In stoichiometry, one mole of water (H2O) corresponds to what mass of water at the macroscopic scale?

Introduction / Context:
The concept of the mole links the microscopic world of atoms and molecules to the macroscopic world of grams and laboratory measurements. Knowing the molar mass of common substances such as water is essential for solving stoichiometry problems, preparing solutions and understanding chemical equations. This question asks for the mass of one mole of water, H2O, which is a basic but very important numerical fact in introductory chemistry.

Given Data / Assumptions:

• Chemical formula of water: H2O.
• Approximate atomic mass of hydrogen is 1 gram per mole.
• Approximate atomic mass of oxygen is 16 grams per mole.
• We are asked for the molar mass of water expressed in grams per mole, or equivalently the mass of one mole of water in grams.

Concept / Approach:
The molar mass of a compound is obtained by adding the molar masses of all the atoms in its formula. For water, each molecule contains two hydrogen atoms and one oxygen atom. Therefore, we multiply the molar mass of hydrogen by two and add the molar mass of oxygen. The result gives the mass of Avogadro number of water molecules. The approach is a straightforward application of formula mass calculation using atomic masses from the periodic table.

Step-by-Step Solution:
Step 1: Note that the formula H2O indicates two hydrogen atoms and one oxygen atom in each molecule. Step 2: Take the approximate molar mass of hydrogen as 1 gram per mole. Step 3: Take the approximate molar mass of oxygen as 16 grams per mole. Step 4: Compute the molar mass of water: (2 * 1) + 16 = 2 + 16 = 18 grams per mole. Step 5: Therefore, one mole of H2O corresponds to a mass of 18 grams of water.

Verification / Alternative check:
Periodic tables often list the atomic mass of hydrogen as about 1.008 and oxygen as about 16.00. Using these more precise values, the molar mass of water is approximately (2 * 1.008) + 16.00 = 18.016 grams per mole, which is usually rounded to 18 grams per mole in introductory problems. This confirms that 18 grams is the correct value to use for one mole of water. Many textbooks and exam papers explicitly quote this value in example calculations, further reinforcing it as standard knowledge.

Why Other Options Are Wrong:
Option A, 16 grams, would correspond more closely to the molar mass of oxygen alone, not water. Option C, 20 grams, and option D, 22 grams, are higher than the correct value and do not match the sum of atomic masses in H2O. Option E, 24 grams, might suggest two oxygen atoms and two hydrogen atoms, which is not the composition of water. Only option B, 18 grams of H2O, correctly matches the calculation based on 2 hydrogens and 1 oxygen per molecule.

Common Pitfalls:
Students sometimes misread the formula and treat water as HO instead of H2O, leading to incorrect mass calculations. Another common mistake is to forget to multiply the hydrogen atomic mass by two. Additionally, some learners confuse molecular mass with formula mass or misapply atomic masses from memory. To avoid errors, always write down the formula, list each element with its subscript, multiply atomic masses by the subscripts and then sum the results. This systematic approach works for any compound, not just water.

Final Answer:
One mole of water corresponds to a mass of 18 grams of H2O under standard molar mass conventions.