The mass of 0.5 mole of N₂ gas is
- (a)7 g
- (b)14 g
- (c)21 g
- (d)28 g
Correct — B, 14 g. Mass = number of moles x molar mass. A nitrogen molecule N2 has two nitrogen atoms, so its molar mass is 2 x 14 = 28 g/mol. Therefore 0.5 mole weighs 0.5 x 28 = 14 g.
- (a)7 g — This uses the atomic mass of nitrogen (14) instead of the molecular mass of N2 (28): 0.5 x 14 = 7 g. But the gas is diatomic, so the molar mass is 28, not 14.
- (c)21 g — 21 g would be 0.75 mole of N2 (0.75 x 28), not 0.5 mole — it does not match the given quantity.
- (d)28 g — 28 g is the mass of one full mole of N2, but the question asks for only half a mole.
The mole links the number of particles to a measurable mass through the molar mass. For any substance, mass (in grams) = number of moles x molar mass (in g/mol). Nitrogen gas is diatomic, so one mole of N2 weighs 28 g (two nitrogen atoms of 14 each).
The classic trap is to use the atomic mass (14) rather than the molecular mass (28), which gives the wrong 7 g. The other wrong answer, 28 g, is the mass of a whole mole. Keep the diatomic nature of N2 in mind and simply halve 28.
- mass (g) = number of moles x molar mass (g/mol).
- Molar mass of N2 = 28 g/mol (nitrogen atomic mass 14 x 2).
- 0.5 mole of N2 = 0.5 x 28 = 14 g.
- One mole of any substance contains 6.022 x 10^23 particles (Avogadro's number).
Use the molecular mass (28), not the atomic mass (14) — half a mole is 14 g.
- Using the atomic mass 14 instead of the molecular mass 28 (gives the wrong 7 g).
- Giving the one-mole mass (28 g) when only half a mole is asked.
Asked as 'mass of X moles of a gas/compound' or, in reverse, 'number of molecules in X g'.
Which one of the following is the correct sequence in increasing order of molecular weights of the hydrocarbons?
- (a) Methane, ethane, propane and butane
- (b) Propane, butane, ethane and methane
- (c) Butane, ethane, propane and methane
- (d) Butane, propane, ethane and methane
Answer(a) Methane, ethane, propane and butane
Same underlying idea — molar (molecular) mass. There the task is to order alkanes by molecular weight; here it is to convert 0.5 mole of N2 to grams using its molar mass, so both reward a firm grip on how molar mass is built from atomic masses.
- practice — not a real PYQ
The number of moles present in 44 g of CO2 is: (molar mass of CO2 = 44 g/mol)
- (a)0.5
- (b)1
- (c)2
- (d)44
Answer(b) 1 — moles = mass / molar mass = 44 / 44 = 1 mole.
- practice — not a real PYQ
The mass of 2 moles of water (H2O) is: (molar mass = 18 g/mol)
- (a)9 g
- (b)18 g
- (c)36 g
- (d)54 g
Answer(c) 36 g — mass = 2 x 18 = 36 g.