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JAMB Chemistry 1994 past questions and answers

48 questions. Try each one, then open the answer to check yourself and read the explanation.

Question 1

Separation of Mixtures

A mixture of sand, ammonium chloride and sodium chloride is best separated by

  1. A.sublimation followed by addition of water and filtration
  2. B.sublimation followed by addition of water and evaporation
  3. C.addition of water followed by filtration and sublimation
  4. D.addition of water followed by crystallization and sublimation
Show the answer and explanation

Answer: A (sublimation followed by addition of water and filtration)

Explanation

Heating first sublimes off the ammonium chloride. Adding water then dissolves the sodium chloride, and filtration removes the insoluble sand.

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Question 2

Separation of Mixtures

A pure solid usually melts

  1. A.over a wide range of temperature
  2. B.over a narrow range of temperature
  3. C.at a lower temperature than the impure one
  4. D.at the same temperature as the impure one
Show the answer and explanation

Answer: B (over a narrow range of temperature)

Explanation

A pure solid has a sharp melting point, melting over a very narrow temperature range. Impurities lower the melting point and spread it over a wider range.

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Question 3

Gas Laws

At the same temperature and pressure, 50 cm3 of nitrogen gas contains the same number of molecules as

  1. A.25 cm3 of methane
  2. B.40 cm3 of hydrogen
  3. C.50 cm3 of ammonia
  4. D.100 cm3 of chlorine
Show the answer and explanation

Answer: C (50 cm3 of ammonia)

Explanation

Avogadro's law: equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. So 50 cm3 of any gas, such as ammonia, matches.

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Question 4

Mole Concept and Stoichiometry

8 g CH4 occupies 11.2 dm3 at s.t.p. What volume would 22 g of CH3CH2CH3 occupy under the same condition? [C = 12, H = 1]

  1. A.3.7 dm3
  2. B.11.2 dm3
  3. C.22.4 dm3
  4. D.33.6 dm3
Show the answer and explanation

Answer: B (11.2 dm3)

Explanation

Propane, C3H8, has M = 44, so 22 g is 0.5 mol, the same number of moles as 8 g of CH4. It therefore occupies the same 11.2 dm3.

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Question 5

Gas Laws

To what temperature must a gas at 273 K be heated in order to double both its volume and pressure?

  1. A.298 K
  2. B.546 K
  3. C.819 K
  4. D.1092 K
Show the answer and explanation

Answer: D (1092 K)

Explanation

PV/T is constant, so doubling both P and V makes PV four times bigger. T must also be four times bigger: 4 × 273 = 1092 K.

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Question 6

Gas Laws

For a gas, the relative molecular mass is equal to 2Y. What is Y?

  1. A.The mass of the gas
  2. B.The vapour density of the gas
  3. C.The volume of the gas
  4. D.The temperature of the gas
Show the answer and explanation

Answer: B (The vapour density of the gas)

Explanation

Relative molecular mass = 2 × vapour density, because vapour density compares the gas with hydrogen (M = 2).

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Question 7

Gas Laws

The densities of two gases, X and Y, are 0.5 g dm-3 and 2.0 g dm-3 respectively. What is the rate of diffusion of X relative to Y?

  1. A.0.1
  2. B.0.5
  3. C.2.0
  4. D.4.0
Show the answer and explanation

Answer: C (2.0)

Explanation

Graham's law: rate is inversely proportional to √density. Rate X/rate Y = √(2.0/0.5) = √4 = 2.0.

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Question 8

Gas Laws

An increase in temperature causes an increase in the pressure of a gas because

  1. A.it decreases the number of collision between the molecules
  2. B.the molecules of the gas bombard the walls of the container more frequently
  3. C.it increases the number of collisions between the molecules
  4. D.it causes the molecules to combine
Show the answer and explanation

Answer: B (the molecules of the gas bombard the walls of the container more frequently)

Explanation

Hotter molecules move faster, so they hit the container walls more often and harder. Pressure comes from these collisions with the walls.

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Question 9

Chemical Bonding

The shape of ammonia molecule is

  1. A.trigonal planar
  2. B.octahedral
  3. C.square planar
  4. D.tetrahedral
Show the answer and explanation

Answer: D (tetrahedral)

Explanation

Nitrogen in NH3 has four electron pairs (three bonding, one lone pair) arranged tetrahedrally, which makes the molecule a pyramid. Of the options given, tetrahedral is the only one that fits.

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Question 10

Atomic Structure

The number of electrons in the valence shell of an element of atomic number 14 is

  1. A.1
  2. B.2
  3. C.3
  4. D.4
Show the answer and explanation

Answer: D (4)

Explanation

Atomic number 14 (silicon) has the arrangement 2, 8, 4, so its outer shell holds 4 electrons.

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Question 11

Periodic Table

Which of the following physical properties decreases down a group in the periodic table?

  1. A.Atomic radius
  2. B.Ionic radius
  3. C.Electropositivity
  4. D.Electronegativity
Show the answer and explanation

Answer: D (Electronegativity)

Explanation

Going down a group, atoms get bigger and the outer electrons are further from the nucleus, so the atom attracts bonding electrons less: electronegativity decreases. The other three increase.

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Question 12

Atomic Structure

The diagram above represents an atom of

Diagram for this question
  1. A.magnesium
  2. B.helium
  3. C.chlorine
  4. D.neon
Show the answer and explanation

Answer: A (magnesium)

Explanation

The diagram shows 12 electrons arranged 2, 8, 2 in three shells. An atom with 12 electrons is magnesium.

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Question 13

Chemical Bonding

Elements X, Y and Z belong to groups I, V and VII respectively. Which of the following is TRUE about the bond types of XZ and YZ3?

  1. A.Both are electrovalent
  2. B.Both are covalent
  3. C.XZ is electrovalent and YZ3 is covalent
  4. D.XZ is covalent and YZ3 is electrovalent
Show the answer and explanation

Answer: C (XZ is electrovalent and YZ3 is covalent)

Explanation

A group I metal and a group VII non-metal transfer an electron to form an ionic (electrovalent) bond, as in NaCl. Two non-metals from groups V and VII share electrons, as in PCl3, giving covalent bonds.

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Question 14

Atomic Structure

Which of the following atoms represents deuterium?
(No. of protons, No. of neutrons, No. of electrons)

  1. A.1, 0, 0
  2. B.1, 0, 1
  3. C.1, 1, 1
  4. D.1, 2, 1
Show the answer and explanation

Answer: C (1, 1, 1)

Explanation

Deuterium is the hydrogen isotope with mass number 2. It has 1 proton, 1 neutron and, as a neutral atom, 1 electron.

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Question 15

Air

The set-up above would be useful for determining the amount of

Diagram for this question
  1. A.oxygen in air
  2. B.water vapour in air
  3. C.CO2 in air
  4. D.argon in air
Show the answer and explanation

Answer: B (water vapour in air)

Explanation

Anhydrous calcium chloride absorbs water. Weighing the U-tube before and after passing a known volume of air through it gives the mass of water vapour in the air.

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Question 16

Water

A solid that absorbs water from the atmosphere and forms an aqueous solution is

  1. A.hydrophilic
  2. B.efflorescent
  3. C.deliquescent
  4. D.hygroscopic
Show the answer and explanation

Answer: C (deliquescent)

Explanation

A deliquescent solid (such as CaCl2 or NaOH) absorbs so much water from the air that it dissolves in it. A hygroscopic substance absorbs water but does not form a solution.

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Question 17

Environmental Pollution

A major effect of oil pollution in coastal waters is the

  1. A.destruction of marine life
  2. B.desalination of the water
  3. C.increase in the acidity of the water
  4. D.detoxification of the water
Show the answer and explanation

Answer: A (destruction of marine life)

Explanation

An oil film stops oxygen dissolving in the water and coats fish, birds and plants, killing marine life.

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Question 18

Solubility and Solutions

Sodium chloride has no solubility product value because of its

  1. A.saline nature
  2. B.high solubility
  3. C.low solubility
  4. D.insolubility
Show the answer and explanation

Answer: B (high solubility)

Explanation

Solubility products are only used for sparingly soluble salts. Sodium chloride is very soluble, so it is not given a Ksp value.

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Question 19

Solubility and Solutions

The solubility, in moles per dm3 of 20.2 g of potassium trioxonitrate (V) dissolved in 100 g of water at room temperature is [K = 39, O = 16, N = 14]

  1. A.0.10
  2. B.0.20
  3. C.1.00
  4. D.2.00
Show the answer and explanation

Answer: D (2.00)

Explanation

KNO3 has M = 101, so 20.2 g is 0.2 mol in 100 g (about 0.1 dm3) of water. Per dm3 that is 0.2 × 10 = 2.00 mol.

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Question 20

Acids, Bases and Salts

A few drops of concentrated HCl are added to about 10 cm3 of a solution of pH 3.4. The pH of the resulting mixture is

  1. A.less than 3.4
  2. B.greater than 3.4
  3. C.unaltered
  4. D.the same as that of pure water
Show the answer and explanation

Answer: A (less than 3.4)

Explanation

Concentrated HCl adds more H+ ions, making the solution more acidic. More acidic means a lower pH.

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Question 21

Acids, Bases and Salts

Which of the following compounds is a base?

  1. A.CO2
  2. B.CaO
  3. C.H3PO3
  4. D.CH3COOCH3
Show the answer and explanation

Answer: B (CaO)

Explanation

Calcium oxide is a metal oxide that reacts with acids to form a salt and water, so it is a base. CO2 is acidic, H3PO3 is an acid and CH3COOCH3 is an ester.

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Question 22

Mole Concept and Stoichiometry

20 cm3 of a 2.0 M solution of ethanoic acid was added to excess of 0.05 M sodium hydroxide. The mass of the salt produced is [Na = 23, C = 12, O = 16, H = 1]

  1. A.2.50 g
  2. B.2.73 g
  3. C.3.28 g
  4. D.4.54 g
Show the answer and explanation

Answer: C (3.28 g)

Explanation

Moles of acid = 0.020 × 2.0 = 0.04 mol, giving 0.04 mol of CH3COONa (M = 82). Mass = 0.04 × 82 = 3.28 g.

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Question 23

Electrolysis

What volume of oxygen measured at s.t.p would be liberated on electrolysis by 9650 coulombs of electricity? [Molar volume of gas = 22.4 dm3, F = 96,500 C mol-1]

  1. A.22.4 dm3
  2. B.11.2 dm3
  3. C.1.12 dm3
  4. D.0.560 dm3
Show the answer and explanation

Answer: D (0.560 dm3)

Explanation

9650 C is 0.1 F. Making one mole of O2 needs 4 F (4OH− → O2 + 2H2O + 4e−), so 0.1 F gives 0.025 mol = 0.025 × 22.4 = 0.560 dm3.

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Question 24

Electrolysis

Crude copper could be purified by the electrolysis of concentrated copper (II) chloride if the crude copper is

  1. A.made both the anode and the cathode
  2. B.made the cathode
  3. C.made the anode
  4. D.dissolved in the solution
Show the answer and explanation

Answer: C (made the anode)

Explanation

The impure copper anode dissolves into the solution as Cu2+, and pure copper is deposited on the cathode. The impurities fall below the anode.

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Question 25

Redox Reactions

H−(s) + H2O(l) → H2(g) + OH−(aq). From the equation above, it can be inferred that the

  1. A.reaction is a double decomposition
  2. B.hydride ion is a reducing agent
  3. C.hydride ion is an oxidizing agent
  4. D.reaction is neutralization
Show the answer and explanation

Answer: B (hydride ion is a reducing agent)

Explanation

Hydrogen in H− goes from −1 to 0 in H2, so the hydride ion loses electrons (is oxidized). A substance that is oxidized acts as the reducing agent.

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Question 26

Energy Changes

The ΔH for the reaction represented by the energy profile above is

Diagram for this question
  1. A.-100 kJ mol-1
  2. B.+100 kJ mol-1
  3. C.+50 kJ mol-1
  4. D.-50 kJ mol-1
Show the answer and explanation

Answer: D (-50 kJ mol-1)

Explanation

ΔH = energy of products − energy of reactants. The curve starts at 100 kJ and ends at 50 kJ, so ΔH = 50 − 100 = −50 kJ mol-1 (exothermic).

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Question 27

Acids, Bases and Salts

An anhydride is an oxide of a non-metal

  1. A.which will not dissolve in water
  2. B.whose solution in water has a pH greater than 7
  3. C.whose solution in water has a pH less than 7
  4. D.whose solution in water has a pH of 7
Show the answer and explanation

Answer: C (whose solution in water has a pH less than 7)

Explanation

An acid anhydride, like SO2 or CO2, dissolves in water to form an acid, so its solution has a pH below 7.

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Question 28

Redox Reactions

MnO4−(aq) + 8H+(aq) + 5Fe2+(aq) → Mn2+(aq) + 5Fe3+(aq) + 4H2O(l). The oxidation number of manganese in the above reaction changed from

  1. A.+7 to +2
  2. B.+6 to +2
  3. C.+5 to +2
  4. D.+4 to +2
Show the answer and explanation

Answer: A (+7 to +2)

Explanation

In MnO4−: Mn + 4(−2) = −1, so Mn = +7. It ends as Mn2+, so the change is +7 to +2.

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Question 30

Chemical Equilibrium

Which of the following is TRUE of Le Chatelier's Principle for an exothermic reaction?

  1. A.Increase in temperature will cause an increase in the equilibrium constant
  2. B.Increase in temperature will cause a decrease in the equilibrium constant
  3. C.Addition of a catalyst will cause an increase in the equilibrium constant
  4. D.Addition of a catalyst will cause a decrease in the equilibrium constant
Show the answer and explanation

Answer: B (Increase in temperature will cause a decrease in the equilibrium constant)

Explanation

For an exothermic reaction, heating favours the backward (endothermic) reaction, so less product forms and K gets smaller. A catalyst speeds up both directions equally and leaves K unchanged.

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Question 31

Nitrogen and its Compounds

Which of the following are produced when ammonium trioxonitrate (V) crystals are cautiously heated in a hard glass roundbottomed flask?

  1. A.N2O and steam
  2. B.NO2 and ammonia
  3. C.N2O4 and NO2
  4. D.NO and NO2
Show the answer and explanation

Answer: A (N2O and steam)

Explanation

Gentle heating of ammonium nitrate gives dinitrogen oxide and water: NH4NO3 → N2O + 2H2O.

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Question 32

Rates of Reaction

2HCl(aq) + CaCO3(s) → CaCl2(aq) + H2O(l) + CO2(g). From the reaction above, which of the following curves represents the consumption of calcium trioxocarbonate (IV) as dilute HCl is added to it?

Diagram for this question
  1. A.L
  2. B.M
  3. C.N
  4. D.P
Show the answer and explanation

Answer: D (P)

Explanation

As acid is added, the amount of CaCO3 left falls, quickly at first and then more slowly. Curve P is the only one that falls with time.

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Question 33

Hydrogen, Oxygen and Oxides

In the diagram above, R is a mixture of

Diagram for this question
  1. A.potassium tetraoxochlorate (VII) and concentrated H2SO4
  2. B.potassium trioxochlorate (V) and concentrated H2SO4
  3. C.potassium tetraoxomanganate (VII) and concentrated HCl
  4. D.manganese (IV) oxide and concentrated HCl
Show the answer and explanation

Answer: D (manganese (IV) oxide and concentrated HCl)

Explanation

The set-up collects dry chlorine. In the laboratory, chlorine is made by heating manganese(IV) oxide with concentrated hydrochloric acid, which oxidizes the HCl to Cl2.

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Question 34

Metals and their Compounds

Which of these metals CANNOT replace hydrogen from alkaline solutions?

  1. A.Aluminium
  2. B.Zinc
  3. C.Tin
  4. D.Iron
Show the answer and explanation

Answer: D (Iron)

Explanation

Aluminium, zinc and tin are amphoteric metals that react with alkalis to give hydrogen. Iron does not react with alkalis.

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Question 35

Halogens

Clothes should be properly rinsed with water after bleaching because

  1. A.the bleach decolourizes the clothes
  2. B.chlorine reacts with fabrics during bleaching
  3. C.the clothes are sterilized during bleaching
  4. D.hydrogen chloride solution is produced during bleaching
Show the answer and explanation

Answer: D (hydrogen chloride solution is produced during bleaching)

Explanation

Chlorine bleaches by forming HOCl, which releases oxygen and leaves hydrochloric acid behind. The acid would weaken the fabric if it were not rinsed out.

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Question 36

Qualitative Analysis

Which of these solutions will give a white precipitate with a solution of barium chloride acidified with hydrochloric acid?

  1. A.Sodium trioxocarbonate (IV)
  2. B.Sodium tetraoxosulphate (VI)
  3. C.Sodium trioxosulphate (IV)
  4. D.Sodium sulphide
Show the answer and explanation

Answer: B (Sodium tetraoxosulphate (VI))

Explanation

Sulphate ions form white BaSO4, which does not dissolve in acid. The acid stops carbonates and sulphites from giving a precipitate.

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Question 37

Sulphur and its Compounds

SO3 is NOT directly dissolved in water in the preparation of H2SO4 by the contact process because

  1. A.the reaction between SO3 and water is violently exothermic
  2. B.acid is usually added to water and never water to acid
  3. C.SO3 does not dissolve in water readily
  4. D.SO3 is an acid gas
Show the answer and explanation

Answer: A (the reaction between SO3 and water is violently exothermic)

Explanation

SO3 reacts with water so violently that it forms a fine acid mist that is hard to condense. It is dissolved in concentrated H2SO4 instead to form oleum.

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Question 38

Electrolysis

In an electrolytic set-up to protect iron from corrosion, the iron is

  1. A.made the cathode
  2. B.made the anode
  3. C.used with a metal of lower electropositive potential
  4. D.initially coated with tin
Show the answer and explanation

Answer: A (made the cathode)

Explanation

Corrosion is oxidation, which happens at the anode. Making the iron the cathode supplies it with electrons so it cannot be oxidized.

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Question 39

Metals and their Compounds

Which of the following is NOT true of metals?

  1. A.They are good conductors of electricity
  2. B.They ionize by electron loss
  3. C.Their oxides are acidic
  4. D.They have high melting points
Show the answer and explanation

Answer: C (Their oxides are acidic)

Explanation

Metal oxides are generally basic (some are amphoteric). Acidic oxides are typical of non-metals.

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Question 40

Metals and their Compounds

Which of the following is the correct order of decreasing activity of the metals Fe, Ca, Al and Na?

  1. A.Fe > Ca > Al > Na
  2. B.Na > Ca > Al > Fe
  3. C.Al > Fe > Na > Ca
  4. D.Ca > Na > Fe > Al
Show the answer and explanation

Answer: B (Na > Ca > Al > Fe)

Explanation

In the activity series the order is K, Na, Ca, Mg, Al, Zn, Fe and so on. These four therefore run Na > Ca > Al > Fe.

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Question 41

Introduction to Organic Chemistry

Which of the following compounds is NOT an isomer of 2,2 dimethylbutane?

  1. A.2-methylbutane
  2. B.3-methylpentane
  3. C.2,3-dimethylbutane
  4. D.2-methylpentane
Show the answer and explanation

Answer: A (2-methylbutane)

Explanation

2,2-dimethylbutane is C6H14. 2-methylbutane has only 5 carbons (C5H12), so it cannot be an isomer; the other three are all C6H14.

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Question 42

Introduction to Organic Chemistry

CH3—C(CH3)2—CH=CH2
The IUPAC name of the compound above is

  1. A.2,2-dimethyl but-1-yne
  2. B.2,2-dimethyl but-1-ene
  3. C.3,3-dimethyl but-1-ene
  4. D.3,3-dimethyl but-1-yne
Show the answer and explanation

Answer: C (3,3-dimethyl but-1-ene)

Explanation

The chain has 4 carbons with a C=C double bond, numbered from the double-bond end so it starts at carbon 1 (but-1-ene). Both methyl groups are then on carbon 3.

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Question 43

Alkanols

When sodium is added to ethanol, the products are

  1. A.sodium hydroxide and water
  2. B.sodium hydroxide and hydrogen
  3. C.sodium ethoxide and water
  4. D.sodium ethoxide and hydrogen
Show the answer and explanation

Answer: D (sodium ethoxide and hydrogen)

Explanation

2C2H5OH + 2Na → 2C2H5ONa + H2. Sodium replaces the hydrogen of the –OH group.

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Question 44

Alkanoic Acids and Esters

The general formula of alkanoates is

  1. A.RCHO
  2. B.R2CO
  3. C.RCOOH
  4. D.RCOOR
Show the answer and explanation

Answer: D (RCOOR)

Explanation

Alkanoates (esters) have the general formula RCOOR'. RCHO is an alkanal, R2CO an alkanone and RCOOH an alkanoic acid.

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Question 45

Alkanoic Acids and Esters

When sodium ethanoate is treated with a few drops of concentrated tetraoxosulphate (VI) acid one of the products is

  1. A.CH3COOH
  2. B.CH3COOCH3
  3. C.CH3COOC2H5
  4. D.C2H5COOCH3
Show the answer and explanation

Answer: A (CH3COOH)

Explanation

A strong acid displaces the weaker ethanoic acid from its salt: CH3COONa + H2SO4 → CH3COOH + NaHSO4.

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Question 46

Hydrocarbons

One mole of a hydrocarbon contains 48 g of carbon. If its vapour density is 28, the hydrocarbon is [C = 12, H = 1]

  1. A.an alkane
  2. B.an alkene
  3. C.an alkyne
  4. D.aromatic
Show the answer and explanation

Answer: B (an alkene)

Explanation

M = 2 × 28 = 56. 48 g of carbon is 4 C atoms, leaving 8 g = 8 H atoms, so the formula is C4H8. This fits CnH2n, an alkene.

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Question 47

Hydrocarbons

Use the diagram above. The reaction taking place in flask G is known as

Diagram for this question
  1. A.hydrolysis
  2. B.double decomposition
  3. C.dehydration
  4. D.pyrolysis
Show the answer and explanation

Answer: C (dehydration)

Explanation

Hot concentrated sulphuric acid removes a water molecule from ethanol to make ethene: C2H5OH → C2H4 + H2O. Removing water is dehydration.

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Question 49

Atomic Structure

Which of the following orbitals of carbon are mixed with hydrogen in methane?

  1. A.1s and 2p
  2. B.1s and 2s
  3. C.2s and 2p
  4. D.2p and 3p
Show the answer and explanation

Answer: C (2s and 2p)

Explanation

Carbon's one 2s and three 2p orbitals hybridize to give four sp3 orbitals, which bond with the hydrogen 1s orbitals.

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Question 50

Hydrocarbons

Which of the following reagents will confirm the presence of unsaturation in a compound?

  1. A.Fehling's solution
  2. B.Bromine water
  3. C.Tollen's reagent
  4. D.Benedict's solution
Show the answer and explanation

Answer: B (Bromine water)

Explanation

Bromine water adds across C=C or C≡C bonds and loses its red-brown colour quickly. The other three reagents test for aldehydes or reducing sugars.

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