The keyword “molecular weight of water” refers to the mass of one mole of water molecules. For water, the chemical formula is H2O, so the calculation uses the atomic masses of hydrogen (H) and oxygen (O) and the subscripts in the formula.
Step 1: Read the formula and count atoms
H2O contains:
- 2 hydrogen atoms
- 1 oxygen atom
Step 2: Use atomic masses (periodic table values)
Standard average atomic masses commonly used in lab calculations are:
- Hydrogen: about 1.008
- Oxygen: about 15.999
Step 3: Add the mass contributions
The molecular weight (molar mass) is the sum of each element’s atomic mass multiplied by the number of atoms in the formula:
\[ M(\mathrm{H_2O}) = 2 \cdot M(\mathrm{H}) + 1 \cdot M(\mathrm{O}) \]
\[ M(\mathrm{H_2O}) = 2 \cdot 1.008 + 15.999 = 2.016 + 15.999 = 18.015 \ \mathrm{g \cdot mol^{-1}} \]
Result: The molecular weight of water is approximately 18.015 g·mol−1 (often rounded to 18.02 g·mol−1 or 18.0 g·mol−1 depending on significant figures).
Element-by-element breakdown
| Element | Atoms in H2O | Atomic mass | Contribution to molar mass |
|---|---|---|---|
| H | 2 | 1.008 | \(2 \cdot 1.008 = 2.016\) |
| O | 1 | 15.999 | \(1 \cdot 15.999 = 15.999\) |
| Total | — | — | \(2.016 + 15.999 = 18.015\ \mathrm{g \cdot mol^{-1}}\) |
Interpretation in biology and lab calculations
Knowing the molecular weight of water supports routine conversions between mass and moles when preparing solutions, checking osmolarity calculations, or interpreting reaction stoichiometry in biochemical contexts. For example, converting a measured mass of water to moles uses:
\[ n = \frac{m}{M} \]
\[ n = \frac{36.0}{18.015} = 1.998\ \mathrm{mol} \approx 2.00\ \mathrm{mol} \]
Common pitfalls and conventions
- “Molecular weight” vs “molar mass”: in strict terminology, molecular weight (relative molecular mass) is dimensionless, while molar mass is in g·mol−1; in lab practice the terms are often used interchangeably for small molecules like water.
- Rounding: 18.015 g·mol−1 is more precise than 18.0 g·mol−1; rounding should match the precision of the atomic masses and the context (introductory vs analytical work).
- Subscripts matter: H2O has two hydrogens; forgetting the “2” produces an incorrect value near 17.0 g·mol−1.