3 free questions from this chapter (the full set has 30). Marking: Multi-correct: +4 all correct, +1 per correct option if none wrong, −2 if any wrong · Numerical: +4 / 0 · Matching: +3 / −1.
Question 1Level 3 · AdvancedSingle correct
A coal-fired power station burns 500 t of coal per day. The coal contains 1.6% sulfur by mass, all of which is converted to SO2 on combustion. The flue gas is scrubbed with a limestone slurry in the presence of air, which removes 90% of the SO2 by the reaction 2CaCO3+2SO2+O2→2CaSO4+2CO2, and all the CaSO4 formed is recovered as gypsum, CaSO4⋅2H2O. (Atomic masses: H 1, C 12, O 16, S 32, Ca 40; 1 t =106 g.) The mass of gypsum recovered per day is ______. Exactly one option is correct.
A43.0 t
B22.5 t
C38.7 t
D30.6 t
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Answer: C
Sulfur burnt per day =0.016×500=8.0 t =8.0×106/32=2.5×105 mol S, giving 2.5×105 mol SO2 (S+O2→SO2).\nSO2 captured =0.90×2.5×105=2.25×105 mol; by the scrubbing equation 1 mol SO2 gives 1 mol CaSO4, i.e. 1 mol gypsum.\nM(CaSO4⋅2H2O)=40+32+64+36=172 g mol−1.\nGypsum =2.25×105×172=3.87×107 g =38.7 t.\nTraps: 43.0 t ignores the 90% capture; 30.6 t is anhydrous CaSO4 (136 g mol−1); 22.5 t is the mass of CaCO3 consumed.\nThis is a control strategy that turns an air pollutant into a usable industrial by-product.\nDifficulty: Very hard
Question 2Level 3 · AdvancedSingle correct
On a clear, warm summer day in a city with heavy morning traffic, a monitoring station records: NO peaks at about 8 a.m.; NO2 peaks at about 10 a.m.; O3 starts low, rises steadily and peaks at about 2 p.m.; after sunset O3 falls again. Which interpretation is consistent with the chemistry of photochemical smog? Exactly one option is correct.
AEngines emit mainly NO, which is oxidised to NO2; sunlight splits NO2 (NO2hνNO+O, then O+O2→O3). Because NO destroys ozone (NO+O3→NO2+O2), O3 can accumulate only after much of the NO has been converted to NO2 (which needs sunlight and reactive unburnt hydrocarbons), so the O3 peak lags behind the NO2 peak; without sunlight no new O atoms form, so O3 falls at night.
BOzone is emitted directly in vehicle exhaust; it peaks at 2 p.m. only because it disperses slowly from the morning rush-hour emissions.
CThe profile is that of classical smog: SO2 and smoke combine with fog, and the resulting reducing mixture converts NO to O3 by midday.
DTropospheric O3 is formed mainly by direct photolysis of O2 by the sunlight reaching the ground, so it simply follows the intensity of sunlight; NO and NO2 play no role in its build-up.
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Answer: A
Photochemical smog forms in warm, dry, sunny conditions from NOx and unburnt hydrocarbons.\nSequence: traffic emits NO (formed from N2+O2→2NO in the hot engine), so NO peaks at rush hour. NO is converted to NO2, which peaks later. Sunlight photolyses NO2: NO2→NO+O; O+O2⇌O3. But O3+NO→NO2+O2 is fast, so ozone cannot build up while NO is plentiful; it accumulates only after the NO has largely been converted to NO2 (hydrocarbon-derived species do this without using up ozone). Hence O3 peaks in the early afternoon, after NO2. At night there is no photolysis, so no new O atoms form while ozone is still removed: O3 falls.\n(B) Ozone is a secondary pollutant, not emitted directly; the lagged sequence NO, then NO2, then O3 is a production chain, not dispersal.\n(C) Classical smog is a cool, humid, reducing smog of smoke, fog and SO2; this is the opposite situation (oxidising smog).\n(D) The UV that splits O2 is absorbed high in the atmosphere; in the troposphere the O atoms come from NO2 photolysis, which is why the O3 peak is tied to the NOx profile.\nInsight: NO is an ozone sink as well as an ozone precursor, so the O3 peak must lag the NO2 peak.\nDifficulty: Peak challenge
Question 3Level 2 · ExamSingle correct
Dichlorodifluoromethane, CF2Cl2 (a freon), is non-toxic, non-flammable and chemically unreactive near the ground, yet it is held responsible for depletion of stratospheric ozone. Which statement gives the correct explanation? Exactly one option is correct.
AIt reacts with tropospheric ozone to give ClO, which then rises into the stratosphere.
BIt is heavier than air, so it stays near the ground and destroys ozone formed in photochemical smog.
CIt dissolves in rain to give HCl, and the HCl evaporates into the stratosphere and destroys ozone directly.
DBeing unreactive, it is not removed in the troposphere; it slowly mixes up into the stratosphere, where UV radiation breaks a C-Cl bond (CF2Cl2hν∙CF2Cl+Cl∙), and the chlorine radical then destroys ozone catalytically.
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Answer: D
The very inertness that made CFCs attractive as refrigerants, propellants and cleaning agents means they survive in the lower atmosphere for a long time and eventually reach the stratosphere. There, strong UV radiation cleaves the weakest bond (C-Cl): CF2Cl2→∙CF2Cl+Cl∙. The chlorine radical starts a chain: Cl∙+O3→ClO∙+O2, ClO∙+O→Cl∙+O2, so one Cl radical destroys many ozone molecules.\n(A) is wrong: CFCs do not react with tropospheric ozone. (B) is wrong: gases mix by turbulence regardless of molar mass, and stratospheric (not smog) ozone is affected. (C) is wrong: CFCs are not significantly soluble or hydrolysed in rain, and the radical mechanism needs UV photolysis in the stratosphere.\nDifficulty: Hard
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