NDA (I) 2023 General Ability Test · Question 114 of 149

Correct — C, Avogadro. Amedeo Avogadro stated in 1811 that equal volumes of all gases, measured at the same temperature and pressure, contain an equal number of molecules (particles). This is Avogadro's law, and it is the reason the volume of a gas at fixed temperature and pressure is proportional to the number of moles present.

• (a)Charles — Charles's law relates the volume of a fixed amount of gas to its temperature at constant pressure (volume is proportional to absolute temperature). It says nothing about equal volumes holding equal numbers of particles.
• (b)Boyle — Boyle's law relates the pressure and volume of a fixed amount of gas at constant temperature (pressure is inversely proportional to volume). That is a different relationship from the equal-number-of-particles statement.
• (d)Lussac — Gay-Lussac gave the law of combining volumes of gases (and the pressure-temperature law), but the hypothesis that equal volumes contain equal numbers of molecules was Avogadro's, put forward to explain Gay-Lussac's observations.

Avogadro's law is one of the basic gas laws. It states that, at the same temperature and pressure, equal volumes of any gas contain the same number of molecules. Because the number of molecules fixes the number of moles, the volume of a gas at fixed temperature and pressure is directly proportional to the amount of gas.

The trap is to reach for the more familiar names Boyle or Charles, or to confuse Avogadro with Gay-Lussac (who studied combining volumes). Each of those laws links a different pair of quantities; only Avogadro's law connects volume with the actual number of particles.

• Avogadro's law states that equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules.
• It was proposed by the Italian scientist Amedeo Avogadro in 1811.
• As a consequence, one mole of any ideal gas occupies about 22.4 litres at standard temperature and pressure.
• Avogadro's number, about 6.022 x 10^23 particles per mole, is named in his honour.

• Confusing Avogadro's law with Gay-Lussac's law of combining volumes.
• Assuming a heavier gas packs more molecules into the same volume.

NDA states a gas law and asks who proposed it, or names the scientist and asks for the law.

No directly related past PYQ was found.

• practice — not a real PYQ

  One mole of any ideal gas at standard temperature and pressure occupies a volume of about:

  • (a)11.2 litres
  • (b)22.4 litres
  • (c)24.4 litres
  • (d)6.022 litres

  Answer(b) 22.4 litres — the molar volume at STP, a direct consequence of Avogadro's law.

• practice — not a real PYQ

  Boyle's law describes the relationship between which two properties of a fixed amount of gas?

  • (a)Volume and temperature
  • (b)Pressure and volume at constant temperature
  • (c)Pressure and temperature
  • (d)Number of moles and volume

  Answer(b) Pressure and volume at constant temperature — pressure is inversely proportional to volume.