Practice MDCAT Chemistry Hard MCQs with explanations — PMC, ETEA, NUMS entry test preparation.
Q1. A laboratory requires 100% pure ethanol for a sensitive reaction. Which substance is used to remove the last traces of water from rectified spirit?
Answer: CaO
Explanation: Absolute alcohol (100%) is obtained by redistilling rectified spirit with CaO, which absorbs the remaining water. Benzene is used for ternary azeotropes.
Q2. An industrial manufacturer produces Bakelite, a thermosetting plastic used for electrical switches. Which aldehyde is reacted with phenol to produce this resin?
Answer: Formaldehyde
Explanation: Bakelite is a polymer formed by the condensation of phenol and formaldehyde. Acetaldehyde is tempting but forms different resins with different properties.
Q3. In the production of explosives and dyes, Picric acid is synthesized from phenol. Which nitrating mixture is required for this specific reaction?
Answer: Concentrated HNO3 and Concentrated H2SO4
Explanation: Phenol reacts with concentrated HNO3/H2SO4 to form 2,4,6-trinitrophenol (Picric acid). Dilute HNO3 would only produce ortho and para nitrophenols.
Q4. The industrial synthesis of salicylic acid, a precursor to aspirin, involves reacting sodium phenoxide with CO2. What is the standard temperature for this Kolbe’s reaction?
Answer: 120-150C
Explanation: Kolbe's reaction occurs at 120-150C and high pressure. 360C is the temperature for Dow's process, which is a common point of confusion.
Q5. A researcher treats phenol with chloroform in the presence of aqueous sodium hydroxide at 60C. What is the major organic product of this Reimer-Tiemann reaction?
Answer: Salicylaldehyde
Explanation: The Reimer-Tiemann reaction introduces an aldehyde group at the ortho position of phenol using chloroform and NaOH, yielding salicylaldehyde.
Q6. Excess ethanol is treated with concentrated sulfuric acid and maintained at exactly 140 degrees Celsius. What is the primary organic product?
Answer: Diethyl ether
Explanation: Dehydration of ethanol at 140 Celsius yields diethyl ether; 170 Celsius is the tempting temperature that yields ethene.
Q7. When a carboxylic acid reacts with an alcohol to form an ester, which specific atoms are removed to form water?
Answer: OH from acid and H from alcohol
Explanation: In esterification, the OH group is removed from the carboxylic acid and the H atom from the alcohol.
Q8. A chemist reacts phenol with concentrated nitric acid in the presence of concentrated sulfuric acid. What is the IUPAC name of the product?
Answer: 2,4,6-trinitrophenol
Explanation: Picric acid is 2,4,6-trinitrophenol, produced by reacting phenol with concentrated nitric acid; dilute acid only gives mono-substituted products.
Q9. When phenol is treated with chloroform in the presence of aqueous sodium hydroxide at 60 degrees Celsius, what is the major product?
Answer: Salicylaldehyde
Explanation: The Reimer-Tiemann reaction involves treating phenol with chloroform and NaOH to introduce an aldehyde group at the ortho position.
Q10. An organic chemist is studying structural isomerism in saturated acyclic compounds. How many total structural isomers (alcohols and ethers) exist for the formula C4H10O?
Answer: 7
Explanation: C4H10O forms 4 alcohols (1-butanol, 2-butanol, isobutanol, t-butanol) and 3 ethers. 4 is the count for alcohols only, not all isomers.
Q11. A student treats 9.4g of phenol with an excess of bromine water. What is the mass of the white precipitate (2,4,6-tribromophenol) formed?
Answer: 33.1 g
Explanation: 9.4g Phenol is 0.1 mol; it reacts to form 0.1 mol of 2,4,6-tribromophenol (M.W 331). 9.4g fails because it ignores bromine mass.
Q12. A mixture of 60g acetic acid and 46g ethanol is heated with H2SO4. What is the theoretical mass of the fruity-smelling ester produced?
Answer: 88 g
Explanation: 60g Acetic acid and 46g Ethanol (1 mole each) produce 1 mole Ethyl acetate (88g). 106g incorrectly sums the reactant masses.
Q13. Comparing the acidic strength, approximately how many times more acidic is phenol than ethanol based on their Ka values?
Answer: 10^6 times
Explanation: Phenol (Ka ~10^-10) is much more acidic than Ethanol (Ka ~10^-16). Ethanol's pKa is roughly 16, while phenol is 10.
Q14. A laboratory technician mixes an unknown alcohol with Lucas reagent at room temperature. No turbidity appears even after 24 hours of standing. Identify the alcohol.
Answer: Methanol
Explanation: Primary alcohols only produce turbidity upon heating with Lucas reagent; methanol is the slowest reacting among them due to its structure.
Q15. When bromine water is added to an aqueous solution of phenol, a characteristic white precipitate is formed immediately. Name this white precipitate.
Answer: 2,4,6-Tribromophenol
Explanation: In aqueous solution, phenol is highly reactive and undergoes trisubstitution, whereas in non-polar CS2, it yields mono-substituted ortho and para products.
Q16. In a laboratory test to distinguish between two primary alcohols, one sample produces yellow crystals with iodine and NaOH. Name this alcohol.
Answer: Ethanol
Explanation: Ethanol and secondary alcohols containing a methyl group adjacent to the carbinol carbon (CH3-CHOH-) give a positive iodoform test.
Q17. A student is given four colorless liquids and told to identify which one is a phenol using aqueous Ferric Chloride. Which color change confirms phenol?
Answer: Phenol
Explanation: Ferric chloride reacts with the hydroxyl group attached to a benzene ring to form a complex, resulting in a characteristic violet or purple color.
Q18. The reaction of phenol with chloroform and NaOH is known as
Answer: Reimer-Tiemann reaction
Explanation: Reimer-Tiemann reaction involves phenol reacting with chloroform and NaOH to form salicylaldehyde.
Q19. Phenol is less soluble in water than ethanol because
Answer: Phenol forms weaker H-bonds
Explanation: Phenol's hydroxyl group forms weaker H-bonds due to delocalization of the lone pair into the benzene ring.
Q20. The iodoform test is given by
Answer: Ethanol
Explanation: Ethanol gives a positive iodoform test as it can be oxidized to acetaldehyde and then to iodoform.
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