All Free Chemistry MCQs with Answers

11. The lines of the Balmer series in the hydrogen spectrum are produced when an electron falls to which energy level?

• A. n = 1
• B. n = 2
• C. n = 3
• D. n = 4

Explanation: The Balmer series arises from transitions ending at n equal to 2 and it is the only hydrogen series that falls in the visible region, which is why it was discovered first. Transitions ending at n equal to 1 give the Lyman series in the ultraviolet, and those ending at n equal to 3 give the Paschen series in the infrared.

12. The line spectrum of hydrogen provides direct evidence that

• A. electrons can have any energy value
• B. electron energies inside an atom are quantised
• C. the nucleus contains neutrons
• D. hydrogen atoms contain more than one electron

Explanation: A hot gas emits light only at particular wavelengths, producing separate bright lines instead of a continuous band, and each line corresponds to one fixed energy difference between two levels. If electrons could take any energy the spectrum would be continuous. Nothing in the spectrum speaks to the neutron, and hydrogen has exactly one electron.

13. Positive rays were used in the discovery of the proton. These rays were produced from

• A. the cathode of the discharge tube
• B. the residual gas atoms losing electrons in a discharge tube
• C. the radioactive decay of uranium
• D. the anode metal evaporating

Explanation: Goldstein observed that cathode rays travelling through a perforated cathode knocked electrons off the residual gas atoms, leaving positive ions that streamed towards the cathode as canal rays. Their charge to mass ratio depended on which gas was in the tube, and it was largest for hydrogen, identifying the proton. Cathode rays themselves are electrons and come from the cathode.

14. Which orbital is filled immediately after the 4s orbital according to the Aufbau principle?

• A. 4p
• B. 3d
• C. 4d
• D. 5s

Explanation: Orbitals fill in order of increasing n plus l, and where that sum ties the lower n goes first. For 3d the sum is 5 and for 4p it is also 5, but 3d has the lower principal quantum number and so fills first, which is why the first transition series follows calcium. The 4p orbitals are filled only after the 3d subshell is complete at zinc.

15. Boyle's law states that at constant temperature, the volume of a fixed mass of gas is

• A. directly proportional to its pressure
• B. inversely proportional to its pressure
• C. independent of its pressure
• D. proportional to the square of its pressure

Explanation: Boyle's law gives PV equal to a constant, so doubling the pressure halves the volume. A graph of P against V is a hyperbola, while a graph of P against 1 over V is a straight line through the origin, and that straight line is the usual way the law is confirmed experimentally. Volume rising with pressure would describe no real gas.

16. A gas occupies 500 cm3 at 2 atm. At constant temperature, what volume will it occupy at 5 atm?

• A. 200 cm3
• B. 250 cm3
• C. 1250 cm3
• D. 100 cm3

Explanation: Applying P1V1 equal to P2V2 gives V2 equal to 2 times 500 divided by 5, which is 200 cm3. The pressure rose by a factor of 2.5, so the volume must fall by the same factor, and 500 divided by 2.5 is 200. Getting 1250 means the ratio was applied the wrong way round.

17. Standard temperature and pressure (STP) is defined as

• A. 25 degrees C and 1 atm
• B. 0 degrees C and 1 atm
• C. 0 K and 1 atm
• D. 25 degrees C and 760 mm of mercury above atmospheric

Explanation: STP is 273.15 K, which is 0 degrees Celsius, at a pressure of 1 atmosphere or 760 mm of mercury. One mole of any ideal gas occupies 22.4 dm3 under these conditions, which is the molar volume used throughout stoichiometry. The value of 25 degrees Celsius belongs to standard state conditions used in thermochemistry, a different convention.

18. Charles's law predicts that a gas cooled at constant pressure would reach zero volume at

• A. 0 degrees C
• B. -100 degrees C
• C. -273.15 degrees C
• D. -373 degrees C

Explanation: Extrapolating the straight line of volume against temperature backwards, every gas reaches zero volume at minus 273.15 degrees Celsius, which is defined as absolute zero or 0 K. The extrapolation is theoretical, since every real gas liquefies well before that point. This is precisely why the Kelvin scale exists and why gas law calculations must use it.

19. A gas at 27 degrees C is heated at constant pressure until its volume doubles. Its final temperature is

• A. 54 degrees C
• B. 300 degrees C
• C. 327 degrees C
• D. 600 degrees C

Explanation: Charles's law must be applied in kelvin, so the initial temperature is 27 plus 273, which is 300 K. Doubling the volume doubles the absolute temperature to 600 K, and converting back gives 600 minus 273, which is 327 degrees Celsius. Doubling 27 to get 54 is the standard mistake and comes from using Celsius directly.

20. Which assumption is made by the kinetic molecular theory of an ideal gas?

• A. Gas molecules attract one another strongly
• B. The volume of the molecules themselves is negligible compared with the volume of the container
• C. Collisions between molecules lose energy as heat
• D. The molecules all move at the same speed

Explanation: The ideal model treats molecules as point masses with negligible volume and no intermolecular forces, colliding perfectly elastically so no kinetic energy is lost. Molecular speeds are not uniform but spread over the Maxwell-Boltzmann distribution, with only the average kinetic energy fixed by temperature. Real gases deviate exactly where these assumptions fail.