool Past Questions Archive (Record #51481)
Chemistry
WAEC / WASSCE 2014
Matter & Separation Techniques
Solution
(a)(i) Outline the steps involved in the purification of water for town supply.
(ii) Give two cations that can cause hardness in water.
(iii) State two disadvantages of hardness of water.
(iv) List two sources of water pollution.
(b)(i) Explain briefly why water is a good solvent for sodium chloride but not for oil.
(ii) State the function of H\(_2\)SO\(_4\) in each of the following reaction equations:
(i) C\(_2\)H\(_5\)OH\(_{(l)}\) \(\to\) C\(_2\)H\(_{4(g)}\)
(ii) MgO\(_{(s)}\) + H\(_2\)SO\(_4\)\(_{(aq)}\) —> MgSO\(_{4(aq)}\) + H\(_2\)O\(_{(l)}\)
(iii) C\(_{(s)}\) + conc. H\(_2\)SO\(_4\) --> CO\(_{(g)}\) + SO\(_{2(g)}\) + H\(_2\)O\(_{(l)}\)
(c) (i) Give the two products formed when chlorine water is exposed to sunlight.
(ii) I. Which of the compounds is suitable for the preparation of standard alkaline solution?
II. Give a reason for your answer in (c)(ii)I
medium Difficulty
Verified Correct Answer: Option
Chemical Principles & Mechanism
(i) (1) Aeration/exposure to sunlight.
(2) Add alum/coagulation /flocculation.
(3) Sedimentation.
(4) Fi!teration.
(5) Add chlorine /disinfectant.
(6) Add flouride/iodide.
(7) Add Ca(OH)\(_2\) / pH control.
(ii) Cations that can cause hardness in water: Mg, Ca, Fe.
(iii)Disadvantages of Hardness of Water:
(1) It wastes soap.
(2) Forms stain on white clothing and material
(3) Forms boiler scale and kettle fur / damage.
(4) It is unsuitable for industrial processes, e.g. dyeing and tanning.
(iv) Sources of Water Pollution:
(1) Wastes from industries, e.g. brewery.
(2) Agricultural activities, e.g. fertilizer.
(3) Waste from quarry activities.
(4) Refuse from plant and animal origin.
(5) Sewage (industrial and domestic).
(6) Oil spillage.
(b)(i) Water is a polar solvent and can therefore easily dissolve sodium chloride which is ionic / polar salt. Oil, on the other hand, is non-polar and, hence cannot dissolve in water which is polar.
(ii) - H\(_2\)SO\(_4\) - As a dehydrating agent.
- H\(_2\)SO\(_2\) - As an acid.
- H\(_2\)SO\(_4\) As an oxidizing agent.
(c)(i) HCl and O\(_2\)
(ii)(I) Na\(_2\)CO\(_3\)
(II) Na\(_2\)CO\(_3\) is stable in air and can be weighed accurately. NaOH absorbs water / deliquescent and cannot be weighed accurately.
Source: Myschool Past Questions Archive (Record #51494)
Chemistry
WAEC / WASSCE 2014
Atomic Structure and Periodic Table
Solution
(a)(i) Explain briefly each of the following terms: I. anode; II. cathode.
(ii) Sodium and aluminium are extracted by the electrolysis of molten sodium chloride and alumina respectively. Write balanced equations for the reactions at the anode and cathode during the extraction of: I. sodium; II. aluminium.
(iii) Explain briefly why extraction of aluminium is considered environmentally friendly while that of sodium is not.
(b) Consider the reaction represented by the following equation:
K\(_2\)Cr\(_2\)O\(_7\) + HC1--> KCI + CrCl\(_3\) + H\(_2\)O + Cl\(_2\)
(i) Explain briefly why this reaction is redox.
(ii) Write balanced half equations for the reaction.
(iii) Write the over-all balanced reaction equation.
(c) During the electrolysis of molten Al\(_2\)O\(_3\), a current of 6A was passed through the electrolyte for 1 hr. 30 mins. Calculate the mass of aluminium deposited at the cathode.
medium Difficulty
Verified Correct Answer: Option
Chemical Principles & Mechanism
(a)(i) I. Anode – Electrode at which oxidation occurs / electron loss
II. Cathode – Electrode at which reduction occurs / electron gain
(ii) I. Cathodic reaction
Na\(^{+_(l)}\) + e\(^{-}\) —> N\(a_s\) .
Anodic reaction
2cl\(^{-_(l)}\) --> C\(l_2\) + 2e\(^{-}\)
II. Cathodic reaction
Al\(^{3+}_{(l)}\) + 3\(^{-}\) —> Al.
Anodic reaction
2O\(^{2-}_{l}\) ----> O\(_2\) + 4e\(^{-}\)
(iii) In the extraction of aluminium, Oxygen is produced whereas in the extraction of sodium, chlorine is produced. Chlorine is an environmental pollutant whereas oxygen is not.
(b)(i) The reaction is a redox because both oxidation and reduction occur simultaneously. The oxidation number of Cr changes from +6 (in K\(_2\)Cr\(_2\)O\(_7\)) to +3 (in CrCl\(_3\)). Cr is therefore reduced. The oxidation number of Cl changes from –1 (in HCl) to 0 (in Cl\(_2\)). CI is therefore oxidized.
OR
Removal of hydrogen from HCl is Oxidation. Removal of Oxygen from K\(_2\)Cr\(_2\)O\(_7\) is reduction.
(ii) Reduction Cr\(_2\)O\(_2\)\(^{2-}\) + 14H\(^{+}\) + 6e\(^{-}\) \(\to\) 2Cr\(^{3+}\) + 7H\(_2\)O Oxidation
2Cl\(^{-}\) \(\to\) Cl\(_2\) + 2e\(^{-}\)
(iii) Cr\(_2\)O\(_7\)\(^{2-}\) + 14H\(^{+}\) + 6Cl\(^{-}\) --> 2Cr\(^{3+}\) + 7H\(_2\)O + 3Cl\(_2\)
OR
K\(_2\)Cr\(_2\)O\(_7\) + 14HCl \(\to\) 2KCl + CrCl\(_3\) + H\(_2\)O + CI\(_2\)
(c) Q = I x T
6 x (30 +60) x 60
= 32400C
Al\(^{3+}\) + 3e\(^{-}\) \(\to\) Al\(_{(l)}\)
Source: Myschool Past Questions Archive (Record #51497)
Chemistry
WAEC / WASSCE 2014
Matter & Separation Techniques
Solution
(a)(i) Define the term functional group.
(ii) Name the functional groups present in the following compound:
(b) Consider the following structure of an organic compound, Q.
(1) Name compound Q.
(ii) Write the balanced equation for the complete combustion of compound Q.
(iii) What type of reaction will compound Q undergo with chlorine?
(vi) Draw the structure of-the alkene that is an isomer of compound Q.
(c) A chemistry student was provided with four samples of organic compounds, A, B, C, and D. Samples A and B each decolourized bromine in tetrachloromethane but only sample B reacted with a solution of ammoniacal silver trioxonitrate (V) to give a white precipitate. Sample D reacted with sodium trioxocarbonate (IV) to liberate carbon (IV) oxide. When heat was applied to the mixture of samples C and D in a test tube and drops of concentrated tetraoxosulphate (VI) acid added, a product with a fruity odour was formed.
(i) Name the family of organic compounds to which samples A, B, C, and D belong.
(ii) State why samples A and B reacted with bromine in tetrachloromethane.
(iii) State the reason why sample B gave a white precipitate with the solution of ammoniacal silver trioxocarbonate (V).
(iv) Give the name o f the reactions between' samples C and D.
(v) State the two roles of the concentrated tetraoxosulphate (VI) acid in the reaction in (a)(iv).
(d) (i) Describe briefly the production of biogas using a biogas generator.
(ii) State two uses of biogas.
medium Difficulty
Verified Correct Answer: Option
Chemical Principles & Mechanism
(a)(i) Functional group is a bond, an atom or group of atoms which give an organic compound its characteristic chemical properties.
(ii) Alkanol or hydroxyl or alkanoate.
(b)(i) Cyclopropane.
(ii) 2C\(_3\)H\(_6\) + O\(_6\) + O\(_2\) —> 6CO\(_2\) + H\(_2\)O.
(iii) Substitution reaction.
(iv)
(c)(i) Sample A – Alkene.
Sample B - Alkyne.
Sample C – Alkanol.
Sample D – Alkanoic acid.
(ii) Both samples A and B are unsaturated compounds.
(iii) Sample B is a terminal alkyne.
(iv) Esterification.
(v) As a catalyst.
(vi) To remove water.
(d)(i) The organic waste is fed into a biodigester where different kinds of bacteria convert the decaying matter into biogas CH\(_4\) / CO\(_2\) / H\(_2\) / CO through the process of anaerobic decomposition.
(ii) (I) For cooking / Fuel.
(2) For • heating / Fuel
(3) For lighting / Fuel
(3) As residue for fertilizer
(4) Generating electricity.
Source: Myschool Past Questions Archive (Record #51501)
Chemistry
WAEC / WASSCE 2014
Matter & Separation Techniques
Solution
Burette readings (initial and final) must be given to two decimal places. Volume of pipette used must also be recorded but no account of experimental procedure is required. All calculations must be done in your answer book.
A is a solution of hydrochloric acid. B is a solution containing 2.45g of anhydrous sodium trioxocarbonate (IV) in 250g of solution.
(a) Put A into the burette and titrate it against 20.0 cm\(^3\) or 25.0 cm\(^3\) portion of B using methyl orange as an indicator. Repeat the exercise to obtain consistent titres. Tabulate your burette reading and calculate the average volume of A used. The equation for the reaction involved in the titration is Na\(_2\)CO\(_{3(aq)}\) + 2HCI\(_{(aq)}\) + H\(_2\)O\(_{(l)}\)
(b) From your results and the information provided, calculate the:
(i) concentration of B in moldm\(^{-3}\)
(ii) concentration of A in moldm\(^{-3}\)
(iii) concentration of A in gdm\(^{-3}\)
(iv) volume of the gas evolved in the reaction at s.t.p.
[H = 1.00; C: 12.0; O = 16.0; Na = 23.0; Na = 23.0; Cl = 35.5; Molar Volume = 22.4 dm\(^3\)mol\(^{-3}\)]
A
carbon (IV) oxide
B
copper (ll) oxide
C
nitrogen (IV) oxide
D
sulphur (IV) oxide
medium Difficulty
Verified Correct Answer: Option C
Chemical Principles & Mechanism
(a)(i)
Burette Reading (cm\(^3\)
Rough Titration (cm\(^3\)
1st 2nd Titration (cm\(^3\)
2nd Titration
Final Burette Reading (cm\(^3\))
Initial Burette Reading (cm\(^3\))
Vol. of Acid Used (cm\(^3\))
21.00
4.00
17.00
38.50
21.00
17.50
23.30
5.70
17.60
Average Titre = \(\frac{17.50 + 17.60}{2}\)
= 17.55 cm\(^3\)
(b)(i) Na\(_2\)CO\(_{3(aq)}\) + 2HCl\(_{(aq)}\) \(\to\) 2NaCl\(_{(aq)}\) + H\(_2\)O\(_{(l)}\) + CO\(_{2(g)}\)
Molar mass (Na\(_2\)CO\(_3\)) = (2 x 23) + 12 + (3 x 15) = 106 gmol\(^{-1}\)
250cm\(^3\) of solution contained 2.45g of Na\(_2\)CO\(_3\)
1000cm\(^3\) of solution contained = \(\frac{2.45 \times 1000}{250}\)
= 2.45 x 4
= 9.80g
Con. B (Na\(_2\)CO\(_3\)) = \(\frac{9.80}{106}\)
= 0.0925 mol dm\(^{-3}\)
(ii) Conc. of A in mol dm\(^{-3}\)
\(\frac{C_AV_A}{C_BV_B} = \frac{N_A}{N_B}\)
\(\frac{C_A \times 17.55}{0.0925 \times 25}\) = \(\frac{2}{1}\)
C\(_A = \frac{0.0925 \times 25 \times 2}{17.55}\)
= 0.264 mol dm\(^{-3}\)
(iii) Conc. of A in gdm\(^{-1}\)
Molar mass HCl = 1 + 35.5 = 36.5 g mol\(^{1}\)
Conc of A(gdm\(^{-3}\)) = 0.264 x 36.5
= 9.64 gdm\(^{-1}\)
(iv) volume of the gas evolved
1 mole of Na\(_2\)CO\(_3\) = 1 mole of CO\(_2\) at s.t.p
n(CO\(_2\)) = n(NaCO\(_3\)) = 0.092.5 mole
vol. of CO\(_2\) = n x volume
= 0.0925 x 22.4 dm\(^{-3}\)
= 2.07dm\(^3\) at s.t.p
Volume of CO\(_2\) liberated in 23 cm\(^3\) of Na\(_2\)O\(_3\) will be \(\frac{2.07 \times 25}{1000}\)
= 0.0518 dm\(^3\) or 51.8 cm\(^3\) at s.t.p
Source: Official Exam / Past Question Archive
Chemistry
WAEC / WASSCE 2014
Matter & Separation Techniques
Solution
Consider the following experimental set-up.
(i) Identify by name: P; Q; R; S and T.
(ii) State the method of collection of gas, S.
(iii) What is the function of R in the experimental set-up?
(iv) Write the balanced equation of the reaction for the preparation of gas S.
medium Difficulty
Verified Correct Answer: Option
Chemical Principles & Mechanism
(i) P - Delivery tube
Q - Drying tower
R - Calcium oxide
S - Ammonia gas
T - Retort stand with clamp
(ii) Upward delivery of gas/downward displacement of air
(iii) To dry the gas
(iv) Ca(OH)\(_{2(aq)}\) + 2NH\(_4\)Cl\(_{(aq)}\) \(\to\) CaCl\(_{2(aq)}\) + 2H\(_2\)O\(_{(l)}\) + 2NH\(_{3(g)}\)
Source: Myschool Past Questions Archive (Record #59998)
Chemistry
WAEC / WASSCE 2014
Matter & Separation Techniques
Solution
Credit will be given for strict adherence to the instructions, for observations, precisely recorded, and for accurate inferences. All tests, observations, and inferences must be clearly entered in your answer book, in ink, at the time they are made.
C contains two cations and two anions. Perform the following exercises on C. Record your observations and identify any gas (es) evolved. State the conclusion you draw from the result of each test.
(a) Dissolve all of C in about 10 cm\(^3\) of distilled water. Stir the resulting solution thoroughly.
(i) To about 2 cm\(^3\) of the solution, add few drops of AgNO\(_3\) solution, followed by HNO\(_{3(aq)}\). To the mixture, add excess NH\(_{3(aq)}\)
(ii) To another 2 cm\(^3\) portion of the solution, add dil. HCl followed by BaCl\(_2\) solution.
(iii) To another 2 cm\(^3\) portion of the solution, add NaOH\(_{3(aq)}\) dropwise and then in excess. Warm the mixture.
(iv) To another 2 cm\(^3\) portion of the solution, add NH\(_{3(aq)}\) dropwise and then in excess.
medium Difficulty
Verified Correct Answer: Option
Chemical Principles & Mechanism
TEST
OBSERVATION
INFERENCE
(i)
C(\(_{aq}\)) + AgNO\(_3\)
+ HNO\(_{3(aq)}\)
+ NH\(_{3(aq)}\) excess
White precipitate formed
precipitate insoluble
Precipitate soluble/dissolved
Cl\(^-\) or S\(^{2-}\) or CO\(_3^{2-}\)
Cl\(^-\) present
(ii)
C\(_{(aq)}\) + HCl\(_{(aq)}\)
+ BaCl\(_{2(aq)}\)
No visible reaction/no gas evolved
White (chalky) precipitate formed
SO\(_4^{2-}\) present
(iii)
C\(_{(aq)}\) + NaOH\(_{(aq)}\) in drops
in excess
Mixed warmed
White gelatinous precipitate
White precipitate dissolved/soluble
Colourless gas evolved pungent/choking/irritating smell evolved
Gas turns red litmus paper blue or
forms dense white fumes with conc. HCl
Zn\(^{2+}\) or Al\(^{3+}\) suspected
Zn\(^{2+}\) or Al\(^{3+}\) suspected
NH\(_3\) gas evolved
NH\(_4^+\) present
(iv)
C\(_{(aq)}\) + NH\(_{3(aq)}\) in drops
in excess
White gelatinous precipitate
Precipitate dissolved/soluble
Zn\(^{2+}\) or Al\(^{3+}\) is suspected
Zn\(^{2+}\) present
Source: Myschool Past Questions Archive (Record #59999)
Chemistry
WAEC / WASSCE 2014
Stoichiometry & Mole Concept
Solution
Burette readings (initial and final) must be given to two decimal places. Volume of pipette used must also be recorded but no account of experimental procedure is required. All calculations must be done in your answer book.
A is a solution of hydrochloric acid. B is a solution containing 2.45g of anhydrous sodium trioxocarbonate (IV) in 250g of solution.
(a) Put A into the burette and titrate it against 20.0 cm\(^3\) or 25.0 cm\(^3\) portion of B using methyl orange as an indicator. Repeat the exercise to obtain consistent titres. Tabulate your burette reading and calculate the average volume of A used. The equation for the reaction involved in the titration is Na\(_2\)CO\(_{3(aq)}\) + 2HCI\(_{(aq)}\) + H\(_2\)O\(_{(l)}\)
(b) From your results and the information provided, calculate the:
(i) concentration of B in moldm\(^{-3}\)
(ii) concentration of A in moldm\(^{-3}\)
(iii) concentration of A in gdm\(^{-3}\)
(iv) volume of the gas evolved in the reaction at s.t.p.
[H = 1.00; C: 12.0; O = 16.0; Na = 23.0; Na = 23.0; Cl = 35.5; Molar Volume = 22.4 dm\(^3\)mol\(^{-3}\)]
medium Difficulty
Verified Correct Answer: Option
Chemical Principles & Mechanism
(a)(i)
Burette Reading (cm\(^3\)
Rough Titration (cm\(^3\)
1st 2nd Titration (cm\(^3\)
2nd Titration
Final Burette Reading (cm\(^3\))
Initial Burette Reading (cm\(^3\))
Vol. of Acid Used (cm\(^3\))
21.00
4.00
17.00
38.50
21.00
17.50
23.30
5.70
17.60
Average Titre = \(\frac{17.50 + 17.60}{2}\)
= 17.55 cm\(^3\)
(b)(i) Na\(_2\)CO\(_{3(aq)}\) + 2HCl\(_{(aq)}\) \(\to\) 2NaCl\(_{(aq)}\) + H\(_2\)O\(_{(l)}\) + CO\(_{2(g)}\)
Molar mass (Na\(_2\)CO\(_3\)) = (2 x 23) + 12 + (3 x 15) = 106 gmol\(^{-1}\)
250cm\(^3\) of solution contained 2.45g of Na\(_2\)CO\(_3\)
1000cm\(^3\) of solution contained = \(\frac{2.45 \times 1000}{250}\)
= 2.45 x 4
= 9.80g
Con. B (Na\(_2\)CO\(_3\)) = \(\frac{9.80}{106}\)
= 0.0925 mol dm\(^{-3}\)
(ii) Conc. of A in mol dm\(^{-3}\)
\(\frac{C_AV_A}{C_BV_B} = \frac{N_A}{N_B}\)
\(\frac{C_A \times 17.55}{0.0925 \times 25}\) = \(\frac{2}{1}\)
C\(_A = \frac{0.0925 \times 25 \times 2}{17.55}\)
= 0.264 mol dm\(^{-3}\)
(iii) Conc. of A in gdm\(^{-1}\)
Molar mass HCl = 1 + 35.5 = 36.5 g mol\(^{1}\)
Conc of A(gdm\(^{-3}\)) = 0.264 x 36.5
= 9.64 gdm\(^{-1}\)
(iv) volume of the gas evolved
1 mole of Na\(_2\)CO\(_3\) = 1 mole of CO\(_2\) at s.t.p
n(CO\(_2\)) = n(NaCO\(_3\)) = 0.092.5 mole
vol. of CO\(_2\) = n x volume
= 0.0925 x 22.4 dm\(^{-3}\)
= 2.07dm\(^3\) at s.t.p
Volume of CO\(_2\) liberated in 23 cm\(^3\) of Na\(_2\)O\(_3\) will be \(\frac{2.07 \times 25}{1000}\)
= 0.0518 dm\(^3\) or 51.8 cm\(^3\) at s.t.p
Source: Myschool Past Questions Archive (Record #60000)