All 20 questions from the Post-UTME Screening (Post-UTME) Chemistry 2017 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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What is quantity of iron is deposited when a current of 5A is passed through a solution containing Fe3+ ions for 19.3 minutes? [Fe=56, F=96,500C mol-1]
Q=It=5x(19.3x60)=5790C. Fe3+ + 3e -> Fe, so moles=Q/(3F)=5790/(3x96500)=0.02mol. Mass=0.02x56=1.12g.
Cu2+(aq) + 2e -> Cu(s), E = +0.34V; Zn2+(aq) + 2e -> Zn(s), E = -0.76V. From the equations above, it can be concluded that
Zinc, being more reactive (more negative electrode potential) than most common metals, readily acts as a reducing agent towards other metal ions.
During the electrolysis of dilute sodium chloride solution, the product at the anode is
In dilute NaCl solution, hydroxide ions are preferentially discharged at the anode, producing oxygen gas.
Palmwine exposed to the atmosphere for sometime becomes sour due to
Exposure to air allows bacteria to oxidise the ethanol in palmwine to ethanoic (acetic) acid, making it sour.
2HI + H2O2 -> 2H2O + I2. In the equation above, the oxidation number of I has changed from
Iodine in HI has oxidation number -1; in I2 it is 0. The oxidation number changes from -1 to 0.
A chemical substance reaction attains equilibrium when
Chemical equilibrium is characterised by the Gibbs free energy change (delta G) being zero.
Substance / delta H kJ mol-1: CH4 -74.8, O2 0, C -393.5, H2O -286. Using the table above, calculate the delta H for the following reaction at 25C: CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g).
delta H = [(-393.5)+2(-286)] - [(-74.8)+2(0)] = -965.5 - (-74.8) = -890.7 kJ.
N2(g) + O2(g) <=> 2NO(g), delta H + 92kJ. The equilibrium represented by the above equation can be shifted to right by
Adding excess N2 (a reactant) shifts the equilibrium to the right (Le Chatelier's principle); the reaction is endothermic, so cooling would not help, and pressure has no effect since moles of gas are equal on both sides.
N2O4(g) <=> 2NO2(g). A high yield of nitrogen (IV) oxide can be obtained the reaction above by
The dissociation of N2O4 to NO2 is endothermic, so raising the temperature shifts equilibrium towards greater NO2 (nitrogen(IV) oxide) yield.
CuSO4(aq) + H2S(g) -> CuS(s) + H2SO4(aq). In the reaction above, hydrogen sulphide acts as
H2S provides sulphide ions that precipitate copper as insoluble CuS, acting as a precipitating agent.
The cleansing action of soap in hard water is not satisfactory because soap
In hard water, soap reacts with calcium and magnesium ions to form insoluble scum (salts), reducing its cleansing effectiveness.
If the solubility of KHCO3 is 0.40 mol/dm3 at room temperature, calculate the mass of KHCO3 in 100cm3 of the solution at that temperature.
Moles = 0.40 x 0.100 = 0.040mol. Molar mass KHCO3 = 100g/mol. Mass = 0.040 x 100 = 4.0g.
An acid salt can be produced using
Acid salts form from polybasic (e.g. dibasic) acids like tetraoxosulphate(VI) acid, which can be only partially neutralised.
Which of the following is deliquescent?
Sodium hydroxide (NaOH) is a classic deliquescent substance, absorbing moisture from the air until it dissolves.
One of the products formed when chlorine is bubbled slowly through a solution of potassium iodide is
Chlorine displaces iodine from potassium iodide (Cl2 + 2KI -> 2KCl + I2), producing iodine, which appears as a solid/dark precipitate.
Which of the following is produced by each of NaNO3, Pb(NO3)2 and AgNO3 when decomposed by heat?
All nitrates release oxygen gas as a common product upon thermal decomposition, alongside other products depending on the metal.
In the extraction of iron, limestone is fed into the furnace to provide
Limestone decomposes in the furnace to form calcium oxide, which reacts with silica impurities to form slag, removing them from the molten iron.
Electrolytic method is usually employed in the extraction of
Aluminium, being highly reactive, is extracted by electrolysis of its molten oxide (in cryolite).
The compound of sodium used in the manufacture of glass is
Sodium carbonate (washing soda, Na2CO3.10H2O) is used as a raw material in glass manufacture.
Which of the following is typical of transition metals?
Transition metals characteristically exhibit variable (multiple) oxidation states/valences.
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