1. What is Molecular Weight?

Molecular weight (MW), also called molecular mass, is the sum of the atomic weights of all atoms in a molecule. It's expressed in atomic mass units (u) or grams per mole (g/mol).

2. How is Molecular Weight Calculated?

The molecular weight is calculated using the formula:

Where:

• \( MW \) — Molecular weight (g/mol)
• \( atomic\_masses \) — Atomic weights of each element (g/mol)
• \( counts \) — Number of atoms of each element in the molecule (unitless)

Example: For water (H₂O):

• 2 Hydrogen atoms: 2 × 1.008 g/mol = 2.016 g/mol
• 1 Oxygen atom: 1 × 15.999 g/mol = 15.999 g/mol
• Total MW = 2.016 + 15.999 = 18.015 g/mol

3. Importance of Molecular Weight

Details: Molecular weight is crucial for stoichiometric calculations, preparing molar solutions, determining physical properties, and in analytical techniques like mass spectrometry.

4. Using the Calculator

Tips: Enter elements and their counts in the format "ElementCount" separated by spaces (e.g., "H2 O1" for water). Element symbols are case-sensitive (first letter capitalized).

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between molecular weight and molar mass?
A: They are numerically identical but molar mass is the mass of one mole of substance (g/mol), while molecular weight is dimensionless (though often expressed in g/mol).

Q2: How accurate are these calculations?
A: Accuracy depends on the precision of the atomic weights used. For most purposes, 2-4 decimal places are sufficient.

Q3: How do I calculate MW for ionic compounds?
A: The same way - sum the atomic masses of all atoms in the formula unit (e.g., NaCl = Na + Cl).

Q4: What about hydrated compounds?
A: Include water molecules in the calculation (e.g., CuSO₄·5H₂O would include 5 water molecules).

Q5: Where can I find accurate atomic weights?
A: The IUPAC publishes standard atomic weights which are updated periodically.