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Chemistry
WAEC / WASSCE 2020
Matter & Separation Techniques
Solution
C is an organic compound. Carry out the following exercises on C. Record your observations and identity any gas(es) evolved. State the conclusions you draw from the results of each test.
(a) Put about 10 drops of C on a watch glass and ignite it using a burning splint.
(b)(i) Put about 1 cm\(^3\) of C in a test tube and add about 1 cm\(^3\) of distilled water. Shake the test tube.
(ii) Put about 1 cm\(^3\) of C in a test tube and add about 2 cm\(^3\) of acidified K\(_2\)Cr\(_2\)O\(_7\) solution. Warm the mixture gently and leave to stand for 5 minutes.
(c) Put few crystals of specimen D in a test tube and add about 2cm\(^3\) of C followed by about 2 cm\(^3\) of 10 % NaOH\(_{(aq)}\) Shake the test tube vigorously.
(d) State the class of compounds to which C belongs.
medium Difficulty
Verified Correct Answer: Option
Chemical Principles & Mechanism
Test
Observation
inference
(a)
C on a water glass + burning splint
Burns with a non-Soothy/blue flame
Saturated organic compound may be present
(b)(i)
C + distilled water + shaking
One layer formed/miscible/viscous mixture/ C dissolved
C is soluble
(ii)
C + acidified K\(_2\)Cr,\(_2\)O\(_{7(aq)}\) warm and leave to stand for 5 minutes
Orange solution turns green
C is oxidized/reducing agent K\(_2\)Cr,\(_2\)O\(_{7(aq)}\) is reduced/Oxidizing agent or Primary or secondary alkanol present
(c)
D\(_{(s)}\) + C + NaOH\(_{(aq)}\) shaking vigorously Yellow
Yellow precipitate forms with antiseptic smell
lod foam produced
(d) Alkanols
Source: Myschool Past Questions Archive (Record #59937)
Chemistry
WAEC / WASSCE 2020
Acids, Bases, and Salts
Solution
(a) A zinc salt, E when heated strongly, produced a brown gas with pungent smell, a colourless gas that rekindled a glowing splint, and a residue that was allowed to cool.
(i) identify the salt E.
(ii) Write an equation for the decomposition of E.
(iii) State what would be observed when the residue was allowed to cool.
(b) Describe how 250cm\(^3\) of 0.2 mol dm\(^3\) H\(_2\)SO\(_4\) could be prepared from 150 cm\(^3\) of a 1.0 mol dm\(^{3}\) stock solution of the acid.
(c) State the effect of aqueous solution of Al\(_2(SO_4)_3\) on litmus paper.
medium Difficulty
Verified Correct Answer: Option
Chemical Principles & Mechanism
(a)(i) Zn(NO\(_3\))\(_2\)
ii) 2Zn(NO\(_3\))2\(_{(s)}\) \(\to\) 2ZnO\(_{(s)}\) + 4NO\(_{2(g)}\) + \(O_2\)
iii) The residue was yellow when lost and turns white when cold.
(b) Using the law of dilution
C\(_1\)V\(_1\) = C\(_2\)V\(_2\)
V\(_1\) = \(\frac{C_2V_2}{C_1} = \frac{0.2 \times 250}{1.0}\)
= 50cm\(^3\)
Procedure: 50cm\(^3\) of the stock solution is pipetted from a burette into a 250cm\(^3\) volumetric flask. The flask is topped up to the mark with distilled water. Flask is stoppered and shaken to obtain a uniform solution.
(c) Turns blue litmus paper red.
Source: Myschool Past Questions Archive (Record #59938)
Chemistry
UTME / JAMB 2020
Matter & Separation Techniques
Solution
To obtain pure carbon(ii)oxide from its mixture with carbon(iv)oxide, the mixture should be?
A
Passed over heated copper(ii)oxide
B
Bubbled through water
C
Bubbled through conc H\(_2\)SO\(_4\)
D
Bubbled through conc NAOH
medium Difficulty
Verified Correct Answer: Option D
Chemical Principles & Mechanism
Carbon(ii)oxide is prepared from carbon(iv)oxide by passing the latter through red-hot carbon and the unchanged CO\(_2\) is removed when the mixture of the gases through conc NaOH.
Source: Official Exam / Past Question Archive
Chemistry
WAEC / WASSCE 2020
Matter & Separation Techniques
Solution
(a) A zinc salt, E when heated strongly, produced a brown gas with pungent smell, a colourless gas that rekindled a glowing splint, and a residue that was allowed to cool.
(i) identify the salt E.
(ii) Write an equation for the decomposition of E.
(iii) State what would be observed when the residue was allowed to cool.
(b) Describe how 250cm\(^3\) of 0.2 mol dm\(^3\) H\(_2\)SO\(_4\) could be prepared from 150 cm\(^3\) of a 1.0 mol dm\(^{3}\) stock solution of the acid.
(c) State the effect of aqueous solution of Al\(_2(SO_4)_3\) on litmus paper.
A
plastic
B
sewage
C
carbon (II) oxide
D
hydrogen sulphide
medium Difficulty
Verified Correct Answer: Option B
Chemical Principles & Mechanism
(a)(i) Zn(NO\(_3\))\(_2\)
ii) 2Zn(NO\(_3\))2\(_{(s)}\) \(\to\) 2ZnO\(_{(s)}\) + 4NO\(_{2(g)}\) + \(O_2\)
iii) The residue was yellow when lost and turns white when cold.
(b) Using the law of dilution
C\(_1\)V\(_1\) = C\(_2\)V\(_2\)
V\(_1\) = \(\frac{C_2V_2}{C_1} = \frac{0.2 \times 250}{1.0}\)
= 50cm\(^3\)
Procedure: 50cm\(^3\) of the stock solution is pipetted from a burette into a 250cm\(^3\) volumetric flask. The flask is topped up to the mark with distilled water. Flask is stoppered and shaken to obtain a uniform solution.
(c) Turns blue litmus paper red.
Source: Official Exam / Past Question Archive
Chemistry
WAEC / WASSCE 2019
Basic Chemistry
Solution
consider the following reaction equation: 2Al(OH)\(_3\) + 3H\(_2\)SO\(_4\) →Al\(_2\)(SO4)\(_3\)(aq)+6H\(_2\)O. How many moles of tetraoxosulphate(vi)acid is needed to produce 0.5mol of aluminium tetraoxosulphate(vi)?
A
0.5
B
1.0
C
1.5
D
3.0
medium Difficulty
Verified Correct Answer: Option C
Chemical Principles & Mechanism
Where 3mols of H2So4 = 1 mol of (Al2SO4)3
y mols of H2SO4 = 0.5 mol of (Al2SO4)3
Then cross multiply to have:
y = \(\frac{3\times0.5}{1}\)
y = 1.5 moles
Source: WAEC / JAMB / NECO Classroom Archive (Record #58126)
Chemistry
UTME / JAMB 2019
Gas Laws & Kinetic Theory
Solution
The velocity, V of a gas is related to its mass, M by (k = proportionality constant)
A
V = \(\frac{k}{M}\)
B
V = \(\frac{k}{M^{\frac{1}{2}}}\)
C
V = \(kM^2\)
D
V = \((\frac{k}{M})^{\frac{1}{2}}\)
medium Difficulty
Verified Correct Answer: Option B
Chemical Principles & Mechanism
Recall:
V = \(\sqrt{\frac{3RT}{M}}\)
\(\therefore V \propto \frac{1}{\sqrt{M}}\)
\(V = \frac{k}{\sqrt{M}}\)
V = \(\frac{k}{M^{\frac{1}{2}}}\)
Source: WAEC / JAMB / NECO Classroom Archive (Record #56012)
Chemistry
WAEC / WASSCE 2019
Periodic Table & Periodicity
Solution
which of the following elements has the highest ionization energy?
A
Calcium
B
Chlorine
C
Flourine
D
Magnesium
medium Difficulty
Verified Correct Answer: Option C
Chemical Principles & Mechanism
The ionisation energy is the minimum energy required to remove the most loosely bound electron of an isolated gaseous atom, positive ion, or molecule.
Fluorine has the highest ionization energy when assessed through the trend on the periodic table.
Source: WAEC / JAMB / NECO Classroom Archive (Record #57924)
Chemistry
WAEC / WASSCE 2019
Water and Solubility Concept
Solution
The properties of a good primary standard solution include the following except?
A
High molecular mass
B
low molecular mass
C
high degree of purity
D
readily available
medium Difficulty
Verified Correct Answer: Option B
Chemical Principles & Mechanism
Properties of a good standard solution
High purity.
Stability (low reactivity)
Low hygroscopicity (to minimize weight changes due to humidity)
High equivalent weight (to minimize weighing errors)
Non-toxicity.
Ready and cheap availability.
High molecular mass
Source: WAEC / JAMB / NECO Classroom Archive (Record #58043)
Chemistry
WAEC / WASSCE 2019
Electrochemistry & Electrolysis
Solution
If the oxidation potential of Mg and Al are +2.37 volts and + 1.66 volts respectively, then the Mg would?
A
not replace Al
B
be replaced by Al
C
remain in the solution
D
replace Al
medium Difficulty
Verified Correct Answer: Option D
Chemical Principles & Mechanism
Cation which is lower in the series (less electropositive) will show a greater tendency to be discharged.
Source: WAEC / JAMB / NECO Classroom Archive (Record #58177)
Chemistry
UTME / JAMB 2019
Organic Chemistry
Solution
2-methylprop-1-ene is an isomer of?
A
3-methyl but-1-ene
B
2-methyl but-1-ene
C
but-2-ene
D
pent-2-ene
medium Difficulty
Verified Correct Answer: Option C
Chemical Principles & Mechanism
Isomers of Butene: are But-1-ene, But-2-ene, 2-Methylpropene / 2-methylprop-1-ene
Source: WAEC / JAMB / NECO Classroom Archive (Record #55522)
Chemistry
UTME / JAMB 2019
Hydrocarbons
Solution
A certain hydrocarbon on complete combustion at s.t.p produced 89.6dm\(^3\) of CO\(_2\) and 54g of water. The hydrocarbon should be?
A
C\(_6\)H\(_6\)
B
C\(_4\)H\(_{10}\)
C
C\(_5\)H\(_{10}\)
D
C\(_4\)H\(_6\)
medium Difficulty
Verified Correct Answer: Option D
Chemical Principles & Mechanism
C\(_x\)H\(_y\) + (x + y/4)O\(_2\) \(\to\) x CO\(_2\) + y/2 H\(_2\)O
89.6 dm\(^3\) \(\to\) x moles CO\(_2\)
22.4 dm\(^3\) \(\to\) 1 mole CO\(_2\)
x = \(\frac{89.6}{22.4}\)
= 4 moles CO\(_2\)
54g of H\(_2\)O \(\to\) k moles H\(_2\)O
18g H\(_2\)O \(\to\) 1 mole H\(_2\)O
k = \(\frac{54}{18}\)
= 3 moles H\(_2\)O
From the equation above,
x = 4
k = y/2 = 3
y/2 = 3 \(\implies\) y = 6
\(\therefore\) The hydrocarbon C\(_x\)H\(_y\) = C\(_4\)H\(_6\)
Source: WAEC / JAMB / NECO Classroom Archive (Record #55521)
Chemistry
UTME / JAMB 2019
Organic Chemistry
Solution
A secondary alkanol can be oxidized to give an?
A
alkanone
B
alkyne
C
alkanoic acid
D
alkanal
medium Difficulty
Verified Correct Answer: Option A
Chemical Principles & Mechanism
secondary alkanols (alcohols) can be oxidised to alkanones (ketones)
Source: Myschool Past Questions Archive (Record #55517)
Chemistry
UTME / JAMB 2019
Atomic Structure and Periodic Table
Solution
Which of the following will give a precipitate with an aqueous solution of copper (I) chloride?
A
But-2-ene
B
But-1-ene
C
But-2-yne
D
But-1-yne
medium Difficulty
Verified Correct Answer: Option D
Chemical Principles & Mechanism
A hydrocarbon that will produce a red precipitate when reacted with an aqueous solution of copper(I) chloride is an alkyne. More specifically, an alkyne with a terminal triple bond, like ethyne (acetylene), will react to form a red precipitate of copper(I) acetylide. The most reactive alkynes are those where the triple bond is at the end of the carbon chain (terminal triple bond).
Source: Myschool Past Questions Archive (Record #55519)
Chemistry
UTME / JAMB 2019
Acids, Bases, and Salts
Solution
The IUPAC name for CH\(_3\)CH\(_2\)COOCH\(_2\)CH\(_3\) is
A
methylpropanoate
B
ethylethanoate
C
ethylpropanoate
D
methylethanoate
medium Difficulty
Verified Correct Answer: Option C
Chemical Principles & Mechanism
While naming the IUPAC nomenclature of hydrocarbon compounds, we usually start with the naming from the rare i.e the functional group from the right hand side.
Consider CH\(_3\)CH\(_2\)COOCH\(_2\)CH\(_3\), this is an ester obtained from two compounds propanoic acid and ethanol.
On splitting CH\(_3\)CH\(_2\)COOCH\(_2\)CH\(_3\), we have CH\(_3\)CH\(_2\)COO / CH\(_2\)CH\(_3\) dividing the compound into two; CH\(_3\)CH\(_2\)COO - Propanoate and CH\(_2\)CH\(_3\) - ethyl
So while naming as mentioned earlier on, we start the naming from the rare(RHS), and then the other(LHS). Thus, the name of the compound is; ethylpropanoate.
Source: Myschool Past Questions Archive (Record #55520)
Chemistry
UTME / JAMB 2019
Acids, Bases, and Salts
Solution
Hydrocarbons which will react with Tollen's reagent conform to the general formula
A
C\(_n\)H\(_{2n}\)
B
C\(_n\)H\(_{2n + 2}\)
C
C\(_n\)H\(_{2n - 2}\)
D
C\(_n\)H\(_{2n + 1}\)OH
medium Difficulty
Verified Correct Answer: Option C
Chemical Principles & Mechanism
Only hydrocarbons containing an aldehyde group (RCHO) or a terminal alkyne (R-C≡C-H) will react with Tollens' reagent, leading to a silver mirror formation or a white precipitate, respectively. Aldehydes are oxidized by Tollens' reagent (ammoniacal silver nitrate solution) to form carboxylic acids, while the silver ions (Ag+) are reduced to metallic silver (Ag), which deposits as a silver mirror on the walls of the test tube.
Terminal alkynes react with Tollens' reagent, producing a white precipitate of silver acetylide.
Source: Myschool Past Questions Archive (Record #55523)