Molecular Weight Calculators

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Molecular weight — also called molecular mass — is the sum of the atomic masses of all atoms in a molecule, expressed in grams per mole (g/mol) or Daltons (Da). It's one of the most fundamental values in chemistry and biochemistry: you need it to prepare molar solutions, calculate enzyme units, determine protein size, and convert between mass and moles. From a simple salt solution to a multi-subunit protein complex, molecular weight underpins nearly every quantitative calculation in the lab.

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What Is Molecular Weight?

Molecular weight (MW) is the mass of one mole of a substance, expressed in grams per mole (g/mol). It is calculated by summing the atomic masses of every atom in the molecule, using values from the periodic table. For example, water (H₂O) has a molecular weight of (2 × 1.008) + 15.999 = 18.015 g/mol.

In biochemistry, molecular weight is often expressed in Daltons (Da) or kilodaltons (kDa), where 1 Da = 1 g/mol. A small protein like insulin has a molecular weight of ~5.8 kDa; a large protein like titin reaches ~3,700 kDa.

Molecular Weight vs. Molecular Mass vs. Molar Mass

These terms are often used interchangeably, but they have subtle differences:

  • Molecular mass: The mass of a single molecule, expressed in Daltons or atomic mass units (u)
  • Molar mass: The mass of one mole (6.022 × 10²³ molecules) of a substance, expressed in g/mol
  • Molecular weight: Technically a dimensionless ratio (mass of the molecule relative to 1/12 the mass of carbon-12), but in practice used identically to molar mass

In everyday lab usage, all three terms are treated as equivalent and expressed in g/mol or Da.

How to Calculate Molecular Weight

To calculate MW manually:

  1. Write out the molecular formula (e.g., glucose = C₆H₁₂O₆)
  2. Multiply the atomic mass of each element by its count in the formula
  3. Sum all values

For glucose: (6 × 12.011) + (12 × 1.008) + (6 × 15.999) = 72.066 + 12.096 + 95.994 = 180.156 g/mol

Molecular Weight in the Lab

Preparing Molar Solutions

To make a 1 M solution of a compound, dissolve 1 mole (= MW in grams) in 1 liter of solvent. For NaCl (MW = 58.44 g/mol): dissolve 58.44 g in water and bring to 1 L total volume.

Protein Molecular Weight

Protein MW is estimated from amino acid sequence (average ~110 Da per residue) or measured experimentally by SDS-PAGE (comparing migration to a protein ladder) or mass spectrometry. The MW determines how proteins migrate in gels and is essential for calculating molar concentrations from absorbance readings.

Average vs. Monoisotopic Mass

For small molecules in mass spectrometry, a distinction is made between average molecular weight (using natural isotope abundance averages) and monoisotopic mass (using the most abundant isotope of each element). For routine lab calculations, average MW is used; for high-resolution mass spec analysis, monoisotopic mass is required.

Molecular Weight of Common Biological Molecules

  • Water (H₂O): 18.02 g/mol
  • Glucose (C₆H₁₂O₆): 180.16 g/mol
  • ATP: 507.18 g/mol
  • Average amino acid: ~110 Da
  • Average nucleotide (DNA): ~330 Da
  • Hemoglobin (tetramer): ~64,500 Da (64.5 kDa)

Glossary

Molecular Weight (MW)
The sum of atomic masses of all atoms in a molecule, expressed in g/mol or Daltons (Da). Used in the lab to calculate molar concentrations, solution preparation, and protein quantification.
Dalton (Da)
A unit of molecular mass equal to 1 g/mol, defined as 1/12 the mass of a carbon-12 atom. Commonly used for proteins and nucleic acids; 1 kDa = 1,000 Da.
Molar Mass
The mass of one mole (6.022 × 10²³ particles) of a substance, expressed in g/mol. Numerically equal to molecular weight and used to convert between grams and moles.

Frequently Asked Questions

In practice, molecular weight and molar mass are used interchangeably and both expressed in g/mol or Da. Technically, molecular weight is a dimensionless ratio relative to 1/12 the mass of a carbon-12 atom, while molar mass is the mass of one mole of a substance in g/mol. For all routine lab calculations, the distinction is irrelevant.

Multiply the atomic mass of each element by the number of that element's atoms in the formula, then sum all values. For example, NaCl: Na (22.99) + Cl (35.45) = 58.44 g/mol. Atomic masses are found on the periodic table.

A Dalton (Da) is a unit of molecular mass equal to 1 g/mol, or 1/12 the mass of a carbon-12 atom. It's commonly used in biochemistry for proteins and nucleic acids. Kilodalton (kDa) = 1,000 Da. A typical protein might be 50 kDa; a DNA plasmid could be several thousand kDa.

To make a 1 molar (1 M) solution, dissolve a mass equal to the molecular weight (in grams) in enough solvent to make 1 liter of solution. For other concentrations, use: mass (g) = molarity (mol/L) × volume (L) × molecular weight (g/mol).