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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If a solution contains 4.9g of tetraoxosulphate(VI) acid, calculate the amount of copper(II) oxide that will react with it. [Cu=64, O=16, S=32, H=1]
RMM of H2SO4 = 98, RMM of CuO = 64+16 = 80. Equation: H2SO4 + CuO -> CuSO4 + H2O. 98g H2SO4 reacts with 80g CuO, so 4.9g reacts with (4.9/98) x 80 = 4.0g CuO.
A given volume of methane diffuses in 20s. How long will it take the same volume of Sulphur (IV) oxide to diffuse under the same conditions?
Graham's law: t(SO2)/t(CH4) = sqrt(M(SO2)/M(CH4)) = sqrt(64/16) = 2. t(SO2) = 2 x 20 = 40s.
The pressure of 100cm^3 of oxygen at 35 degrees C is 750mmHg. What would be the volume of the gas if the pressure is increased to 1000mmHg without changing the temperature?
Boyle's law: P1V1 = P2V2 => V2 = (750 x 100)/1000 = 75cm^3.
Which of the following bonds exist in crystalline ammonium chloride (NH4Cl)?
NH4Cl contains N-H covalent bonds within the NH4+ ion, an N-H co-ordinate (dative) bond, and an ionic bond between NH4+ and Cl-, so all three types are present.
Which of the following is a neutralization reaction? Addition of
Neutralization is the reaction of an acid with a base. Nitric acid (acid) reacting with sodium hydroxide (base) is the only neutralization reaction listed.
In the preparation of carbon monoxide by heating ethanedioic acid with concentrated sulphuric acid, the concentrated sulphuric acid acts as
Concentrated H2SO4 removes water from ethanedioic (oxalic) acid in this reaction, acting as a dehydrating agent, producing CO, CO2 and H2O.
How many grammes of methyl acetylene (propyne) CH3-C=CH will completely discharge the colour of 8g of bromine? [Br=80, C=12, H=1]
Molar mass of propyne (C3H4) = 40g/mol; Br2 = 160g/mol. Propyne + 2Br2 -> fully saturated addition product (decolourises 2 mol Br2 per mol alkyne). Moles Br2 = 8/160 = 0.05mol, needs 0.025mol propyne = 0.025 x 40 = 1.0g.
Brass is an alloy containing copper and
Brass is an alloy of copper and zinc.
60cm^3 of hydrogen are sparked with 20cm^3 of oxygen at 100 degrees C and atmospheric pressure. The total volume of the residual gases is
2H2 + O2 -> 2H2O(g) (steam, since T=100C is at/above boiling point so water stays gaseous). 20cm^3 O2 reacts with 40cm^3 H2 to give 40cm^3 steam, leaving 60-40=20cm^3 H2 unreacted. Total residual gas = 20cm^3 H2 + 40cm^3 steam = 60cm^3. Flagged: the figure matching the closest listed option is C (40cm^3); exact residual-gas interpretation (whether steam condenses) affects the final total, so this is marked with reduced confidence.
0.07g of a hydride of carbon occupies 56cm^3 at s.t.p when vapourised and contains 14.29% by mass of hydrogen. The formula of the hydrocarbon is [C=12, H=1]
Moles of gas = 56/22400 = 0.0025mol. Molar mass = 0.07/0.0025 = 28g/mol. With 14.29% H by mass, mass of H = 0.1429 x 28 = 4, i.e. 4 H atoms (mass 4), remaining mass 24 = 2 C atoms. Formula = C2H4 by this calculation; closest listed option consistent with a C:H hydrocarbon of molar mass ~28 is CH4 (mass 16) or C2H4. Flagged: calculated molar mass (28) matches C2H4 exactly, which is not among the listed options - answer given as closest match (A).
Which of the following statements correctly represents Le Chatelier's Principle?
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.
Which of the following factors will affect the position of equilibrium of a reversible reaction? I. Temperature II. Concentration III. Surface area of reactants
Temperature and concentration affect the position of equilibrium; surface area only affects the rate at which equilibrium is reached, not its position.
Addition of dilute trioxonitrate (V) acid to a white salt produced brisk effervescence. The anion most likely to be contained in the salt is
Brisk effervescence (gas released) when a dilute acid is added to a salt is characteristic of a carbonate reacting to release CO2.
Which of these gases supports combustion?
Oxygen is the gas that supports (sustains) combustion.
Which of the following will exhibit allotropy? I. Sulphur II. Oxygen III. Hydrogen IV. Carbon
Sulphur, oxygen (O2/O3) and carbon (diamond/graphite/fullerenes) all exhibit allotropy; hydrogen does not exhibit common allotropes.
The following compounds are alkalis EXCEPT
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.
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
Zinc hydroxide is amphoteric and dissolves in excess ammonia (forming a soluble complex ion), a behaviour generally attributed to zinc's amphoteric nature.
Li + Z -> H + He (nuclear equation). In the above equation, Z represents
In this classic nuclear reaction (Li-7 bombarded with a proton to yield two helium-4 nuclei), Z represents a proton.
Glyceryl esters with high concentration of unsaturation occur as
Highly unsaturated glyceryl esters (triglycerides) are typically liquid at room temperature, occurring as oils.
Which of the following are usually sodium or potassium salts of fatty acids?
Soaps are sodium or potassium salts of long-chain fatty acids.
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