1. Identify the Elements and Their Quantities:
* From the Chemical Formula: Look at the chemical formula of the compound. It tells you which elements are present and how many atoms of each element are in one molecule.
* For example, H₂O (water) contains 2 hydrogen atoms (H) and 1 oxygen atom (O).
2. Look Up Atomic Weights:
* Periodic Table: Use the periodic table to find the atomic weight (also called atomic mass) of each element in the compound. Atomic weights are usually listed below the element's symbol.
* Example: The atomic weight of hydrogen (H) is approximately 1.008 amu (atomic mass units). The atomic weight of oxygen (O) is approximately 15.999 amu.
3. Multiply Atomic Weights by Subscripts:
* Chemical Formula: Multiply the atomic weight of each element by its subscript (the small number written below and to the right of the element symbol) in the chemical formula.
* Example: In H₂O, you have:
* 2 hydrogen atoms * 1.008 amu/hydrogen atom = 2.016 amu
* 1 oxygen atom * 15.999 amu/oxygen atom = 15.999 amu
4. Add the Results Together:
* Formula Weight: Add up the results from step 3 to get the formula weight of the compound.
* Example: 2.016 amu + 15.999 amu = 18.015 amu (approximately)
Therefore, the formula weight of water (H₂O) is approximately 18.015 amu.
Important Note: The term "molecular weight" is specifically used for covalent compounds (those formed by sharing electrons). "Formula weight" is a more general term that can be used for both covalent and ionic compounds.
