The phrase molecular weight of h2o refers to the mass of one mole of water molecules, commonly called the molar mass or formula mass of \(\mathrm{H_2O}\).
Direct result: \(M(\mathrm{H_2O}) = 18.016\ \text{g/mol} \approx 18.02\ \text{g/mol}\) using \(M(\mathrm{H}) = 1.008\ \text{g/mol}\) and \(M(\mathrm{O}) = 16.00\ \text{g/mol}\).
Step 1: Read the formula \(\mathrm{H_2O}\)
The subscript “2” means each water molecule contains:
- 2 hydrogen atoms (\(\mathrm{H}\))
- 1 oxygen atom (\(\mathrm{O}\))
Step 2: Use atomic masses from the periodic table
A standard set of rounded atomic masses used in general chemistry is:
- \(M(\mathrm{H}) = 1.008\ \text{g/mol}\)
- \(M(\mathrm{O}) = 16.00\ \text{g/mol}\)
Step 3: Add contributions to get the molecular weight of \(\mathrm{H_2O}\)
Multiply each atomic mass by how many atoms appear in the formula, then add:
\[ M(\mathrm{H_2O}) = 2\cdot M(\mathrm{H}) + 1\cdot M(\mathrm{O}) \]
\[ M(\mathrm{H_2O}) = 2\cdot 1.008 + 16.00 = 2.016 + 16.00 = 18.016\ \text{g/mol} \]
| Element | Count in \(\mathrm{H_2O}\) | Atomic mass (g/mol) | Contribution (g/mol) |
|---|---|---|---|
| Hydrogen (H) | \(2\) | \(1.008\) | \(2\cdot 1.008 = 2.016\) |
| Oxygen (O) | \(1\) | \(16.00\) | \(1\cdot 16.00 = 16.00\) |
| Total | \(18.016\ \text{g/mol}\) |
Step 4: Use the result in mole–mass conversions
The molar mass connects mass and amount of substance:
\[ n = \frac{m}{M} \quad\text{and}\quad m = n\cdot M \]
Example: the number of moles in \(36.0\ \text{g}\) of water is
\[ n = \frac{36.0}{18.016} = 1.998 \approx 2.00\ \text{mol} \]
Terminology note
“Molecular weight of h2o” is commonly used in place of “molar mass of \(\mathrm{H_2O}\).” In general chemistry calculations, the numerical value is the same, and the standard unit is \(\text{g/mol}\).