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Post-UTME Chemistry 2019 Objective Past Questions

All 20 questions from the Post-UTME Screening (Post-UTME) Chemistry 2019 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.

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Chemistry 2019 Objective — Question 1

The number of moles of solute present in 50cm3 of a 0.25M H2SO4 solution is

  • A. 0.025
  • B. 0.022
  • C. 0.0125Correct
  • D. 0.010

Explanation

Moles = Molarity x Volume(L) = 0.25 x 0.050 = 0.0125 mol.

Chemistry 2019 Objective — Question 2

If 100cm3 of a 2.0M HNO3 solution is mixed with 150cm3 of a 1.8M KOH solution, the resulting aqueous solution is

  • A. weakly acidic
  • B. strongly acidic
  • C. neutral
  • D. strongly basicCorrect

Explanation

Moles HNO3 = 0.1x2.0=0.2mol; moles KOH=0.15x1.8=0.27mol. Excess KOH (0.07mol) remains, making the solution strongly basic.

Chemistry 2019 Objective — Question 3

Use the figure below (electrolysis of molten sodium chloride, with electrodes labelled and Z marking one electrode) to answer this question. Z is the

  • A. anode where the Cl- ions are oxidizedCorrect
  • B. cathode where the Cl- ions are reduced
  • C. anode where the Na+ ions are reduced
  • D. cathode where the Na+ ions are oxidized

Explanation

In electrolysis of molten NaCl, oxidation (loss of electrons from Cl-) occurs at the anode, while reduction of Na+ occurs at the cathode; Z corresponds to the anode where Cl- ions are oxidized.

Chemistry 2019 Objective — Question 4

A copper voltameter is set up with a copper cathode and an anode of carbon. A current of 4 amperes is passed through the voltameter for 2 hours. The mass of copper deposited during this period is

  • A. 6.96 g
  • B. 7.60 g
  • C. 8.76 g
  • D. 9.50 gCorrect

Explanation

Q=It=4x7200=28800C. Moles Cu = Q/(zF) = 28800/(2x96500) = 0.149mol. Mass = 0.149x64 = 9.5g.

Chemistry 2019 Objective — Question 5

An atom, P reacts by losing two electrons on the outermost shell which are accepted by two Q atoms. The oxidation states of P and Q are respectively

  • A. +1 and -1
  • B. +2 and -2
  • C. -2 and +2
  • D. +2 and -1Correct

Explanation

P loses 2 electrons to become +2. Since 2 electrons are shared between two Q atoms (one each), each Q atom gains 1 electron to become -1.

Chemistry 2019 Objective — Question 6

A metal, X does not react with cold water but reacts with steam. In which of the following is X correctly placed in order of decreasing activity?

  • A. Potassium, X, calcium, copper, silver
  • B. Potassium, Calcium, X, copper and silverCorrect
  • C. Potassium, calcium, copper, silver, X
  • D. Potassium, calcium, copper, X, silver

Explanation

A metal reacting with steam but not cold water is less reactive than calcium (which reacts with cold water) but more reactive than copper/silver (which don't react with steam), placing X between calcium and copper.

Chemistry 2019 Objective — Question 7

The enthalpy of combustion of graphite is 394 KJ mol-1. The enthalpy change when 3.6g of graphite is burnt is

  • A. 120.1 KJ mol-1
  • B. 119.8 Kj mol-1
  • C. 118.2 KJ mol-1Correct
  • D. 110.3 KJ mol-1

Explanation

Moles of graphite = 3.6/12 = 0.3mol. Enthalpy change = 0.3 x 394 = 118.2 KJ mol-1.

Chemistry 2019 Objective — Question 8

A piece of metal reacts with 2M H2SO4 at 20C to give 50cm3 of H2 in 5 minutes. The same piece reacts with the same acid at the temperature of tC to give 50 cm3 of H2 in 10 minutes. This implies that tC is

  • A. greater than 20C
  • B. lower than 20CCorrect
  • C. equal to 20C
  • D. equal to 40C

Explanation

The reaction took longer (10 vs 5 minutes) to produce the same volume of gas, meaning it proceeded more slowly - consistent with a lower temperature.

Chemistry 2019 Objective — Question 9

A chemical reaction is said to have attained a state of chemical equilibrium when the

  • A. concentrations of the reactants are equal to those of the products
  • B. reagents in the system stop reacting
  • C. concentrations of the products are greater than those of the reactants
  • D. concentrations of the reactants and products remain constantCorrect

Explanation

At equilibrium, the forward and reverse reaction rates are equal, so the concentrations of reactants and products remain constant (though not necessarily equal).

Chemistry 2019 Objective — Question 10

Use the table below (Initial conc. of H2 in moles: 1.00, 2.00, 3.00, 4.00; Initial Rate: 20.00, 40.00, 60.00, 80.00) to answer this question. These results suggest that when the initial concentration of hydrogen is doubled, the rate of reaction is

  • A. doubledCorrect
  • B. tripled
  • C. quadrupled
  • D. unchanged

Explanation

Doubling the H2 concentration from 1.00 to 2.00 doubles the rate from 20.00 to 40.00, showing rate is directly proportional to (doubled with) concentration.

Chemistry 2019 Objective — Question 11

Which of the following statements correctly represents Le Chatelier's Principle?

  • A. If a stress is applied to a system, the system readjusts to reduce the stressCorrect
  • B. If a stress is applied to a system in equilibrium, the system will readjust to counter the stress
  • C. If a stress is applied to a system which is not at equilibrium, the system will readjust to counter the stress
  • D. If a stress is applied to a system, the stress will readjust for the reaction to take place spontaneously

Explanation

Le Chatelier's Principle states that a system subjected to a stress (change in conditions) will readjust itself to counteract/reduce the effect of that stress.

Chemistry 2019 Objective — Question 12

Which of the following factors will affect the position of equilibrium of a reversible reaction? I. Temperature II. Concentration III. Surface area of reactants

  • A. II and III only
  • B. I and II onlyCorrect
  • C. III only
  • D. I only

Explanation

Temperature and concentration affect the position of equilibrium; surface area only affects the rate at which equilibrium is reached, not its position.

Chemistry 2019 Objective — Question 13

Addition of dilute trioxonitrate (V) acid to a white salt produced brisk effervescence. The anion most likely to be contained in the salt is

  • A. SO4 2-
  • B. CO3 2-Correct
  • C. NO3 -
  • D. PO4 3-

Explanation

Brisk effervescence (gas released) when a dilute acid is added to a salt is characteristic of a carbonate reacting to release CO2.

Chemistry 2019 Objective — Question 15

Which of the following will exhibit allotropy? I. Sulphur II. Oxygen III. Hydrogen IV. Carbon

  • A. I only
  • B. I and II only
  • C. I, II and III only
  • D. I, II and IV onlyCorrect

Explanation

Sulphur, oxygen (O2/O3) and carbon (diamond/graphite/fullerenes) all exhibit allotropy; hydrogen does not exhibit common allotropes.

Chemistry 2019 Objective — Question 16

The following compounds are alkalis EXCEPT

  • A. Ca(OH)2
  • B. Mg(OH)2Correct
  • C. NaOH
  • D. KOH

Explanation

Mg(OH)2 is only sparingly soluble in water and is not classified as an alkali (a soluble base), unlike Ca(OH)2, NaOH and KOH.

Chemistry 2019 Objective — Question 17

The solution of a zinc salt and a white precipitate which dissolved in excess ammonia solution. The solubility of the zinc salt in aqueous ammonia can be explained by the

  • A. amphoteric nature of zincCorrect
  • B. formation of the zincate ion
  • C. solubility product of the zinc salt
  • D. electrovalent nature of the zinc salt

Explanation

Zinc hydroxide is amphoteric and dissolves in excess ammonia (forming a soluble complex ion), a behaviour generally attributed to zinc's amphoteric nature.

Chemistry 2019 Objective — Question 18

Li + Z -> H + He (nuclear equation). In the above equation, Z represents

  • A. an alpha particle
  • B. a protonCorrect
  • C. a neutron
  • D. an electron

Explanation

In this classic nuclear reaction (Li-7 bombarded with a proton to yield two helium-4 nuclei), Z represents a proton.

Chemistry 2019 Objective — Question 19

Glyceryl esters with high concentration of unsaturation occur as

  • A. fat
  • B. margarine
  • C. oilsCorrect
  • D. glycerine

Explanation

Highly unsaturated glyceryl esters (triglycerides) are typically liquid at room temperature, occurring as oils.

Chemistry 2019 Objective — Question 20

Which of the following are usually sodium or potassium salts of fatty acids?

  • A. Polymers and plastics
  • B. Fats and oils
  • C. Butter and margarine
  • D. SoapsCorrect

Explanation

Soaps are sodium or potassium salts of long-chain fatty acids.

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