The molecular mass of H2O (water) is calculated by summing the atomic weights of each atom in the molecule.
Understanding Atomic and Molecular Mass
- Atomic Weight: The average mass of an atom of an element, typically found on the periodic table.
- Molecular Mass: The sum of the atomic weights of all the atoms in a molecule.
Calculation Steps
- Identify the Atoms: A water molecule (H2O) consists of two hydrogen (H) atoms and one oxygen (O) atom.
- Find Atomic Weights: From the periodic table:
- Hydrogen (H) has an atomic weight of approximately 1 g/mol.
- Oxygen (O) has an atomic weight of approximately 16 g/mol.
- Calculate the Molecular Mass:
- Multiply the atomic weight of each element by the number of atoms of that element in the molecule.
- Add the results together.
Detailed Calculation
Here's how to calculate the molecular mass of H2O:
| Element | Number of Atoms | Atomic Weight (g/mol) | Contribution (g/mol) |
|---|---|---|---|
| Hydrogen | 2 | 1 | 2 x 1 = 2 |
| Oxygen | 1 | 16 | 1 x 16 = 16 |
| Total | 2 + 16 = 18 |
Therefore, the molecular mass of H2O is approximately 18 g/mol. According to Carnegie Mellon University, a more precise calculation using more accurate atomic weights yields 18.01528 g/mol. The calculation remains the same: 2(1) + 1(16) = 18 grams/mole.
