At normal temperature and pressure (NTP), what is the mass (in grams) of 56 litres of ammonia gas (NH3)? Show understanding of molar volume and molar mass before selecting the value.

Introduction / Context:
Gas stoichiometry questions at normal temperature and pressure (NTP) test your ability to combine the molar volume concept with molar mass. At NTP (or STP in many texts), one mole of an ideal gas occupies a standard molar volume, which lets us convert from a measured volume to moles and then to mass for any gas, including ammonia (NH3).

Given Data / Assumptions:

• NTP molar volume = 22.4 L per mol (ideal-gas assumption).
• Given gas volume of NH3 = 56 L.
• Molar mass of NH3 = 14 (N) + 3*1 (H) = 17 g/mol.
• Gas behaves ideally for this calculation, which is the standard exam convention.

Concept / Approach:
Use two conversions in sequence: volume → moles using the molar volume; then moles → mass using the molar mass. Keep units explicit to avoid errors and round only at the end.

Step-by-Step Solution:

Verification / Alternative check:
You can scale from the known: 22.4 L weighs 17 g. 44.8 L would weigh 34 g; add another 11.2 L (half of 22.4 L) which weighs 8.5 g. Total 34 + 8.5 = 42.5 g, confirming the result without calculators.

Why Other Options Are Wrong:

• 2.5 g: Confuses moles (2.5 mol) with grams.
• 56 g: Treats density as 1 g/L, which is not valid for NH3.
• 2800 g: An off-by-two-orders-of-magnitude error.
• 17 g: Mass of 22.4 L, not 56 L.

Common Pitfalls:
Using the wrong molar volume, ignoring units, or mixing up moles and grams. Always convert volumes to moles before applying molar mass.

Final Answer:
42.5 g