All 20 questions from the Post-UTME Screening (Post-UTME) Chemistry 2016 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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The correct decreasing order of conductivity for the compounds below is
HNO3 (strong acid, fully ionised) conducts best, CH3COOH (weak acid, partially ionised) conducts less, and C2H5OH (non-electrolyte) conducts least.
Calculate the pH of an aqueous solution of a 0.01M dibasic acid.
A dibasic acid releases 2 H+ per molecule, so [H+] = 2 x 0.01 = 0.02M. pH = -log(0.02) = 1.7.
During the electrolysis of acidified water, how many Faraday(s) will liberate 5.6dm3 of oxygen gas, measured at s.t.p? [Molar volume of a gas at s.t.p = 22.4 dm3]
Moles O2 = 5.6/22.4 = 0.25mol. Since 4 electrons (Faradays) are needed per mole of O2 (2H2O -> O2 + 4H+ + 4e-), Faradays = 0.25 x 4 = 1.00F.
In an electrolytic process, a metal X is deposited according to the equation X2+ + 2e(aq) -> X. Calculate the quantity of electricity required to deposit 0.5 mole of X. [F = 96,500 C mol-1]
Q = nzF = 0.5 x 2 x 96500 = 96500 C.
The oxidation number of Sulphur in the ion S2O3 2- is
Let x = oxidation state of S: 2x + 3(-2) = -2, so 2x = 4, x = +2.
Cu2+(aq) [pale blue] + 4NH3(aq) <=> [Cu(NH3)4]2+(aq) [Deep blue solution]. In the equation above, addition of a few drops of dilute hydrochloric acid will produce
HCl reacts with (protonates) free NH3, reducing its concentration and shifting equilibrium back towards pale blue Cu2+, decreasing the deep blue colour.
A galvanic cell makes use of the following half reactions: Li(s) -> Li+(aq) + e, E = +3.05V; F2(g) + 2e(aq) -> 2F-(aq), E = +2.85V. What is the voltage of the cell?
Cell voltage = sum of the given oxidation potential (Li) and reduction potential (F2): 3.05 + 2.85 = 5.90V.
The rate of reaction between zinc and sodium hydroxide can be increased by using
Both a more concentrated reagent and a greater surface area (granulated zinc) increase reaction rate.
CO2(g) + H2(g) <=> CO(g) + H2O(g), delta H = +41kJ. In the reaction above, increase in temperature will
Since the forward reaction is endothermic, an increase in temperature shifts the equilibrium to the right (towards products).
In the preparation of carbon (II) oxide from ethanedioic acid, concentrated tetraoxosulphate (VI) acid acts as
Concentrated H2SO4 acts as a dehydrating agent in this preparation, removing water from ethanedioic acid to yield CO, CO2 and H2O.
The reaction of methane with steam in the presence of nickel catalyst at 800C produces
Steam reforming of methane: CH4 + H2O -> CO + 3H2, producing hydrogen and carbon monoxide.
Trioxonitrate (V) acid prepared in the laboratory has a yellow colour due to the presence of dissolved
Dissolved NO2 (formed from partial decomposition of the acid) gives laboratory-prepared concentrated trioxonitrate(V) acid its characteristic yellow tinge.
Hydrogen sulphide is produced in the laboratory using
Kipp's apparatus is commonly used to generate gases like hydrogen sulphide in the laboratory.
The formation of a dense white precipitate, on addition of dilute hydrochloric acid and barium chloride solution to an unknown solution on warming indicates the presence of
Barium sulphate (BaSO4) is an insoluble white precipitate that remains even in the presence of dilute HCl, indicating the presence of sulphate ions.
A substance that could be used to displace bromine from a solution of sodium bromide is
Chlorine, being more reactive than bromine, displaces bromine from a sodium bromide solution.
The use of calcium in the electrolytic extraction of calcium from calcium chloride is to
In this extraction process, the use of an appropriate electrode/collection method helps obtain pure calcium metal as the final product.
Which impurity in sodium chloride causes it to be deliquescent?
Magnesium chloride, a common impurity in crude sodium chloride, is deliquescent and causes the salt to absorb moisture and become damp.
On the addition of excess aqueous ammonia to a mixture of Zn2+(aq) and Al3+(aq), the precipitate obtained is
Zn(OH)2 dissolves in excess ammonia (forming a soluble complex), while Al(OH)3 does not, so the precipitate remaining is aluminium hydroxide.
In the extraction of aluminium from bauxite, molten cryolite is added to
Cryolite is added to lower the melting point of alumina (Al2O3), reducing the energy needed for electrolysis.
Metals are good conductors of electricity because they contain mobile
Metals conduct electricity due to the presence of a 'sea' of mobile (delocalised) electrons.
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