Molecular Weight Calculator

What is Molecular Weight?

Molecular weight, also known as molecular mass, is the sum of the atomic weights of all atoms in a molecule. It is a fundamental property of a chemical compound that determines many of its physical and chemical characteristics. Molecular weight is typically expressed in atomic mass units (amu) or grams per mole (g/mol).

Formula

The general formula for calculating molecular weight is:

\[ MW = \sum_{i=1}^{n} (AW_i \times N_i) \]

Where:

• \(MW\) is the molecular weight
• \(AW_i\) is the atomic weight of element \(i\)
• \(N_i\) is the number of atoms of element \(i\) in the molecule
• \(n\) is the total number of different elements in the molecule

Calculation Steps

Let's calculate the molecular weight of water (H₂O):

1. Identify the chemical formula: H₂O
2. Determine atomic weights: H = 1.008 g/mol, O = 15.999 g/mol
3. Apply the formula: \[ MW_{H_2O} = (AW_H \times 2) + (AW_O \times 1) \]
4. Calculate: \[ MW_{H_2O} = (1.008 \times 2) + (15.999 \times 1) \] \[ MW_{H_2O} = 2.016 + 15.999 = 18.015 \text{ g/mol} \]

Example and Visual Representation

Let's visualize the molecular weight calculation for methane (CH₄):

This diagram illustrates:

• The structure of a methane molecule (CH₄)
• One carbon atom (black) bonded to four hydrogen atoms (blue)
• The calculation of molecular weight by summing the atomic weights
• Carbon contributes 12.011 g/mol, and the four hydrogen atoms contribute 4.032 g/mol
• The total molecular weight of methane is 16.043 g/mol