In chemistry, FW stands for formula weight, which is the sum of the atomic weights of all atoms present in a given chemical formula. This term is particularly useful for substances that do not exist as individual molecules, such as ionic compounds.
Understanding Formula Weight
Definition
Formula weight (FW) represents the total mass of all atoms in a chemical formula. Unlike molecular weight, which specifically applies to molecules, formula weight is generally used for:
- Ionic compounds (e.g., sodium chloride, NaCl)
- Other non-molecular substances (e.g., network solids)
Calculation
To determine the formula weight, you add the atomic weights of each atom in the chemical formula. You can find the atomic weight of each element on the periodic table.
Example 1: Sodium Chloride (NaCl)
| Atom | Atomic Weight (amu) | Number of Atoms | Total Weight (amu) |
|---|---|---|---|
| Sodium (Na) | 22.99 | 1 | 22.99 |
| Chlorine (Cl) | 35.45 | 1 | 35.45 |
| Total | 58.44 |
Formula weight of NaCl = 22.99 amu (Na) + 35.45 amu (Cl) = 58.44 amu.
Example 2: Calcium Chloride (CaCl2)
| Atom | Atomic Weight (amu) | Number of Atoms | Total Weight (amu) |
|---|---|---|---|
| Calcium (Ca) | 40.08 | 1 | 40.08 |
| Chlorine (Cl) | 35.45 | 2 | 70.90 |
| Total | 110.98 |
Formula weight of CaCl2 = 40.08 amu (Ca) + (2 x 35.45 amu) (Cl) = 110.98 amu.
Key Points
- Formula weight is expressed in atomic mass units (amu).
- It helps to quantify the mass of compounds, crucial in chemical calculations.
- The formula weight concept can be extended to any chemical formula, including molecular formulas (although the term "molecular weight" is more appropriate in that context).
- When you are given the molar mass, formula mass and molecular mass can be expressed as grams per mole.
Significance in Chemistry
- Stoichiometry: Formula weight is vital in stoichiometric calculations, allowing chemists to determine the amount of reactants needed and products produced in a chemical reaction.
- Solution Preparation: It is essential when calculating molar concentrations of solutions, which require knowing the amount of solute present.
- General Chemical Calculations: Understanding formula weight is foundational for numerous quantitative analyses and procedures in chemistry.
Formula weight provides a fundamental measurement that is widely applicable in the field of chemistry.
