501. Assertion: Old chloroform that has been exposed to air and light may be unsafe to use.
Reason: Chloroform can be oxidised by oxygen in the presence of light to form poisonous phosgene.
ⓐ. Both Assertion and Reason are true, and Reason explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: Air provides the oxygen needed for oxidation of chloroform. Light promotes the chemical change leading to phosgene formation. Phosgene is much more acutely toxic than the original solvent and may be present without an obvious change that guarantees safe detection. Prolonged exposure to both air and light therefore increases the storage hazard. The Reason directly accounts for the safety concern stated in the Assertion.
502. In the balanced oxidation equation
\[
\mathrm{2CHCl_3+O_2\xrightarrow{light}2COCl_2+\Box HCl}
\]
the coefficient represented by \(\Box\) is:
ⓐ. \(1\)
ⓑ. \(3\)
ⓒ. \(2\)
ⓓ. \(4\)
Correct Answer: \(2\)
Explanation: \( \textbf{Hydrogen atoms on the reactant side:} \)
\[
2\mathrm{CHCl_3}\text{ contains }2\ \mathrm{H\ atoms}
\]
\( \textbf{Hydrogen balance:} \) Each \(\mathrm{HCl}\) molecule contains one hydrogen atom, so two \(\mathrm{HCl}\) molecules are required.
\( \textbf{Chlorine check:} \)
\[
2\mathrm{CHCl_3}\text{ contains }6\ \mathrm{Cl\ atoms}
\]
Two \(\mathrm{COCl_2}\) molecules use four chlorine atoms, leaving two chlorine atoms for \(2\mathrm{HCl}\).
\( \textbf{Balanced equation:} \)
\[
\mathrm{2CHCl_3+O_2\xrightarrow{light}2COCl_2+2HCl}
\]
\( \textbf{Final answer:} \) The coefficient represented by \(\Box\) is \(2\).
503. Inspect the following chloroform-storage measures.
| Row | Storage measure | Purpose |
|---|
| P | Dark-coloured bottle | Reduces exposure to light |
| Q | Tightly closed container | Limits contact with air |
| R | Bottle filled with little empty space | Reduces the amount of trapped oxygen |
| S | Small amount of ethanol | Helps destroy traces of phosgene |
The appropriate evaluation is:
ⓐ. only P and Q are correct
ⓑ. P, Q, R, and S are correct
ⓒ. only Q, R, and S are correct
ⓓ. only P, R, and S are correct
Correct Answer: P, Q, R, and S are correct
Explanation: A dark bottle suppresses the light-dependent oxidation of chloroform. Tight closure reduces entry of fresh air and also limits loss of the volatile liquid. Leaving little headspace decreases the amount of oxygen initially present above the liquid. Ethanol is commonly included as a stabiliser because it reacts with traces of phosgene and converts them into less dangerous products. The four precautions address different parts of the same storage problem.
504. Ethanol is often added in small quantity to stored chloroform because ethanol:
ⓐ. scavenges traces of phosgene formed by oxidation
ⓑ. supplies oxygen needed to keep chloroform colourless
ⓒ. converts chloroform into iodoform during storage
ⓓ. makes the bottle transparent to ultraviolet light
Correct Answer: scavenges traces of phosgene formed by oxidation
Explanation: Ethanol serves as a stabilising additive rather than as a preparation reagent for chloroform. If small amounts of phosgene form, ethanol can react with them and produce less harmful derivatives. This does not remove the need for dark, tightly closed storage. The additive provides an extra protective measure against trace oxidation products. It should not be interpreted as making aged or poorly stored chloroform automatically safe.
505. Read the following statements about chloroform.
Statement I: Chloroform was formerly used as an anaesthetic.
Statement II: Its anaesthetic use became limited because of serious toxicity concerns.
Statement III: Its volatility guarantees that it cannot cause harmful exposure.
ⓐ. Statements I and III only
ⓑ. Statements I and II only
ⓒ. Statements II and III only
ⓓ. Statements I, II and III
Correct Answer: Statements I and II only
Explanation: Chloroform has historical importance as an anaesthetic. Its medical use declined because it can produce serious toxic effects and safer alternatives became available. Volatility does not eliminate exposure risk; it allows vapour to enter the surrounding air readily. Inhalation can therefore become significant in an enclosed or poorly ventilated space. Statement III reverses the actual safety implication of volatility.
506. Two identical chloroform samples are stored with equal amounts of air.
Sample P is kept in a transparent bottle under strong light.
Sample Q is kept in a dark bottle.
A graph plots phosgene concentration against storage time. Which pattern is most reasonable?
ⓐ. The curve for P rises faster than the curve for Q
ⓑ. The curve for Q rises faster because darkness accelerates oxidation
ⓒ. Both curves remain identical because light has no role
ⓓ. Both curves fall below zero as chloroform evaporates
Correct Answer: The curve for P rises faster than the curve for Q
Explanation: Both samples contain oxygen, so oxidation is chemically possible in each bottle. Strong light increases the likelihood and rate of chloroform oxidation to phosgene. Sample P should therefore show a faster increase in phosgene concentration. The dark bottle suppresses the photochemical contribution and keeps the curve for Q lower. The comparison assumes that all other factors, including temperature and air content, remain the same.
507. Which option correctly describes iodoform?
ⓐ. \(\mathrm{CHCl_3}\), a colourless volatile liquid
ⓑ. \(\mathrm{CCl_4}\), a pale-yellow crystalline solid
ⓒ. \(\mathrm{CH_2I_2}\), a colourless gas
ⓓ. \(\mathrm{CHI_3}\), a pale-yellow crystalline solid
Correct Answer: \(\mathrm{CHI_3}\), a pale-yellow crystalline solid
Explanation: Iodoform is triiodomethane and has the molecular formula \(\mathrm{CHI_3}\). It is observed as a pale-yellow crystalline solid. Its characteristic smell and colour make it recognisable in the iodoform test. Chloroform is the chlorine analogue \(\mathrm{CHCl_3}\), but it differs in physical appearance. The halogen identity changes both formula mass and observable properties.
508. A compound is most directly expected to give the iodoform test when it contains:
ⓐ. only a terminal \(\mathrm{-CH_2OH}\) group with no suitable oxidation pathway
ⓑ. a \(\mathrm{CH_3CO-}\) unit or a \(\mathrm{CH_3CH(OH)-}\) unit
ⓒ. only a benzene ring bonded to chlorine
ⓓ. a carboxyl group with no methyl group nearby
Correct Answer: a \(\mathrm{CH_3CO-}\) unit or a \(\mathrm{CH_3CH(OH)-}\) unit
Explanation: Methyl ketones contain the structural unit \(\mathrm{CH_3CO-}\) and characteristically give the iodoform reaction. Secondary alcohols containing \(\mathrm{CH_3CH(OH)-}\) can be oxidised under the test conditions to corresponding methyl ketones. Ethanol and ethanal also give the test through the appropriate two-carbon oxidation relationship. The test is therefore based on a specific structural pattern rather than merely the presence of oxygen. Compounds lacking this connection generally do not form the yellow iodoform precipitate.
509. Which pair gives a positive iodoform test under standard iodine–alkali conditions?
ⓐ. Propanone and propan-\(2\)-ol
ⓑ. Propan-\(1\)-ol and benzaldehyde
ⓒ. Benzaldehyde and propanone
ⓓ. Propan-\(1\)-ol and propan-\(2\)-ol
Correct Answer: Propanone and propan-\(2\)-ol
Explanation: Propanone is a methyl ketone and contains the required \(\mathrm{CH_3CO-}\) unit directly. Propan-\(2\)-ol is oxidised under the test conditions to propanone, so it also produces iodoform. Propan-\(1\)-ol is oxidised to propanal rather than to a methyl ketone or ethanal and therefore does not give the test. Benzaldehyde lacks a methyl group attached to the carbonyl carbon. This leaves propanone and propan-\(2\)-ol as the only pair in which both members respond positively.
510. An unknown liquid is warmed with iodine and alkali. A pale-yellow crystalline precipitate with a characteristic smell appears. The observation most strongly indicates formation of:
ⓐ. chloroform
ⓑ. carbon tetrachloride
ⓒ. iodoform
ⓓ. dichloromethane
Correct Answer: iodoform
Explanation: Iodoform is recognised by its pale-yellow crystalline precipitate. The characteristic smell provides an additional observational clue. Its formula is \(\mathrm{CHI_3}\), showing that three iodine atoms are attached to one carbon atom. The test suggests that the original compound contained, or could be converted into, a suitable methyl-carbonyl structural unit. The observation identifies the product of the test rather than directly proving only one possible starting compound.
511. Assertion: Propan-\(2\)-ol gives a positive iodoform test.
Reason: Under the reaction conditions, propan-\(2\)-ol can be oxidised to propanone, which contains a \(\mathrm{CH_3CO-}\) unit.
ⓐ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓑ. Assertion is true, but Reason is false
ⓒ. Assertion is false, but Reason is true
ⓓ. Both Assertion and Reason are true, and Reason explains Assertion
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: Propan-\(2\)-ol contains the \(\mathrm{CH_3CH(OH)-}\) structural arrangement. Oxidation converts its hydroxyl-bearing carbon into a carbonyl carbon. The resulting compound is propanone, a methyl ketone. Propanone then undergoes the reactions that produce yellow iodoform. The Reason traces the chemical conversion responsible for the positive result.
512. Review the following properties of carbon tetrachloride.
| Row | Feature | Description |
|---|
| P | Formula | \(\mathrm{CCl_4}\) |
| Q | Bond character | Individual \(\mathrm{C-Cl}\) bonds are polar |
| R | Molecular dipole | Approximately zero because of symmetry |
| S | Combustibility | Nonflammable under ordinary conditions |
The appropriate evaluation is:
ⓐ. only P and Q are correct
ⓑ. only Q, R, and S are correct
ⓒ. P, Q, R, and S are correct
ⓓ. only P, R, and S are correct
Correct Answer: P, Q, R, and S are correct
Explanation: Carbon tetrachloride contains one carbon atom bonded to four chlorine atoms. Each carbon–chlorine bond is polar because of the electronegativity difference between the atoms. The tetrahedral symmetry causes the bond dipoles to cancel, making the molecule nonpolar overall. Its nonflammability contributed to some former industrial uses. None of these properties implies that the compound is safe to inhale or release into the environment.
513. The claim “Carbon tetrachloride is nonflammable, so it is harmless” is:
ⓐ. correct because only flammable substances can be toxic
ⓑ. incorrect; nonflammability does not remove toxic hazards
ⓒ. correct provided the liquid is colourless
ⓓ. incorrect only because carbon tetrachloride burns explosively in air
Correct Answer: incorrect; nonflammability does not remove toxic hazards
Explanation: Flammability describes whether a substance readily supports combustion. Toxicity describes its harmful effects on living organisms, and these are separate properties. Carbon tetrachloride can damage health through exposure even though it does not burn readily. Highly chlorinated compounds can also persist and contribute to environmental harm. Treating nonflammability as a complete safety test confuses fire risk with chemical and biological risk.
514. Carbon tetrachloride was formerly used in some fire-extinguishing applications mainly because it:
ⓐ. was nonflammable and its vapour could suppress combustion
ⓑ. released oxygen and increased the burning temperature
ⓒ. converted every fuel into water without forming vapour
ⓓ. reacted with flames to produce harmless elemental carbon only
Correct Answer: was nonflammable and its vapour could suppress combustion
Explanation: Carbon tetrachloride does not ignite easily under ordinary conditions. Its vapour could interfere with the supply of oxygen and reactive flame species near a fire. These properties led to its former use in some fire-extinguishing fluids. Such use became unacceptable because the compound is toxic and can form dangerous decomposition products when strongly heated. A useful fire-related property therefore did not make the material suitable for continued widespread use.
515. A \(15.4\,\mathrm{g}\) sample of carbon tetrachloride has molar mass \(154\,\mathrm{g\,mol^{-1}}\). How many moles of chlorine atoms are present?
ⓐ. \(0.10\,\mathrm{mol}\)
ⓑ. \(0.20\,\mathrm{mol}\)
ⓒ. \(0.40\,\mathrm{mol}\)
ⓓ. \(0.80\,\mathrm{mol}\)
Correct Answer: \(0.40\,\mathrm{mol}\)
Explanation: \( \textbf{Molecular formula:} \)
\[
\mathrm{CCl_4}
\]
\( \textbf{Moles of carbon tetrachloride:} \)
\[
n=\frac{m}{M}
\]
\( \textbf{Substitution:} \)
\[
n(\mathrm{CCl_4})=\frac{15.4}{154}
\]
\[
n(\mathrm{CCl_4})=0.10\,\mathrm{mol}
\]
\( \textbf{Chlorine-atom relation:} \)
\[
1\,\mathrm{mol\ CCl_4}\text{ contains }4\,\mathrm{mol\ Cl\ atoms}
\]
\( \textbf{Required amount:} \)
\[
n(\mathrm{Cl\ atoms})=(0.10)(4)
\]
\[
n(\mathrm{Cl\ atoms})=0.40\,\mathrm{mol}
\]
The molecular subscript must be applied after determining the amount of carbon tetrachloride molecules.
516. Freons are broadly classified as compounds containing:
ⓐ. only carbon and hydrogen
ⓑ. carbon, sulfur, and sodium
ⓒ. nitrogen, hydrogen, and iodine
ⓓ. carbon, chlorine, and fluorine
Correct Answer: carbon, chlorine, and fluorine
Explanation: Freons are members of the chlorofluorocarbon family or closely related halogenated refrigerant families. Classical chlorofluorocarbons contain carbon, chlorine, and fluorine. Their stability, volatility, and heat-transfer properties led to widespread technological use. The same stability also allows some of them to persist long enough to reach the upper atmosphere. The term does not describe ordinary hydrocarbons containing only carbon and hydrogen.
517. The formula of the chlorofluorocarbon commonly represented as Freon-12 is:
ⓐ. \(\mathrm{CH_2ClCF_3}\)
ⓑ. \(\mathrm{CCl_4}\)
ⓒ. \(\mathrm{CH_2F_2}\)
ⓓ. \(\mathrm{CCl_2F_2}\)
Correct Answer: \(\mathrm{CCl_2F_2}\)
Explanation: Freon-12 is dichlorodifluoromethane. It contains one carbon atom, two chlorine atoms, and two fluorine atoms. Its formula is therefore \(\mathrm{CCl_2F_2}\). Carbon still forms four bonds in the molecule, with all four hydrogen positions of methane replaced by halogens. \(\mathrm{CH_2ClCF_3}\) contains two carbon atoms, while \(\mathrm{CCl_4}\) contains no fluorine; neither is Freon-12.
518. Freons were formerly used extensively as refrigerants and aerosol propellants mainly because they were:
ⓐ. highly reactive with metals and water
ⓑ. stable, volatile, and suitable for heat transfer
ⓒ. strongly acidic and completely nonvolatile
ⓓ. designed to decompose instantly in the lower atmosphere
Correct Answer: stable, volatile, and suitable for heat transfer
Explanation: A useful refrigerant must undergo controlled evaporation and condensation during a cooling cycle. Many chlorofluorocarbons were chemically stable and noncorrosive under ordinary operating conditions. Their volatility also made them useful as aerosol propellants. These practical advantages encouraged large-scale use. Their environmental problem arose partly because the same stability allowed them to survive in the lower atmosphere for long periods.
519. Assertion: The chemical stability of chlorofluorocarbons in the lower atmosphere contributes to their environmental persistence.
Reason: Their slow removal allows them to survive long enough to reach regions where ultraviolet radiation can release chlorine radicals.
ⓐ. Both Assertion and Reason are true, and Reason explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: Chlorofluorocarbons were valued because they did not react readily during normal use. This stability also means that they are not quickly destroyed after release into the atmosphere. They can persist for years and be transported to the stratosphere. Ultraviolet radiation there can break a carbon–chlorine bond and generate reactive chlorine species. The Reason explains how lower-atmosphere stability produces long-term environmental consequences.
520. Assess the following stages of chlorofluorocarbon-related ozone depletion.
| Row | Stage | Description |
|---|
| P | Lower atmosphere | Chlorofluorocarbon remains relatively stable |
| Q | Stratospheric photolysis | Ultraviolet radiation releases a chlorine radical |
| R | Ozone reaction | \(\mathrm{Cl\cdot}\) converts ozone into molecular oxygen |
| S | Radical cycle | Chlorine radical is regenerated and can react again |
The appropriate evaluation is:
ⓐ. only P and Q are correct
ⓑ. only Q, R, and S are correct
ⓒ. P, Q, R, and S are correct
ⓓ. only P, R, and S are correct
Correct Answer: P, Q, R, and S are correct
Explanation: Atmospheric persistence allows chlorofluorocarbons to reach the stratosphere. Ultraviolet light then produces chlorine radicals by bond cleavage. A chlorine radical reacts with ozone and forms molecular oxygen along with a radical intermediate. Subsequent reaction regenerates the chlorine radical. Every row represents one necessary link in the complete cause-and-effect chain from chemical stability to catalytic ozone depletion.
