JAMB Chemistry Previous Year Questions: 2020 | Chemistry for JAMB PDF Download

Q1: The electronic configuration of an element is 1S² 2S² 2P⁶ 3S² 3P³. How many unpaired electrons are there in the element?
(a) 5
(b) 4
(c) 3
(d) 2
Ans: (c)

• Once you figure out the electron configuration, you fill up the corresponding orbitals with electrons, any left with one is considered unpaired. Since 1s can only hold 2 electrons, and P has 15, that's obviously filled and has no unpaired electrons. The same is for 2s which holds 2, 2p which holds 6, 3s which holds 2.
• However 3p can hold 6 electrons and in order for that to be filled up you would need to have an element of 18 electrons. So you fill up as much as you can in 3p by first adding 1 electron to each energy level. 3p has 3 energy levels and there are only 3 electrons left to distribute, so each of those energy levels only gets 1, because you have to fill them all with one before you can start adding a second.
• So since you are only able to fill one electron in each of the three energy levels of the 3p orbital, that leaves the orbital open for 1 more electron in each of its energy levels. So there are 3 unpaired electrons in P.

Q2: Which of the following can be obtained by fractional distillation?
(a) Nitrogen from liquid air
(b) Sodium Chloride from sea water
(c) Iodine from solution of Iodine in carbon tetrachloride
(d) Sulphur from the solution of sulphur in carbon disulphide
Ans: (a)
The Fractional Distillation of Liquid Air

• An air separation unit is often called an oxygen or nitrogen generator since its purpose is to extract one or both of these elements from the air. In the distillation process, the air is first passed through a filter that absorbs all the water vapour.
• An air separation plant separates atmospheric air into its primary components, typically nitrogen and oxygen, and sometimes also argon and other rare inert gases. The most common method for air separation is fractional distillation.

Q3: Duralumin consists of aluminum, copper?
(a) Zinc and Gold
(b) Lead and Manganese
(c) Nickel and Silver
(d) Manganese and Magnesium
Ans: (d)
Duralumin is an alloy made up of 90% aluminium, 4% copper, 0.51% magnesium, and less than 1% manganese.

Q4: An example of a polysaccharide is?
(a) Dextrose
(b) Mannose
(c) Glucose
(d) Starch
Ans: (d)
A polysaccharide is a large molecule made of many smaller monosaccharides. Monosaccharides are simple sugars, like glucose. Special enzymes bind these small monomers together creating large sugar polymers, or polysaccharides. A polysaccharide is also called a glycan.

Q5: 8g of CH₄ occupies 11.2 at S.T.P. What volume would 22g of CH₃CH₂CH₃ occupy under the same condition?
(a) 3.7dm³
(b) 11.2dm³
(c) 22.4dm³
(d) 33dm³
Ans: (b)
Molar mass of methane, CH4 = 16g/mol
At standard conditions, S.T.P., 8g CH₄ occupies 11.2
Hence, 16g occupies 22.4 dm³
Molar mass of propane = 44g/mol^-1
 occupies 22.4dm^3.  at S.T.P
Hence,
22g will occupy 11.2dm³

Q6: The best treatment for a student who accidentally poured conc tetraoxosulphate(vi) on his skin in the laboratory is to wash his skin with?
(a) with cool running water
(b) sodium hydroxide solution
(c) iodine solution
(d) sodium trioxonitrate(v) solution
Ans: (a)

The treatment: Flushed Sulfuric acid with a mild, soapy solution if the burns are not severe.
Note: Sulfuric acid feels hot when water is added to the acid, but it is better to flush the area and not leave the acid on the skin.
Warning: Don't try to neutralize the burn with acid or alkali. This could cause a chemical reaction that worsens the burn!

Q7: Which of the gas laws does this graph illustrate?
(a) Boyle
(b) Charles
(c) Gay-Lussac
(d) Graham
Ans: (b)
Charles' law states that the volume occupied by a fixed amount of gas is directly proportional to its absolute temperature, if the pressure remains constant.

Q8: What are the possible oxidation numbers of an element if its atomic number is 17?
(a) -1 and 7
(b) -1 and 6
(c) -3 and 5
(d) -2 and 6
Ans: (a)
The element with atomic number 17 is Chlorine which has a valency of -1.
Hence, it needs to receive an electron or lose seven electrons to obey the octet rule which refers to the tendency of atoms to prefer to have eight electrons in the valence shell. When atoms have fewer than eight electrons, they tend to react and form more stable compounds

Q9: What process would coal undergo to give coal gas, coal tar, ammoniacal liquor and coke?
(a) Steam distillation
(b) Destructive distillation
(c) Liquefication
(d) Hydrolysis
Ans: (b)
Destructive distillation is a chemical process in which decomposition of unprocessed material is achieved by heating it to a high temperature; the term generally applies to the processing of organic material in the absence of air or in the presence of limited amounts of oxygen or other reagents, catalysts, or solvents, such as steam or phenols.

Q10: Liquid black soap is made by boiling palm oil with liquid extract of ash. The function of the ash is to provide the ?
(a) Acid
(b) Ester of alkanoic acid
(c) Alkali
(d) Alkanol
Ans: (c)
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations Na? and hydroxide anions OH? .

Q11: When water drops are added to calcium carbide in a container and the gas produced is passed through a jet and lighted, the resultant flame is called an?
(a) Oxyethene
(b) Oxyhydrocarbon flame
(c) Oxyacetylene flame
(d) Oxymethane flame
Ans: (c)
The reaction of calcium carbide with water, producing acetylene and calcium hydroxide, was discovered by Friedrich Wöhler in 1862. This reaction was the basis of the industrial manufacture of acetylene and is the major industrial use of calcium carbide.

Q12: Which of the following substances is a mixture?
(a) Granulated sugar
(b) Seawater
(c) Sodium chloride
(d) Iron filling
Ans: (b)
In chemistry, a mixture is a material made up of two or more different substances which are not chemically combined. A mixture is the physical combination of two or more substances in which the identities are retained and are mixed in the form of solutions, suspensions and colloids.
Examples include Air, Soil, Palm oil, Seawater, Blood, Crude oil, etc

Q13: If the quantity of oxygen occupying 2.76L container at a pressure of 0.825 atm and 300k is reduced by one-half, what is the pressure exerted by the remaining gas?
(a) 1.650atm
(b) 0.825atm
(c) 0.413atm
(d) 0.275atm
Ans: (a)
Using the combined gas law formula,
Given that P₁ = 0.825,   V₁ = 2.76 L, V₂ = 1.38 L, P₂ = ?
And T₁ = T₂; we would have P₂

 = [ P_{1 x} V₁ ] ÷ V2

: [0.825 x 2.76]  ÷ 1.38
= 1.650atm

Q14: Four elements P, Q, R and S have atomic numbers of 4, 10, 12 and 14 respectively. Which of these elements is noble gas?
(a) P
(b) Q
(c) R
(d) S
Ans: (b)
They are the most stable because they have the maximum number of valence electrons their outer shell can hold.
Hence, they rarely react with other elements since they are already stable.

Q15: The products of the thermal decomposition of ammonium trioxonitrate(v) are?
(a) Nitrogen (I) Oxide and Water
(b) Ammonia and Oxide
(c) Nitrogen and Water
(d) Nitrogen (IV) Oxide and Water
Ans: (a)

NH₄NO₃ Ammonium Trioxonitrate(V) , unlike all other metallic trioxonitrates(v), decomposes on heating to give nitrogen(i)oxide N₂O and water.

Q16: How many valence electrons are contained in the element ?
(a) 3
(b) 4
(c) 15
(d) 31
Ans: (a)
The element is Phosphorus in group 5 with valencies of 3 and 5. These are contained in the third energy level of the atom. Due to the number of valence electrons, Phosphorus is capable of forming three bonds with other elements.

Q17: The number of molecules of Carbon(iv)Oxide produced when 10.0g of CaCO₃  is treated with 0.2dm of 1 Mole of HCL in the equation CaCO₃ + 2HCL ?  CaCl₂  + H₂O + CO₂  , is ?
(a) 1.00 X 10²³
(b) 6.02 X 10²³
(c) 6.02 X 10²²
(d) 6.02 X 10²⁴
Ans: (c)
1 mole of CaCO₃ = 100g or  6.02 X 10²³

liberated 1 mole of CO₂ or 44g or 6.02 X 10²³
:100g =  6.02 X 10²³
10g of CaCO₃  = x
cross multiply
[10g X 6.02 X 10²³] / 100g = x
⇒ x = 6.02 X 10²²
Since
6.02 X 10²³  of CaCO₃  liberated 6.02 X 10²³ of CO₂
⇒ 6.02 X 10²²  of CaCO₃ will liberate 6.02 X 10²²  of CO₂

Q18: The volume occupied by 1.58g of a gas at S.T.P is 500cm³. What is the relative molecular mass of the gas?  [ G.M.V at S.T.P = 22.4dm³ ]
(a) 28
(b) 32
(c) 44
(d) 74
Ans: (d)
500cm³  of the gas weighed 1.58
22400cm³ of the gas weight: [22400 X 1.58g] ÷ [ 500 ]
= 74

Q19: Which of the following gases will rekindle a brightly splint?
(a) NO₂
(b) NO
(c) N₂O
(d) CL₂
Ans: (c)
Nitrogen(I)Oxide rekindles a brightly glowing splinter but extinguishes a feebly glowing one which is not hot enough to decompose the gas to liberate oxygen

Q20: The shape of ammonia molecule is?
(a) Trigonal planar
(b) Octahedral
(c) Square planar
(d) Tetrahedral
Ans: (a)
The ammonia molecule has a trigonal pyramidal shape with three hydrogen atoms and an unshared pair of electrons attached to the nitrogen atom. It is a polar molecule and is highly associated because of strong intermolecular hydrogen bonding.

Q21: A few drop of conc HCL are added to about 10cm of a solution of PH 3.4. The PH of the resulting mixture is?
(a) less than 3.4
(b) greater than 3.4
(c) unaltered
(d) the same as that of the pure water
Ans: (a)
The addition of conc HCL makes the solution more acidic and the solution falls below the original value

Q22: Chromatography is used to separate components of mixture which differ in their rates of ?
(a) diffusion
(b) migration
(c) reaction
(d) sedimentation
Ans: (b)
Chromatography is a method of separating mixtures by using a moving solvent on filter paper.
It is a laboratory technique for the separation of a mixture. The mixture is dissolved in a fluid called the mobile phase, which carries it through a system on which is fixed a material called the stationary phase. Wikipedia

Q23: Deliquescent substances are also?
(a) efflorescent
(b) anhydrous
(c) hygroscopic
(d) insoluble
Ans: (c)
Deliquescence is the process by which a substance absorbs moisture from the atmosphere until it dissolves in the absorbed water and forms a solution. Deliquescence occurs when the vapour pressure of the solution that is formed is less than the partial pressure of water vapour in the air.

Q24: Helium is used in observation of ballons because it is?
(a) Light and combustible
(b) light and non-combustible
(c) heavy and combustible
(d) heavy and non-combustible
Ans: (b)
Helium floats because it is buoyant;
The molecules in Helium are lighter than the nitrogen and oxygen molecules of our atmosphere and so they rise above it

Q25: 32g of anhydrous copper(ii)tetraoxosulphate(vi) dissolved in 1dm  of water generated 13.0kj of heat. The heat solution is?
(a) 26.0kj/mol
(b) 65.0kj/mol
(c) 130.0kj/mol
(d) 260.0kj/mol
Ans: (b)
32g of CUSO liberate 13.0kj of heat
Molar mass of CUSO₄ = 160g
No. of moles of CUSO₄  = 32g
No. of moles = 32(mass) / 160(molar mass)
= 0.2 moles
i.e 0.2 = 13kj
1 moles = xkj
= 65KJ 

Q26: A gaseous metallic chloride MCl consists of 20.22% of M by mass. The formula of the chloride is? [ M = 27, Cl = 35.5]
(a) MCl
(b) MCl₂
(c) MCl₃
(d) M₂Cl₆
Ans: (c)

Which gives formula of the Chloride = MCl₃

Q27: What quantity of electricity will liberate 0.125 mole of oxygen molecules during the electrolysis of dilute sodium chloride solution?
(a) 24,125 C
(b) 48,250 C
(c) 72,375 C
(d) 96,500 C
Ans: (b)
4OH ⇒ 2H₂O + O + 4e-  ........ (i)
From equation (i):
1 mole of Oxygen requires 4e
i.e 1 mole of Oxygen requires 4F,
where 1F = 96,500 C
[ 4 x 96,500 x 0.125 ] ÷ 1
= 48,250 C

Q28: Which of the following types of bonding does not involve the formation of new substances?
(a) Metallic
(b) Covalent
(c) Co-ordinate
(d) Electrovalent
Ans: (a)
In metallic bonding, the valence electrons of metal atoms are delocalized and form a "sea" of electrons that are free to move throughout the metal lattice. This creates a bond between the metal ions and the delocalized electrons, resulting in the characteristic properties of metals such as conductivity, malleability, and ductility.

Q29: An organic compound that does not undergo a reaction with both hydrogen cyanide and hydroxylamine can be?
(a) Alkene
(b) Alkanal
(c) Alkanone
(d) Alkanoic Acid
Ans: (c)
The addition of hydrogen cyanide to a carbonyl group of an aldehyde or most ketones produces a cyanohydrin.
Sterically hindered ketones, however, don't undergo this reaction. Aldehydes and ketones react with primary amines to form a class of compounds called imines, however, Imines of aldehydes are relatively stable while those of ketones are unstable.

Q30: The substance often used for vulcanization of rubber is?
(a) Chlorine
(b) Hydrogen peroxide
(c) Sulphur
(d) Tetraoxosulphate(vi) acid
Ans: (c)
Vulcanization of rubber is among the several uses of sulfur

Q31: The chlorinated alkane often used industrially to remove grease is?
(a) Tetrachloromethane
(b) Chloromethane
(c) Trichloromethane
(d) Dichloromethane
Ans: (a)
Tetrachloromethane is an effective solvent within the chemical industry and is used to clean machinery and electrical equipment. That is to say that it functions as a solvent for oils, fats, lacquers, varnishes, rubber waxes, and resins, and as a grain fumigant.

Q32: What mass of water is produced when 8.0g of hydrogen reacts with excess oxygen?
(a) 8.0g
(b) 16.0g
(c) 36.0g
(d) 72.0g
Ans: (d)
2H₂ + O₂  ⇒ 2H₂O
From the equation:
4g of Hydrogen produces 36g of water
8g of Hydrogen will produce 72g of water

Q33: Proteins in acid solution undergo?
(a) Substitution
(b) Fermentation
(c) Hydrolysis
(d) Polymerisation
Ans: (c)
Acid hydrolysis refers to a hydrolysis reaction occurring under acidic conditions. Acid hydrolysis of a protein breaks the polymer down into its constituent amino acids.
In organic chemistry, acid hydrolysis is a hydrolysis process in which a protic acid is used to catalyze the cleavage of a chemical bond via a nucleophilic substitution reaction, with the addition of the elements of water. For example, in the conversion of cellulose or starch to glucose.

Q34: The general formula for the Alkanals is?
(a) R₂CO
(b) RCHO
(c) ROH
(d) RCOOR
Ans: (b)
Alkanals (Aldehydes) have the general formula RCHO while Alkanones or Ketones have the general formula RCOR'.
Where R and R' may be alkyl or aryl groups.

Q35: When cathode rays are deflected onto the electrode of an electrometer, the instrument becomes?
(a) Bipolar
(b) Negatively charged
(c) Positively charged
(d) Neutral
Ans: (c)
When these cathode rays are deflected onto an electrode of an electrometer, the instrument measured a negative charge. The rays tested also were deflected by the application of an electric field.

Q36: When a solid substance disappears completely as a gas on heating, the substance is said to have undergone?
(a) Sublimation
(b) Crystallization
(c) Distillation
(d) Evaporation
Ans: (a)
Sublimation is the transition of a substance directly from the solid to the gas state, without passing through the liquid state

Q37: An electron can be added to a halogen atom to form a halide ion with?
(a) 8 valence electrons
(b) 7 valence electrons
(c) 2 valence electrons
(d) 3 valence electrons
Ans: (a)
A halide ion is a halogen atom bearing a negative charge. The halide anions are fluoride (F^-), chloride (Cl^-), bromide (Br^-), iodide (I^-) and astatide (At^-). Such ions are present in all ionic halide salts. Halide minerals contain halides.

Q38; Aluminum hydroxide is used in the dyeing industry as a?
(a) Dye
(b) Dispersant
(c) Salt
(d) Mordant
Ans: (d)
Aluminium hydroxide, Al(OH)? is used as a mordant in dyeing.

Q39: The most important element on the earth's crust is?
(a) Nitrogen
(b) Hydrogen
(c) Oxygen
(d) Fluorine
Ans: (c)
Oxygen, O₂, is by far the most abundant element in earth's crust at 46.6 percent, nearly half of the mass of the crust.

Q40: 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 H₂SO₄
(d) Bubbled through conc NAOH
Ans: (d)
Carbon monoxide, that is Carbon(ii)oxide is prepared from carbon(iv)oxide by passing the latter through red-hot carbon and the unchanged CO₂  is removed when the mixture of the gases through conc NAOH.