Molecular Weight

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 Oxygen (O): 1 atom × 15.999 = 15.999 Total 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 H: 12 × 1.008 = 12.096 O: 6 × 15.999 = 95.994 Total = 180.156 g/mol %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 S: 1 × 32.065 = 32.065 O: 4 × 15.999 = 63.996 Total = 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.

How accurate are calculated molecular weights?

Very accurate for exact isotope composition. Natural samples vary slightly due to isotope ratios. Carbon's average mass (12.011) accounts for ~99% C-12 and ~1% C-13.

What is formula weight vs molecular weight?

Molecular weight applies to covalent molecules. Formula weight applies to ionic compounds (like NaCl) which don't form discrete molecules. Numerically calculated the same way.

Background & Theory

Molecular weight (MW) is the sum of atomic weights of all atoms in a molecule. It's fundamental to chemistry calculations and essential for converting between mass and moles. **Formula:** MW = Σ(atomic mass × number of atoms) **Units:** - Atomic mass units (amu or u) - Daltons (Da) - same as amu - g/mol (numerically equal) **Key Relationships:** - Moles = mass (g) / MW - Mass = moles × MW - Molecules = moles × 6.022×10²³ **Common Atomic Masses:** | Element | Symbol | Mass | |---------|--------|------| | Hydrogen | H | 1.008 | | Carbon | C | 12.011 | | Nitrogen | N | 14.007 | | Oxygen | O | 15.999 | | Sulfur | S | 32.065 | | Chlorine | Cl | 35.453 | | Sodium | Na | 22.990 | **Percent Composition:** %Element = (element mass in formula / MW) × 100 **Applications:** - Stoichiometric calculations - Solution preparation - Mass spectrometry interpretation - Empirical formula determination

History

The concept of molecular weight emerged in the early 19th century alongside atomic theory. John Dalton proposed that elements consist of atoms with specific weights in 1803, laying the groundwork for molecular calculations. Amedeo Avogadro's 1811 hypothesis—that equal volumes of gases contain equal numbers of molecules—was crucial but wasn't widely accepted until Stanislao Cannizzaro championed it in 1860. This allowed chemists to determine relative molecular weights from gas densities. Determining accurate atomic weights was a 19th-century obsession. Jöns Jacob Berzelius and others meticulously measured combining ratios in thousands of reactions. Their work established the atomic weight scale, anchored initially to hydrogen (=1) and later to oxygen (=16) and finally carbon-12 (=12). Modern mass spectrometry, developed in the early 20th century, revolutionized molecular weight determination. Today's instruments can measure masses to parts per million accuracy and identify isotopic patterns that reveal molecular formulas.