All Free Chemistry MCQs with Answers

481. The value of Planck's constant is:

• A. 6.6262x10-30 J.sec
• B. 6.6262x10-32 J.sec
• C. 6.6262x10+34 J.sec
• D. 6.6262x10-34 J.sec

Explanation: Planck's constant is 6.626 times 10 to the minus 34 joule seconds, and its extreme smallness is why quantisation is invisible in everyday life and only matters at atomic scale. The positive exponent in one option would make it enormous, which a moment's thought rules out. It appears in E equals hf, in the de Broglie relation and in the uncertainty principle.

482. Final equation for the representation of rate of reaction in term of concentration is called

• A. Rate law
• B. Rate constant
• C. Reaction rate
• D. Reaction order

Explanation: The rate law, or rate equation, states the rate as the rate constant multiplied by concentration terms raised to their experimentally determined powers. The rate constant is only the proportionality factor within it and the order is the exponent, so both are components of the rate law rather than the equation itself. Orders must be found by experiment, since they need not match the coefficients in the balanced equation.

483. Poly vinyl chloride (PVC) is the type of:

• A. Homopolymer
• B. Thermosetting polymer
• C. Thermoplastic polymer
• D. Copolymer

Explanation: This is the option marked correct in the official UHS answer key. PVC softens on heating and can be remoulded repeatedly because its chains are separate and held only by weak intermolecular forces, which is what thermoplastic means and is why it is recyclable. It is also a homopolymer, since it is built from one monomer, chloroethene, so option (a) is true as well and this question turns on which classification the examiner intended.

484. Which of the following is true about pressure for an ideal gas at -273 degrees Celsius?

• A. P = 1 atm
• B. P = 2 atm
• C. P = 3 atm
• D. P = 0 atm

Explanation: Absolute zero is minus 273 degrees Celsius, and extrapolating a pressure against temperature graph for a fixed volume of ideal gas back to that point gives zero pressure, since molecular motion reaches its minimum and no collisions occur with the walls. No real gas actually reaches this, because every gas liquefies first, so the value is an extrapolation rather than an observation. It is exactly how the absolute scale is defined.

485. Which of the following is basic buffer?

• A. NH4OH/NH4Cl
• B. NaOH/NaCl
• C. NaOH/HCl
• D. H2CO3/NaHCO3

Explanation: A basic buffer needs a weak base together with the salt of that base, and ammonium hydroxide with ammonium chloride is the standard example, holding the pH around 9. Carbonic acid with sodium hydrogen carbonate is also a buffer but an acidic one, built from a weak acid and its salt. Sodium hydroxide is a strong base and is fully ionised, so it has no reservoir of un-ionised base and cannot buffer.

486. What happens when H+ is added to ammonium hydroxide and ammonium chloride buffer?

• A. More ammonium hydroxide is formed
• B. Reaction will move reverse
• C. Reaction will move forward
• D. No effect on equilibrium

Explanation: The added hydrogen ions are mopped up by the weak base ammonium hydroxide, which reacts forward to give ammonium ions and water, so the pH barely shifts. This is exactly what buffering means: the reservoir of base neutralises added acid. If alkali were added instead, the ammonium ions from the salt would supply protons to neutralise it.

487. What is general requirement for free radical substitution reaction of ethane and halogen?

• A. Low pressure
• B. Low temperature
• C. High pressure
• D. Heat and UV light

Explanation: Initiation requires the halogen molecule to split homolytically into two radicals, and only ultraviolet light or a high temperature supplies enough energy to break that bond. In the dark and in the cold, an alkane and a halogen simply do not react, which is a standard demonstration of how unreactive alkanes are. Once started, the chain propagates on its own until two radicals combine.

488. Which one of the following is not a characteristic of benzene?

• A. Aromaticity
• B. Stability
• C. To follow Huckel rule
• D. High reactivity

Explanation: The delocalised ring of six pi electrons makes benzene unusually stable, so it resists addition and reacts only under forcing conditions, preferring substitution which preserves the aromatic system. Aromaticity, stability and obedience to Huckel's 4n plus 2 rule are all genuine features. Expecting benzene to behave like a reactive alkene is precisely the misconception this question targets.

489. Correct arrangement of orbital according to size of hybridized orbitals?

• A. sp > sp2 > sp3
• B. sp3 > sp > sp2
• C. sp2 > sp > sp3
• D. sp3 > sp2 > sp

Explanation: The greater the s character, the closer the orbital is held to the nucleus and the smaller it is, and s character rises from 25 per cent in sp3 through 33 per cent in sp2 to 50 per cent in sp. So sp3 is the largest and sp the smallest. The same trend explains why an sp carbon holds its bonding electrons most tightly, making terminal alkynes weakly acidic.

490. Consider the equation H2 + O2 gives H2O. What volume of hydrogen gas is required to produce 1 mol of water at standard temperature and pressure?

• A. 11.2 dm3
• B. 22.4 dm3
• C. 10 dm3
• D. 58 dm3

Explanation: Balanced properly the equation is 2H2 plus O2 giving 2H2O, so one mole of water needs one mole of hydrogen, and one mole of any gas occupies 22.4 dm3 at STP. The figure 11.2 dm3 is half a mole and is the trap for anyone who halves the ratio the wrong way. Molar volume is independent of the identity of the gas, which is Avogadro's law.