How to Calculate Formula Weight of an Unknown Compound?

The formula weight of an unknown compound is calculated by summing the atomic masses of all the atoms in the compound's formula.

Steps to Calculate Formula Weight

Here's a breakdown of how to determine the formula weight of an unknown compound:

1. Identify the Elements: Determine all the elements present in the compound.
2. Get Atomic Masses: For each element, find its atomic mass on the periodic table. The atomic mass is usually found below the element's symbol.
3. Count Atoms: Identify how many atoms of each element are present in the compound's chemical formula.
4. Multiply and Sum: Multiply the atomic mass of each element by the number of atoms of that element in the compound. Then, add up all the results to get the total formula weight.

Example

Let's calculate the formula weight of water (H₂O).

• Elements: Hydrogen (H) and Oxygen (O)
• Atomic Masses:
  • Hydrogen (H): 1.008 atomic mass units (amu)
  • Oxygen (O): 16.00 amu
• Number of Atoms:
  • Hydrogen (H): 2
  • Oxygen (O): 1
• Calculation:
  • (2 x 1.008 amu) + (1 x 16.00 amu) = 2.016 amu + 16.00 amu = 18.016 amu

Therefore, the formula weight of water is approximately 18.016 amu.

Summary of Calculation

To calculate the formula weight for a compound: select the atomic mass of each of the elements in the compound from the periodic table, multiply by the number of atoms, and sum the result.

Formula Weight in Practice

• Formula weight, also known as molecular weight (for molecules), is crucial for various chemical calculations, such as stoichiometry and determining mole quantities.
• The units are typically expressed in atomic mass units (amu) for single molecules or grams per mole (g/mol) for a mole of the substance.

By following these steps, you can accurately calculate the formula weight of any unknown compound.