Practice MDCAT Chemistry Medium MCQs with explanations — PMC, ETEA, NUMS entry test preparation.
Q1. A brewery uses molasses as a raw material for ethanol production. Which enzyme specifically converts sucrose into glucose and fructose during fermentation?
Answer: Invertase
Explanation: Invertase hydrolyzes molasses (sucrose) into glucose and fructose. Zymase is tempting but it only converts glucose directly into ethanol and CO2.
Q2. During the industrial synthesis of phenol via the Dow's process, chlorobenzene is reacted with 10% NaOH. What are the required temperature and pressure?
Answer: 360C and 150 atm
Explanation: Dow's process requires 360C and 150 atm to convert chlorobenzene to phenol. Higher pressures like 300 atm are typical for Haber's process, not Dow's.
Q3. In the energy sector, a mixture of 80% petrol and 20% alcohol is used as a fuel. Which alcohol is used in this 'power alcohol' mixture?
Answer: Ethanol
Explanation: Ethanol is used as a fuel in some internal combustion engines when mixed with gasoline. Methanol is toxic and less common for 'power alcohol'.
Q4. A technician performs the Lucas test on a sample of 2-propanol. What observation would confirm the presence of this secondary alcohol in the lab?
Answer: Oil layer formation after 5-10 minutes
Explanation: The Lucas reagent (HCl/ZnCl2) reacts with secondary alcohols like 2-propanol to form an oily layer within 5-10 minutes. Primary alcohols require heating.
Q5. During a chemical identification test, a student adds iodine and sodium hydroxide to an unknown alcohol, resulting in yellow crystals. Which alcohol is it?
Answer: Ethanol
Explanation: Ethanol reacts with I2 and NaOH to form yellow iodoform crystals. Methanol fails this test because it lacks the CH3-CH(OH)- group.
Q6. A mechanic adds a specific alcohol to an automobile radiator to prevent the cooling water from freezing in sub-zero temperatures. Which alcohol is used?
Answer: Ethylene glycol
Explanation: Ethane-1,2-diol (Ethylene glycol) is used as an antifreeze because it lowers the freezing point of water. Ethanol is too volatile for this.
Q7. A chemical engineer wants to produce diethyl ether by dehydrating ethanol with concentrated H2SO4. Which temperature must be maintained to maximize ether yield?
Answer: 140C
Explanation: At 140C with excess alcohol, ethanol undergoes intermolecular dehydration to form diethyl ether. At 170C, ethene would be the major product.
Q8. A student adds bromine water to an aqueous solution of phenol in a test tube. What is the characteristic observation for this reaction?
Answer: White precipitate
Explanation: Phenol reacts with bromine water to give a white precipitate of 2,4,6-tribromophenol. Bromine in CCl4 would give a different product (monobrominated).
Q9. Methanol is restricted in industrial use because it causes blindness upon ingestion. This toxicity is due to its oxidation in the liver to which compound?
Answer: Formic acid
Explanation: Methanol is oxidized to formic acid (methanoic acid) and formaldehyde in the liver, which are toxic to the optic nerve. Acetic acid is non-toxic.
Q10. A chemical engineer is optimizing the industrial production of 'wood spirit' from water gas. Which catalyst combination is essential for this process?
Answer: ZnO + Cr2O3
Explanation: The industrial synthesis of methanol from water gas requires ZnO-Cr2O3; V2O5 is tempting but used for sulfuric acid production.
Q11. A researcher compares the relative acidic strengths of water, phenol, and ethanol. Which sequence correctly represents their decreasing order of acidity?
Answer: Phenol > Water > Ethanol
Explanation: Phenol is more acidic than water and ethanol due to phenoxide resonance; ethanol is the least acidic of the group.
Q12. During the commercial preparation of phenol via Dow's process, what are the specific temperature and pressure conditions required for the reaction?
Answer: 360 Celsius and 150 atm
Explanation: In Dow's process, chlorobenzene reacts with 10% NaOH at 360 Celsius and 150 atmospheres to produce sodium phenoxide.
Q13. When methanol vapors are passed over platinum wire or treated with acidic K2Cr2O7, what is the initial organic oxidation product?
Answer: Formaldehyde
Explanation: Methanol oxidizes to formaldehyde (methanal), while ethanol oxidizes to acetaldehyde (ethanal), which is the most common distractor.
Q14. Comparing the boiling points of the first four primary alcohols, which one will require the highest temperature to boil at 1 atm?
Answer: Butan-1-ol
Explanation: Boiling points of alcohols increase with chain length due to increased London forces; butanol has a higher boiling point than propanol.
Q15. An unknown alcohol is tested with Lucas reagent and shows no turbidity even after 20 minutes at room temperature. Which alcohol could it be?
Answer: 1-butanol
Explanation: Primary alcohols like 1-butanol show no reaction with Lucas reagent at room temperature and only react upon heating.
Q16. A small piece of sodium metal is dropped into a beaker of liquid phenol. What is the name of the salt formed?
Answer: Sodium phenoxide
Explanation: Phenol reacts with sodium metal to form sodium phenoxide and hydrogen gas, confirming its acidic nature.
Q17. A laboratory technician reacts 2.3g of metallic sodium with an excess of absolute ethanol. What volume of hydrogen gas is evolved at STP?
Answer: 1.12 dm3
Explanation: 2.3g Na is 0.1 moles; since 2 moles Na produce 1 mole H2, 0.05 moles H2 occupies 1.12 dm3. 2.24 dm3 assumes 1:1 ratio.
Q18. During the industrial fermentation of molasses, 180g of glucose is converted into ethanol by zymase. What is the maximum theoretical mass of ethanol produced?
Answer: 92 g
Explanation: One mole of glucose (180g) yields two moles of ethanol (92g). 46g is the yield from only half a mole of glucose.
Q19. A sample of 2-methyl-2-propanol is tested with a mixture of anhydrous ZnCl2 and concentrated HCl. When will the oily layer of alkyl halide appear?
Answer: Immediately upon addition
Explanation: Tertiary alcohols like 2-methyl-2-propanol react immediately with Lucas reagent. 5-10 minutes is characteristic of secondary alcohols like 2-propanol.
Q20. In a combustion analysis, 1 mole of liquid 1-propanol is completely burned in oxygen. How many moles of oxygen gas are consumed?
Answer: 4.5 moles
Explanation: Combustion of 1 mole of propanol (C3H8O) requires 4.5 moles of O2. 3 moles only covers the carbon oxidation, ignoring hydrogen.
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