In basic chemistry, which one of the following one mole samples is heaviest in terms of mass: water molecules, ammonia molecules, sodium atoms, or hydrogen molecules?

Introduction / Context:
This chemistry question tests your understanding of the concept of the mole and molar mass. Although one mole always represents the same number of particles, namely Avogadros number, the mass of one mole depends on the molar mass of the substance. Being able to compare molar masses quickly is a fundamental skill in stoichiometry, chemical calculations and exam problem solving.

Given Data / Assumptions:
- Four different one mole samples are given: water molecules, ammonia molecules, sodium atoms and hydrogen molecules.
- Each sample contains exactly one mole, that is the same number of particles.
- We use standard approximate atomic masses: H ≈ 1 u, O ≈ 16 u, N ≈ 14 u, Na ≈ 23 u.
- We are asked to identify which one mole sample has the greatest mass in grams.

Concept / Approach:
The key ideas are the mole concept and molar mass. One mole of any substance contains the same number of particles, but the gram mass of that mole equals the molar mass in grams per mole. Therefore, we must calculate or recall the molar masses of water, ammonia, sodium and hydrogen gas and compare them. The substance with the highest molar mass will give the heaviest one mole sample.

Step-by-Step Solution:
Step 1: Calculate molar mass of water (H2O): 2 * 1 (for two H atoms) + 16 (for one O atom) = 18 g mol^-1. Step 2: Calculate molar mass of ammonia (NH3): 14 (for N) + 3 * 1 (for three H atoms) = 17 g mol^-1. Step 3: Determine molar mass of sodium atoms (Na): approximately 23 g mol^-1 for metallic sodium. Step 4: Calculate molar mass of hydrogen molecules (H2): 2 * 1 = 2 g mol^-1. Step 5: Compare results: 18 g, 17 g, 23 g and 2 g. The largest value is 23 g mol^-1, which corresponds to one mole of sodium atoms.

Verification / Alternative check:
You can quickly verify by remembering periodic table trends. Sodium is a metal in the third period with atomic mass around 23, while the other species involve mostly light non metals (H, N, O). The molar mass of Na is clearly larger than that of typical small covalent molecules like H2O, NH3 and H2. Therefore, one mole of sodium atoms has the highest mass in grams among the given options.

Why Other Options Are Wrong:
1 mole of water (18 g) is lighter than 1 mole of sodium (23 g).
1 mole of ammonia (17 g) is also lighter than 1 mole of sodium.
1 mole of hydrogen molecules (2 g) is the lightest of all the given samples.

Common Pitfalls:
A common mistake is to assume that one mole of any substance always has the same mass because the particle count is the same. In reality, the mass depends on the atomic or molecular composition. Another error is to miscalculate molar masses by forgetting to multiply by the number of atoms in the formula. To avoid such issues, always write out the formula, multiply each atomic mass by the number of atoms and sum accurately.

Final Answer:
The heaviest one mole sample among the options is 1 mole of sodium (Na) atoms.