Formula weight

Formula weight is a term used in chemistry to represent the sum of the atomic weights of the atoms in a chemical formula. It is a measure of the mass of the chemical substances and can be used in various calculations in chemistry, such as determining the proportions of elements in a compound or the molar mass of a substance. The concept of formula weight is particularly important in the study of stoichiometry, the branch of chemistry that deals with the quantitative relationships between the reactants and products in a chemical reaction.

Definition

The formula weight of a substance is calculated by adding up the atomic weights of each atom in its chemical formula. For molecular substances, the term molecular weight is often used interchangeably with formula weight, although they technically refer to different measurements. Molecular weight specifically refers to the mass of a molecule, measured in atomic mass units (amu), and is calculated based on the molecular formula of a substance. For ionic compounds, which do not have molecules as their basic units, the term formula weight is more appropriate.

Calculation

To calculate the formula weight of a compound, one must know the chemical formula of the substance and the atomic weights of the elements involved. The atomic weights of elements are determined experimentally and are available on the periodic table of elements. The formula weight is calculated by multiplying the atomic weight of each element in the formula by the number of atoms of that element present in the formula and then adding all these values together.

For example, the formula weight of water (H₂O) can be calculated as follows:

• The atomic weight of hydrogen (H) is approximately 1.008 amu.
• The atomic weight of oxygen (O) is approximately 16.00 amu.
• Water has two hydrogen atoms and one oxygen atom.
• Therefore, the formula weight of water = (2 × 1.008) + (1 × 16.00) = 18.016 amu.

Applications

Formula weight is a fundamental concept in chemistry that has various applications, including:

• Calculating the molar mass of a compound, which is essential for converting between grams and moles of a substance.
• Determining the percentage composition of elements in a compound.
• Calculating the empirical and molecular formulas of a substance from experimental data.
• Stoichiometric calculations in chemical reactions, which involve balancing chemical equations and determining the proportions of reactants and products.

See Also

• Atomic weight
• Molecular weight
• Stoichiometry
• Molar mass
• Chemical formula

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