NDA (I) 2025 General Ability Test · Question 79 of 150

Correct — D, '4 gram of hydrogen gas contains 6.022 x 10^23 molecules' is the statement that is NOT correct. Hydrogen gas is H2 with molar mass 2 g, so 4 g is 2 moles, and 2 moles hold 2 x 6.022 x 10^23 = 1.2 x 10^24 molecules — twice Avogadro's number, not one. The statement undercounts the molecules by a factor of two.

• (a)Half mole of nitrogen gas is measured 11.2 litre at STP — This is correct. One mole of any gas occupies 22.4 L at STP, so half a mole occupies 11.2 L. The statement is true and therefore not the exception.
• (b)17 gram of ammonia gas contains 6.022 x 10^23 molecules at STP — This is correct. Ammonia NH3 has molar mass 17 g, so 17 g is exactly one mole, which contains 6.022 x 10^23 molecules — Avogadro's number.
• (c)22.4 litre of CO2 gas at STP contains 44 gram of molecules — This is correct. At STP 22.4 L is one mole of gas, and one mole of CO2 (molar mass 44 g) weighs 44 g, so the statement holds.

One mole of any substance contains Avogadro's number, 6.022 x 10^23, of particles and has a mass in grams equal to its molar mass. For a gas at STP one mole also occupies 22.4 litres. These three bridges — mass, number and volume — all pass through the mole.

Each option converts between mass, volume and number, so the check is to find the number of moles first. The catch in (d) is that hydrogen gas is diatomic H2 (molar mass 2 g), so 4 g is two moles; students who treat hydrogen as atomic mass 1 or forget it is H2 wrongly get one mole.

• One mole contains 6.022 x 10^23 particles (Avogadro's number).
• Molar mass in grams equals one mole: NH3 = 17 g, CO2 = 44 g, H2 = 2 g.
• At STP one mole of any gas occupies 22.4 litres.
• 4 g of H2 = 2 moles = 1.2 x 10^24 molecules, so statement (d) is wrong.

• Treating hydrogen as atomic (1 g) instead of molecular H2 (2 g).
• Forgetting 22.4 L per mole applies only at STP.
• Mixing up number of molecules with number of atoms.

NDA/UPSC give mass, volume or molecule statements and ask which is NOT correct — always convert to moles first, watching for diatomic gases.

No directly related past PYQ was found.

• practice — not a real PYQ

  The number of molecules in 8 g of oxygen gas (O2) at STP is

  • (a)6.022 x 10^23
  • (b)1.505 x 10^23
  • (c)3.011 x 10^23
  • (d)1.2 x 10^24

  Answer(b) 1.505 x 10^23 — O2 is 32 g/mol, so 8 g = 0.25 mole = 0.25 x 6.022 x 10^23.

• practice — not a real PYQ

  What volume does 2 moles of any ideal gas occupy at STP?

  • (a)11.2 L
  • (b)22.4 L
  • (c)44.8 L
  • (d)5.6 L

  Answer(c) 44.8 L — one mole occupies 22.4 L at STP, so two moles occupy 44.8 L.