All 27 questions from the Post-UTME Screening (Post-UTME) Chemistry 2005 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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During the electrolysis of copper (II) sulphate solution, an ammeter showed a steady current reading of 1.0A for 1/2 hr. 6.6x10^-4 kg of copper were liberated. Calculate the % error in ammeter reading
Theoretical mass (using Faraday's laws) is about 5.97x10^-4 kg; comparing to the actual 6.6x10^-4 kg gives an error of approximately 11%.
With the Daniel cell the depolarizer is
In a Daniell cell, copper(II) sulphate solution acts as the depolarizer at the copper electrode.
Which of the following is a physical change?
Dissolving sodium chloride in water is a physical (reversible) change, unlike the chemical reactions described in the other options.
A mixture of oil and water can be easily separated by
Immiscible liquids like oil and water are easily separated using a separating funnel.
Bronze is a/an
Bronze is an alloy - a mixture (solid solution) of copper and tin.
To separate a mixture of benzene, ethanol and water use
These three liquids have different boiling points, so fractional distillation is used to separate them.
Hydrogen forms two oxides. In the first oxide, 2g of hydrogen combine with 16g of oxygen. In the second oxide, 2g of hydrogen combine with 32g of oxygen. What law is illustrated?
Fixed masses of one element (hydrogen) combining with different masses of another (oxygen) in a simple whole-number ratio (16:32=1:2) illustrates the law of multiple proportions.
Lead forms two chlorides. The ratio of chloride atoms in the two chlorides is 1:2. If the mass of chlorine in the second chloride is 42.0g, find the mass of chlorine in the first compound
Since the ratio is 1:2, the first chloride contains half the chlorine mass of the second: 42/2 = 21g.
According to the balanced equation below, which gas is in excess and by how much? H2(g) + F2(g) -> 2HF(g); 50dm^3, 60dm^3
The reaction is 1:1; with 50dm^3 H2 and 60dm^3 F2, F2 is in excess by 60-50=10dm^3.
Which is heavier: 0.2mol SO2 or 0.2mol CO2? [S=32.0, O=16, C=12]
Mass SO2 = 0.2 x 64 = 12.8g. Mass CO2 = 0.2 x 44 = 8.8g. Difference = 4g, with SO2 heavier.
A gas jar of air was placed on top of a gas jar of nitrogen IV oxide. After sometime, a uniform colour was seen throughout both gas jars. What process has occurred?
The gradual mixing/spreading of gas molecules to achieve a uniform colour/concentration is diffusion.
At 17 degrees C, a sample of hydrogen occupies 125cm^3. What will the volume be at 100 degrees C. If the pressure remains constant?
V2 = 125 x (373/290) = 160.8cm^3, approximately 161cm^3.
Which of the following atoms A,B,C and D would readily form an ion with a charge of 2? Mass No.: (a)12 (b)16 (c)24 (d)31; Atomic No.: (a)6 (b)8 (c)12 (d)15
An atomic number of 12 (magnesium, a group 2 metal) readily loses 2 electrons to form a 2+ ion.
The ion X contains 23 particles in the nucleus and 10 electrons outside the nucleus. What does the nucleus of the ion X contain?
A sodium (Na) atom, mass number 23, has 11 protons and 12 neutrons; losing one electron to form Na+ gives 10 electrons, matching the description.
Which of the following are properties of carbon in the form of diamond?
Diamond does not conduct electricity (nil conductivity) and has an extremely high melting point, around 3500 degrees C.
What particles are present in solid bromide and molten lead II bromide?
PbBr2 is an ionic compound; it consists of ions in both the solid lattice (immobile) and the molten state (mobile, hence conducting).
The non-luminous Bunsen flame has the following characteristics except
The flame does not enter the burner tube to consume incoming fuel; this describes flashback, an abnormal/undesirable condition, not a normal characteristic of the non-luminous flame.
Water of crystallization is responsible for
Water of crystallization, incorporated in a fixed ratio within the crystal lattice, is responsible for the characteristic shape of hydrated crystals.
The substance which loses its water of crystallization on exposure to air is
An efflorescent substance loses its water of crystallization when exposed to air.
15g of water dissolves 3.5g of sodium chloride at 25 degrees C. What is the solubility of the sodium chloride at that temperature? [Na=23, Cl=35.5]
Solubility = 3.5/15 x 100 = 23.3g per 100g water. In 0.1dm^3 (approx 100g water), moles = 23.3/58.5 = 0.399mol, giving a concentration of about 4.0mol/dm^3.
To determine the solubility of a salt in the laboratory, you need a/an
Solubility determination requires a saturated solution (containing the maximum dissolved solute) at a given temperature.
The major water pollutant is
Sewage (from the sheer volume discharged) is generally regarded as the major water pollutant overall.
Which of the following acids is monobasic?
Ethanoic (acetic) acid, CH3COOH, has only one replaceable/ionizable hydrogen, making it monobasic.
Which is the function of silica SiO2 in the equation shown? CaO + SiO2 -> CaSiO3
Silica acts as an acidic oxide, reacting with the basic oxide CaO.
Which of the following substances is not a polymer
Propane is a simple small hydrocarbon molecule, not a polymer, unlike nylon, polyethene, and starch.
The oxidizing agent in the reaction H2S(g) + Br2(aq) -> 2HBr(aq) + S(s) is
Bromine (Br2) is reduced to Br- (in HBr), meaning it is the oxidizing agent.
The preferential discharge of sodium during the electrolysis of brine using mercury as cathode is due to one of the following
Mercury forms a sodium amalgam, which significantly lowers the energy required to discharge sodium at the cathode compared to hydrogen.
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