Molecular mass, also known as molecular weight, is determined by summing the atomic masses of all atoms present in a molecule.
Steps to Calculate Molecular Mass
- Identify the Formula: Determine the chemical formula of the molecule.
- Find Atomic Masses: Use the periodic table to find the atomic mass of each element in the formula.
- Calculate the Total: Multiply the atomic mass of each element by the number of atoms of that element in the molecule, then add these values together.
Example Calculation: Water (H₂O)
| Element | Atomic Mass (amu) | Number of Atoms | Total Mass (amu) |
|---|---|---|---|
| Hydrogen | 1.008 | 2 | 2.016 |
| Oxygen | 15.999 | 1 | 15.999 |
| Total | 18.015 |
Explanation
- Hydrogen (H): The atomic mass of hydrogen is approximately 1.008 amu. In water, there are two hydrogen atoms, so we multiply 1.008 by 2, resulting in 2.016 amu.
- Oxygen (O): The atomic mass of oxygen is approximately 15.999 amu. There is one oxygen atom in water, contributing 15.999 amu.
- Adding the Masses: We add the total mass of hydrogen (2.016 amu) and oxygen (15.999 amu) to get the molecular mass of water, which is 18.015 amu.
Additional Tips
- Use Accurate Values: Always refer to a reliable periodic table for the most accurate atomic masses.
- Units: Molecular mass is typically expressed in atomic mass units (amu) or Daltons (Da).
- For complex molecules: You can break down the calculation into smaller parts for easier computation.
Reference: You can find the atomic mass of each element on the periodic table, and you will add the atomic masses of the elements that constitute the molecule to calculate the molecular mass.
