Molecular Weight
Free Molecular Weight for chemistry. Enter variables to compute results using verified scientific formulas with step-by-step explanations.
Formula
MW = Σ(Atomic Mass × Number of Atoms)
Add the atomic masses of all atoms in the molecular formula. Multiply each element's atomic mass by its subscript count.
Worked Examples
Example 1: Calculate Water MW
Problem:Calculate the molecular weight of water (H₂O).
Solution:Hydrogen (H): 2 atoms × 1.008 = 2.016\nOxygen (O): 1 atom × 15.999 = 15.999\n\nTotal MW = 2.016 + 15.999 = 18.015 g/mol
Result:18.015 g/mol
Example 2: Glucose Composition
Problem:Find the molecular weight and percent carbon in glucose (C₆H₁₂O₆).
Solution:C: 6 × 12.011 = 72.066\nH: 12 × 1.008 = 12.096\nO: 6 × 15.999 = 95.994\n\nTotal = 180.156 g/mol\n\n%C = (72.066/180.156) × 100 = 40.0%
Result:180.156 g/mol, 40% carbon
Example 3: Sulfuric Acid
Problem:Calculate MW of sulfuric acid (H₂SO₄).
Solution:H: 2 × 1.008 = 2.016\nS: 1 × 32.065 = 32.065\nO: 4 × 15.999 = 63.996\n\nTotal = 98.077 g/mol
Result:98.08 g/mol
Frequently Asked Questions
How do I calculate molecular weight?
Add the atomic masses of all atoms in the formula. For H₂O: 2×(1.008) + 1×(15.999) = 18.015 g/mol. Use a periodic table for atomic masses.
What's the difference between molecular weight and molar mass?
They're numerically equal but have different units. Molecular weight is in atomic mass units (amu or Da), molar mass is in g/mol. For water: MW = 18.015 amu, molar mass = 18.015 g/mol.
Why is molecular weight important?
It's essential for: calculating moles from mass, stoichiometric calculations, determining solution concentrations, identifying unknown compounds, and understanding physical properties.
What's the molecular weight of proteins?
Proteins range from ~5,000 to over 1,000,000 Da. Insulin is ~5,800 Da, hemoglobin is ~64,500 Da. Calculated from amino acid sequence using residue masses.