JEE Chemistry Environmental Chemistry MCQs Set 10

Answer: (a) Chlorofluorocarbons

Explanation:
1. Photochemical smog contains ozone, PAN, aldehydes (acrolein, formaldehyde) and nitrogen oxides.
2. CFCs are involved in ozone depletion, not in smog.

Teacher's Note:
1. Photochemical smog is oxidising.
2. CFCs damage the stratospheric ozone layer.

Answer: (a) A dissolved oxygen concentration below 6 ppm is good for the growth of fish

Explanation:
1. Fish need a dissolved oxygen concentration of 6 ppm or more.
2. Below 6 ppm, fish growth is hampered, so the statement is not true.

Teacher's Note:
1. Higher DO means cleaner water.
2. Higher BOD means more pollution.

Answer: (c) reduce the use and production of hazardous chemicals

Explanation:
1. Green chemistry aims to design processes that reduce or eliminate the use and generation of hazardous substances.

Teacher's Note:
1. It focuses on pollution prevention at the source.
2. Atom economy is a key principle.

Answer: (c) CO

Explanation:
1. Carbon monoxide does not absorb enough infrared radiation to be a greenhouse gas.
2. CO2, CH4, O3, N2O, CFCs and water vapour are greenhouse gases.

Teacher's Note:
1. CO is toxic but not a major greenhouse gas.
2. It binds to haemoglobin.

Answer: (a) \(\ce{SO2}\)

Explanation:
1. Photochemical smog forms from nitrogen oxides and hydrocarbons under sunlight, giving O3 and PAN.
2. SO2 is a component of classical (London) smog.

Teacher's Note:
1. Classical smog is reducing and contains SO2.
2. Photochemical smog is oxidising.

Answer: (c) Its main components are produced by the action of sunlight on emissions of automobiles and factories

Explanation:
1. Classical smog is formed from smoke, fog and SO2 in cold, humid weather and is reducing.
2. The sunlight-driven formation described in (a) belongs to photochemical smog.

Teacher's Note:
1. Classical smog: reducing, cold, SO2. Photochemical smog: oxidising, warm, O3.
2. Smog is smoke plus fog.

Answer: (d) rich in dissolved oxygen

Explanation:
1. A low BOD means little oxygen is consumed by the decomposition of organic matter.
2. The water is clean and rich in dissolved oxygen.

Teacher's Note:
1. BOD is the oxygen needed by bacteria to break down organic matter.
2. Polluted water has a BOD of 17 ppm or more.

Answer: (b) Ozone is not responsible for the greenhouse effect

Explanation:
1. Ozone absorbs infrared radiation and does contribute to the greenhouse effect.
2. The other statements are correct.

Teacher's Note:
1. Tropospheric ozone is a greenhouse gas and a pollutant.
2. Stratospheric ozone absorbs harmful UV.

Answer: (b) a decrease in the amount of dissolved oxygen in water

Explanation:
1. Bacteria decompose organic waste and use up the dissolved oxygen.
2. The fall in dissolved oxygen kills fish.

Teacher's Note:
1. Eutrophication also reduces oxygen.
2. High BOD means low dissolved oxygen.

Answer: (a) It has a low concentration of oxidising agents

Explanation:
1. Photochemical smog is oxidising in nature because of ozone, PAN and NO2.
2. So the statement of low concentration of oxidising agents is wrong.

Teacher's Note:
1. Photochemical smog irritates the eyes.
2. Catalytic converters reduce its precursors.

Answer: (c) troposphere

Explanation:
1. The troposphere is the lowest layer, extending to about 10 km, and contains most of the air and weather.

Teacher's Note:
1. Layers from the bottom: troposphere, stratosphere, mesosphere, thermosphere.
2. The ozone layer is in the stratosphere.

Answer: (d) the reaction is endothermic and requires a very high temperature

Explanation:
1. The reaction \(\ce{N2 + O2 -> 2NO}\) is endothermic (+180 kJ/mol) and needs a very high temperature such as in automobile engines or lightning.

Teacher's Note:
1. Bond energy of N2 is very high.
2. This is the source of NOx in vehicle exhaust.

Answer: (b) Peroxyacetyl nitrate

Explanation:
1. Primary pollutants (CO, hydrocarbons, NO) are emitted directly.
2. PAN is formed in the atmosphere from reactions of primary pollutants in sunlight, so it is a secondary pollutant.

Teacher's Note:
1. Ozone, PAN and acid rain are secondary pollutants.
2. Primary pollutants come directly from sources.

Answer: (d) The ozone hole is the thinning of the ozone layer of the stratosphere at some places

Explanation:
1. The ozone hole refers to a region of the stratosphere, mostly over Antarctica, where the ozone concentration is greatly reduced.

Teacher's Note:
1. It is caused mainly by CFCs and chlorine radicals.
2. It is not a physical hole.

Answer: (b) Using plastic cans for storing substances

Explanation:
1. Using plastic cans increases non-biodegradable waste, so it is not green chemistry.
2. The other three reduce hazardous chemicals or pollution.

Teacher's Note:
1. Green chemistry reduces hazardous substances at source.
2. H2O2 is a clean bleaching agent.

Answer: (a) 5

Explanation:
1. Clean water has a BOD value below 5 ppm.
2. Highly polluted water has a BOD of 17 ppm or more.

Teacher's Note:
1. BOD is a measure of organic pollution.
2. Low BOD means clean water.

Answer: (a) 5

Explanation:
1. Fe in haem is coordinated to four nitrogen atoms of the porphyrin ring and one nitrogen of the histidine of globin.
2. That is 5 groups, with the sixth site left for O2 or CO.

Teacher's Note:
1. Haem iron is six-coordinate in total.
2. CO poisoning arises because CO binds about 200 times more strongly.

Answer: (b) 1

Explanation:
1. \(\ce{C4H10}\) gives 5 H2O, using 5 O atoms. 6 mol O2 is 12 O atoms, leaving 7 O atoms for CO2 and CO.
2. Let x = CO2 and y = CO: x + y = 4 and 2x + y = 7, giving x = 3 and y = 1.

Teacher's Note:
1. Balance C, H and O in turn.
2. Insufficient oxygen gives CO.

Answer: (b) +3

Explanation:
1. Dichromate (Cr +6) is reduced to \(\ce{Cr^{3+}}\) in acidic medium.
2. The oxidation number is +3.

Teacher's Note:
1. Cr(VI) is toxic and Cr(III) is less so.
2. Orange to green is the colour change.

Answer: (d) 4

Explanation:
1. \(\ce{5SO2 + 2MnO4- + 2H2O -> 5SO4^{2-} + 2Mn^{2+} + 4H+}\).
2. Moles of SO2 = 2.5 x 7.143e-5 = 1.786e-4 mol, and its volume at STP is 1.786e-4 x 22.4 = 4.0e-3 L.
3. ppm = (4.0e-3/1000) x 10^6 = 4.

Teacher's Note:
1. Use the mole ratio 5 : 2 for SO2 : MnO4-.
2. ppm by volume equals the volume fraction times 10^6.

Answer: (a) 8

Explanation:
1. Moles of SO2 = 1.867/22.4 = 0.0833.
2. 3 mol S form per mole of SO2, so S = 0.25 mol = 0.25 x 32 = 8 g.

Teacher's Note:
1. Use the mole ratio from the equation.
2. Sulphur is recovered by the Claus process.

Answer: (b) 6

Explanation:
1. In the Lewis structure the central O has 1 lone pair, the terminal O with a double bond has 2, and the singly bonded terminal O has 3.
2. Total = 1 + 2 + 3 = 6.

Teacher's Note:
1. Count lone pairs in one resonance structure.
2. Ozone is a bent molecule with a resonance hybrid.

Answer: (d) 5

Explanation:
1. Greenhouse gases among those listed: CO2, N2O, CH4, O3 and CFCs.
2. N2, O2 and H2 do not absorb infrared radiation, so the count is 5.

Teacher's Note:
1. Diatomic homonuclear gases are not greenhouse gases.
2. They have no dipole change in vibration.

Answer: (b) 5

Explanation:
1. From the balanced equation, 1 mol of I2O5 oxidises 5 mol of CO.

Teacher's Note:
1. Iodine (+5) is reduced to 0, gaining 10 electrons.
2. Each CO loses 2 electrons.

Answer: (b) 6

Explanation:
1. The oxides formed are NO (+2) and NO2 (+4).
2. The sum of the oxidation states is 2 + 4 = 6.

Teacher's Note:
1. NO forms first and is oxidised in air to NO2.
2. These oxides contribute to smog and acid rain.