1. Understand the Concept of Molecular Mass
* Molecular Mass: The molecular mass is the sum of the atomic masses of all the atoms in a molecule. It's expressed in atomic mass units (amu).
* Atomic Mass Unit (amu): One amu is approximately equal to the mass of a proton or neutron.
2. Calculate Molecular Mass
* Use the Periodic Table: The periodic table lists the atomic masses of each element.
* Multiply by Subscripts: Look at the chemical formula of the molecule. The subscripts indicate the number of atoms of each element in the molecule. Multiply the atomic mass of each element by its subscript.
* Sum the Results: Add up the results for all the elements in the molecule.
Example: Water (H₂O)
1. Atomic Masses:
* Hydrogen (H): 1.008 amu
* Oxygen (O): 15.999 amu
2. Calculate:
* 2 Hydrogen atoms * 1.008 amu/atom = 2.016 amu
* 1 Oxygen atom * 15.999 amu/atom = 15.999 amu
* Total molecular mass = 2.016 amu + 15.999 amu = 18.015 amu
3. Convert to Grams
* Avogadro's Number: There are 6.022 x 10²³ molecules in one mole of a substance.
* Molar Mass: The molar mass is the mass of one mole of a substance, which is numerically equal to the molecular mass in grams.
* Calculation: Divide the molar mass (in grams) by Avogadro's number to find the mass of a single molecule.
Example: Water (H₂O)
* Molar Mass: 18.015 g/mol
* Mass of one water molecule: (18.015 g/mol) / (6.022 x 10²³ molecules/mol) ≈ 2.992 x 10⁻²³ g
Key Point: The mass of a single molecule is incredibly small, typically expressed in grams per molecule or in units of amu.
