47 questions. Try each one, then open the answer to check yourself and read the explanation.
A mixture contains 20 cm3 of hydrogen, 35 cm3 of oxygen, 15 cm3 of carbon monoxide and 10 cm3 of nitrogen at S.T.P. Which of the following gives the mole fraction of hydrogen in this mixture?
Answer: D (0.25)
For gases at the same conditions, volume is proportional to moles. Total volume = 80 cm3, so the mole fraction of hydrogen = 20/80 = 0.25.
0.07 g of a hydride of carbon occupies 56.0 cm3 at S.T.P. when vaporized and contains 14.29% by mass of hydrogen. The formula of the hydrocarbon is (C = 12, H = 1)
Answer: C (C2H4)
Molar mass = 0.07 × 22400/56 = 28 g. H : C = 14.29/1 : 85.71/12 = 2 : 1, so the empirical formula is CH2 (14) and the molecule is (CH2)2 = C2H4.
The pressure on 100 cm3 of oxygen gas at 35°C is 750 mm of Hg. What would be the volume of the gas if the pressure is increased to 1000 mm of Hg without changing the temperature?
Answer: C (75 cm3)
Boyle's law: P1V1 = P2V2, so V2 = 750 × 100/1000 = 75 cm3.
When a pressure cooker is half filled with water, and heated to boiling point, then the pressure inside the cooker will
Answer: E (increase, because the water vapour molecules now strike the walls of the tube more frequently because of their increased velocity)
Heating turns more water into vapour and makes the vapour molecules move faster, so they hit the walls more often and harder: the pressure rises.
Which of the following bonds exist in crystalline ammonium chloride (NH4Cl)?
Answer: C (Ionic, covalent and co-ordinate)
NH4+ and Cl− are held by ionic bonds; inside NH4+ three N–H bonds are covalent and one is a co-ordinate (dative) bond.
An element X has two isotopes, 2010X and 2210X, present in the ratio 1:3. The relative atomic mass of X would be
Answer: C (21.5)
Average mass = (1 × 20 + 3 × 22)/4 = 86/4 = 21.5.
Which of the following is a neutralization reaction? Addition of
Answer: D (nitric acid to sodium hydroxide)
Neutralization is an acid reacting with a base to give salt and water: HNO3 + NaOH → NaNO3 + H2O.
A current of 10.0A was passed for 1 hour through a solution of copper (II) sulphate between the copper electrodes. Which of the following observations would be correct? (1 Faraday = 96,500 coulombs)
(i) 0.186 mole of copper is dissolved from the anode
(ii) 0.372 mole of copper is deposited on the cathode
(iii) the original Cu2+ ion concentration of the solution remains unchanged.
(iv) the original Cu2+ ion concentration of the solution decreases
Answer: B ((i) and (iii))
Charge = 10 × 3600 = 36,000 C = 0.373 F, which moves 0.186 mol Cu (2 F per mole). With copper electrodes the anode dissolves as fast as the cathode gains copper, so [Cu2+] stays the same.
In the preparation of carbon monoxide by heating ethanedioic acid with concentrated sulphuric acid, the concentrated sulphuric acid acts as
Answer: C (a dehydrating agent)
The acid removes water from ethanedioic acid: H2C2O4 → CO + CO2 + H2O, so it acts as a dehydrating agent.
When copper powder is heated with concentrated sulphuric acid, the gas produced is
Answer: B (sulphur dioxide)
Hot concentrated sulphuric acid oxidises copper and is itself reduced to sulphur dioxide: Cu + 2H2SO4 → CuSO4 + SO2 + 2H2O.
Magnesium ribbon was added to each of the following aqueous solutions in turn: H2SO4, AgNO3, HNO3, HCl, NH3. In which case does the magnesium become covered with a dark precipitate?
Answer: B (AgNO3)
Magnesium is above silver in the activity series, so it displaces silver, which coats the ribbon as a dark deposit. The acids just give a gas.
How many grams of methylacetylene (propyne) CH3—C≡CH will completely discharge the colour of 8 g of bromine? (Br = 80, C = 12, H = 1)
Answer: B (1.0)
8 g Br2 = 8/160 = 0.05 mol. A triple bond adds two Br2, so 0.025 mol propyne is needed: 0.025 × 40 = 1.0 g.
Which of the following is NOT a redox reaction?
Answer: C (Ca(HCO3)2 → CaCO3 + H2O + CO2)
In the decomposition of calcium hydrogen carbonate no element changes its oxidation number (Ca +2, C +4, O −2, H +1 throughout), so it is not redox.
What weight of sodium hydroxide is required to make 500 cm3 of 0.2 M solution? (Na = 23, O = 16, H = 1)
Answer: D (4 g)
Moles = 0.5 × 0.2 = 0.1 mol; mass = 0.1 × 40 = 4 g.
Which of the following combinations of conditions may increase the rate of a chemical reaction?
Answer: C (Increase in temperature, increase in concentration, addition of a catalyst and increase in the surface area of the reactants)
Higher temperature, higher concentration, a catalyst and a larger surface area all speed up a reaction; option C is the only one with every factor in the helpful direction.
In the manufacture of ammonia by the reaction N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = −92.37 kJ, the amount of ammonia formed at equilibrium will increase if
Answer: C (the temperature decreases)
The forward reaction is exothermic, so lowering the temperature shifts the equilibrium to the right (Le Chatelier). A catalyst only speeds up reaching equilibrium.
0.16 g of methane when burnt raises the temperature of 100 g of water by 40°C. What is the heat of combustion of methane if the heat capacity of water is 4.2 Jg−1°C−1? (CH4 = 16)
Answer: E (1,680 kJ mol−1.)
Heat = 100 × 4.2 × 40 = 16,800 J = 16.8 kJ from 0.16/16 = 0.01 mol, so 16.8/0.01 = 1,680 kJ mol−1.
Which of the following processes does NOT lead to a chemical change?
Answer: B (Stiring sodium carbonate in water.)
Sodium carbonate simply dissolves in water, a physical change. Each of the other processes is a reaction (with an acid, dehydration, or neutralisation).
Brass is an alloy containing copper and
Answer: A (zinc)
Brass is copper with zinc; bronze is copper with tin.
The number of grammes of potassium hydroxide in 250 cm3 of one molar solution is (K = 39, O = 16, H = 1)
Answer: E (14 g)
Moles = 0.25 × 1 = 0.25 mol; KOH = 56 g/mol, so mass = 0.25 × 56 = 14 g.
1.00 g of a mixture of calcium carbonate and calcium oxide liberated 0.33 g of carbondioxide on strong heating. The percentage of calcium oxide in the mixture is (Ca = 40, C = 12, O = 16)
Answer: C (25)
100 g CaCO3 gives 44 g CO2, so 0.33 g CO2 came from 0.75 g CaCO3. The rest, 0.25 g, is CaO: 25%.
If the rate of diffusion of oxygen gas is taken as 1, what will be the rate of diffusion of methane whose relative molar mass is 16?
Answer: C (1.4)
Graham's law: rate ∝ 1/√M. Rate(CH4)/rate(O2) = √(32/16) = √2 ≈ 1.4.
If the temperature of a gas is increased
Answer: C (the gas will occupy a greater volume if the pressure is constant)
By Charles' law, at constant pressure the volume of a gas increases as its temperature rises.
When a little brine was added to a metal nitrate a white precipitate was formed which first turned violet and then black on exposure to air. The metal nitrate is therefore
Answer: D (silver nitrate)
Chloride ions with silver ions give white silver chloride, which darkens (violet, then black) in light as silver is set free.
If atom X with six electrons in its outermost shell combines with atom Y with only one electron in its outermost shell to form an ionic compound, then
Answer: C (one atom of X will combine with two of Y)
X needs 2 electrons (forming X2−) and each Y gives 1 (forming Y+), so the formula is Y2X: one X to two Y.
The nitrogen gas obtained by burning an excess of phosphorus in air has a density higher than the nitrogen from nitrogen compounds because the nitrogen from air is contaminated with
Answer: D (rare gases)
Removing oxygen from air leaves nitrogen mixed with about 1% of rare gases, mainly argon, which is denser than nitrogen.
9.8g of solid potassium chlorate (KClO3) were added to 40 cm3 of water and heated to dissolve all the solid. As the solution cooled, crystals of potassium chlorate started forming at 66°C. The solubility of potassium chlorate at 66°C is therefore
Answer: E (2.0 moles per dm3.)
At 66°C the solution is just saturated: 9.8 g in 40 cm3 = 245 g per dm3. Dividing by KClO3 = 122.5 g/mol gives 2.0 mol dm−3.
100 cm3 of oxygen is made to react with 50 cm3 of hydrogen, and the whole reaction mixture was then passed through anhydrous calcium chloride. What is the volume of gas left?
Answer: D (75 cm3)
50 cm3 H2 needs only 25 cm3 O2; the water formed is absorbed by the calcium chloride, leaving 100 − 25 = 75 cm3 of oxygen.
An acid is said to be weak if
Answer: E (its ions are incompletely dissociated.)
A weak acid ionises only partly in water, so most of it stays as molecules.
Which of the following statements is correct during the electrolysis of a caustic soda solution using platinum electrodes?
Answer: E (Alkalinity at the cathode increases.)
At the cathode water is reduced to hydrogen gas and OH− ions, so the solution there becomes more alkaline. Oxygen comes off at the anode and no sodium is deposited.
An amphoteric oxide is an
Answer: D (oxide that behaves both as a base as well as an acid)
Amphoteric oxides such as ZnO and Al2O3 react with both acids and alkalis.
The solubility curve of a solid X (molecular mass = 160) is as shown below. 1,000 cm3 of a saturated solution of X at 80°C is cooled to 40°C. The weight of X crystalized out would be
Answer: D (16 g)
From the graph the solubility is 0.4 mol/dm3 at 80°C and 0.3 mol/dm3 at 40°C. The 0.1 mol that comes out of 1 dm3 weighs 0.1 × 160 = 16 g.
Water is poured over a white solid and a colourless neutral gas is evolved which burns with a sooty flame. The white solid is
Answer: C (calcium carbide)
Calcium carbide and water give ethyne (acetylene), a neutral gas with a high carbon content that burns with a sooty flame.
Cotton is almost pure
Answer: D (cellulose)
Cotton is a plant fibre made almost entirely of cellulose.
Which of the following types of reactions takes place between C2H4 and the halogens?
Answer: C (Addition)
Ethene is unsaturated, so a halogen adds across the C=C double bond, e.g. C2H4 + Br2 → C2H4Br2.
Which of the following equations represents an oxidizing action of sulphur dioxide?
Answer: D (SO2 + 2H2S → 3S + 2H2O)
With H2S, the sulphur in SO2 drops from +4 to 0, so SO2 is reduced and acts as the oxidizing agent. In the other equations SO2 is oxidised to sulphate.
The product formed at the cathode during the electrolysis of sodium chloride solution with carbon electrodes is
Answer: C (hydrogen)
H+ (from water) is discharged in preference to Na+, so hydrogen gas forms at the cathode.
Sodium carbonate crystals on exposure to a dry atmosphere change to a white powder. This phenomenon is known as
Answer: D (efflorescence)
Losing water of crystallization to the air and crumbling to a powder is called efflorescence.
Which of the following statements is incorrect?
Answer: E (Copper reacts with steam.)
Copper is below hydrogen in the activity series, so it does not react with water or steam.
In the reaction NH3 + H+ → NH4+, the bond formed results from
Answer: B (an electron pair donated by NH3)
The lone pair on nitrogen is given to the H+, which has no electrons, forming a dative (co-ordinate) bond.
Which of the following represents ethyl ethanoate?
Answer: D (CH3CO2CH2CH3)
Ethyl ethanoate is the ester of ethanoic acid (CH3CO2–) and ethanol (–CH2CH3): CH3CO2CH2CH3.
In the reaction N2(g) + O2(g) ⇌ 2NO(g) an increase in the total pressure will
Answer: E (have no effect.)
There are 2 moles of gas on each side, so changing the pressure does not shift the equilibrium.
In the chemical reaction A + B ⇌ C + D, more of D is formed
Answer: E (if D is continuously removed from the reaction mixture.)
Removing a product makes the equilibrium shift to the right to replace it, so more D keeps forming.
1.00 g of the oxide of metal X (atomic mass = 64) gave 0.80 g of the metal when reduced with hydrogen. The formula of the oxide is
Answer: B (XO)
Oxygen = 0.20 g = 0.0125 mol; metal = 0.80/64 = 0.0125 mol. The 1 : 1 ratio gives XO.
When a solution of silver (I) nitrate is saturated with hydrogen sulphide, a black precipitate forms which is
Answer: B (Ag2S)
Silver ions combine with sulphide ions to form black, insoluble silver sulphide, Ag2S.
The production of ammonia by the Haber process may be represented by the equilibrium equation N2(g) + 3H2(g) ⇌ 2NH3 ΔH = −22kJ. Therefore the production of ammonia will be favoured by
Answer: D (a combination of low temperature and high pressure)
The forward reaction is exothermic (favoured by low temperature) and reduces 4 moles of gas to 2 (favoured by high pressure).
The I.U.P.A.C name for the compound CH3—C(Cl)(CH3)—CH2—CH3 is
Answer: C (2-chloro-2-methylbutane)
The longest chain has four carbons (butane); carbon 2 carries both the Cl and a CH3 branch, so the name is 2-chloro-2-methylbutane.