All Free Chemistry MCQs with Answers

461. Value of R gas constant in J mol-1 K-1 is

• A. 8.314
• B. 62.4
• C. 0.821
• D. 62400

Explanation: The universal gas constant is 8.314 joules per mole per kelvin, and this is the value to use whenever pressure is in pascals and volume in cubic metres. The figure 0.0821 dm3 atm per mole per kelvin is the same constant expressed for pressure in atmospheres, which is why 0.821 appears as a distractor. Always match the value of R to the units of the other quantities in the problem.

462. Which of the following best explains why phenol reacts with aqueous NaOH, but alcohols do not?

• A. Phenol has a lower boiling point than alcohol
• B. Alcohols contain a strong O-H bond that cannot be broken by weak bases
• C. Phenol form hydrogen bonding that facilitates ionization
• D. Phenol is weakly acidic due to resonance stabilization of phenoxide ion

Explanation: Losing the hydroxyl hydrogen gives phenol an ion whose negative charge is delocalised into the benzene ring, so the phenoxide ion is far more stable than an alkoxide and the acid dissociates readily enough to be neutralised by sodium hydroxide. An alkoxide has no such delocalisation, so ethanol is not acidic enough to react. Both compounds hydrogen bond, so that cannot be the deciding factor.

463. In propene pi-bond is formed by sideway overlap of:

• A. s-orbitals
• B. p-orbitals
• C. sp3 hybrid orbitals
• D. sp2 hybrid orbitals

Explanation: Each doubly bonded carbon uses three sp2 hybrids for its sigma bonds and keeps one unhybridised p orbital, and the sideways overlap of those two p orbitals above and below the plane forms the pi bond. Hybrid orbitals point along the internuclear axis and so can only make sigma bonds. This is why rotation about the double bond is blocked.

464. An enzyme used to decompose the lipids into fatty acids in our alimentary canal is

• A. Amylase
• B. Protease
• C. Lipase
• D. Urease

Explanation: Pancreatic lipase hydrolyses triglycerides into fatty acids and glycerol in the duodenum, working on the enormous surface created after bile salts have emulsified the fat. Amylase acts on starch and protease on protein, each enzyme being specific to one class of substrate. Urease breaks urea down and is found in bacteria and plants rather than the human gut.

465. Molecules having one lone pair and three bond pairs have geometrical shape

• A. Bent or angular
• B. Trigonal
• C. Trigonal pyramidal
• D. Trigonal planar

Explanation: Four regions of electron density arrange themselves tetrahedrally, but only the three bonded atoms are visible, so the shape is a pyramid, as in ammonia with its bond angle of about 107 degrees. Trigonal planar at 120 degrees would require three bond pairs and no lone pair, as in boron trifluoride, which is the trap here. A bent shape comes from two bond pairs with two lone pairs, as in water.

466. Conversion of dihaloalkane into alkyne does not involve

• A. Addition
• B. Elimination
• C. Base
• D. Heat

Explanation: A vicinal dihalide is converted to an alkyne by removing two molecules of hydrogen halide with hot alcoholic potassium hydroxide, which is a double dehydrohalogenation and therefore an elimination. Addition is the opposite process and would move back towards the saturated compound. The base and the heat are both required conditions for the elimination.

467. Majority of reactions taking place at ordinary temperatures with ssH are

• A. Endothermic
• B. Exothermic
• C. Thermally unstable
• D. Reversible

Explanation: The symbol in the original stem is the enthalpy change, which did not print cleanly in the paper. Reactions that proceed readily at ordinary temperature without a continuous supply of heat are overwhelmingly those that release energy, that is exothermic ones with a negative enthalpy change. Endothermic reactions generally need heating to be maintained, which is why combustion and neutralisation happen spontaneously while thermal decomposition does not.

468. Which one of the following is a coinage metal

• A. Pd
• B. Cu
• C. Cd
• D. Hg

Explanation: Copper, silver and gold form group IB and are called the coinage metals because their unreactivity and durability made them the traditional choice for currency. Palladium is a platinum group metal, and cadmium and mercury sit in group IIB with completely filled d subshells. All four are d block elements, which is what makes the distractors plausible.

469. Consider the given balanced chemical equation: 2H2 + O2 gives 2H2O. If 4 g of H2 reacts with 32 g of O2 to produce 28 g of H2O, what is the percentage yield of the reaction? (Molar mass of H2 = 2 g/mol, O2 = 32 g/mol and H2O = 18 g/mol)

• A. 63.6%
• B. 77.8%
• C. 87.5%
• D. 92.5%

Explanation: Four grams of hydrogen is 2 moles and 32 g of oxygen is 1 mole, which is exactly the 2 to 1 ratio the equation requires, so neither is in excess and 2 moles of water should form, that is 36 g. The percentage yield is therefore 28 divided by 36 multiplied by 100, which is 77.8 per cent. Checking for a limiting reactant before computing the theoretical yield is the step most often skipped.

470. Amount of energy needed to weaken the existing bonds to an extent that can be broken through collision is called:

• A. Bond energy
• B. Activation energy
• C. Average energy
• D. Enthalpy

Explanation: Activation energy is the barrier reactant molecules must climb to reach the transition state, and only collisions carrying at least this much energy, with the right orientation, lead to reaction. Bond energy is the energy needed to break a particular bond completely in the gaseous state, a different quantity. A catalyst works by lowering the activation energy, not the bond energy.