Actual Yield Calculator
Calculate actual yield from theoretical yield and percent yield. Determine real product amounts accounting for reaction efficiency and side reactions.
Molecular Weight Calculator
Calculate molecular weight from chemical formula. Supports elements, compounds, hydrates, and polymers with a full periodic table database.
Preset Compounds
Element Atom Counts
Results
Molecular Weight⧉
0.0000 g/mol
Sum of all atomic mass contributions
Total Atoms⧉
0
Number of atoms per formula unit
1 mole mass⧉
0.000 g
Mass of exactly one mole of this compound
Moles per gram⧉
N/A
Reciprocal of molecular weight
Mass per molecule⧉
N/A
Molecular weight divided by Avogadro's number
Molecules per gram⧉
N/A
Number of molecules in one gram
Planning notes, formulas, and examples
About the Molecular Weight Calculator
The molecular weight calculator determines the molar mass of any chemical compound from its elemental composition. By summing the atomic masses of all atoms in a molecular formula, you obtain the molecular weight in grams per mole (g/mol) — the mass of one mole or 6.022 × 10²³ molecules of the substance.
Molecular weight is arguably the most frequently needed value in chemistry. It appears in virtually every calculation: converting grams to moles, determining molarity, calculating yields, performing dilutions, and balancing equations quantitatively. Having an accurate molecular weight is the first step in almost any stoichiometry problem.
This calculator accepts common chemical formulas, breaks them down into their constituent elements, and computes the total molecular weight using IUPAC-recommended atomic masses. It also shows the percent composition of each element — useful for empirical formula determination and analytical chemistry.
When This Page Helps
It gives instant molecular weight and percent composition calculations for any compound. It eliminates the need to look up individual atomic masses and perform tedious summations.
How to Use the Inputs
- Enter the number of atoms for each element present in your compound.
- Alternatively, select a preset compound to auto-fill element counts.
- The calculator sums atomic masses to give the molecular weight.
- Review the percent composition breakdown for each element.
- Use the elemental analysis table for detailed mass contributions.
- Add elements as needed for complex compounds.
- Reference the common compounds table for quick lookups.
Formula used
Molecular Weight = Σ(number of atoms × atomic mass) for all elements
For example, H₂O = 2(1.008) + 1(16.00) = 18.015 g/mol
Percent Composition = (atoms × atomic mass / MW) × 100%Example Calculation
Result: 98.079 g/mol
For H₂SO₄: H = 2(1.008) = 2.016, S = 1(32.06) = 32.06, O = 4(16.00) = 64.00. Total = 2.016 + 32.06 + 64.00 = 98.079 g/mol.
Tips & Best Practices
- Always use the standard atomic weight, not the mass number, for molecular weight calculations.
- For isotopically enriched compounds, adjust individual atomic masses accordingly.
- Percent composition is useful for verifying the identity and purity of synthesized compounds.
- Remember to account for all atoms including those in polyatomic groups.
- Check your formula carefully — a single wrong subscript changes the molecular weight significantly.
- For proteins and polymers, molecular weight is often given in kilodaltons (kDa) where 1 kDa = 1000 g/mol.
Atomic Weights and the Periodic Table
Standard atomic weights are published by IUPAC and updated periodically as measurements improve. These weighted averages account for the natural isotopic distribution of each element. For example, chlorine's atomic weight is 35.45, not 35 or 37, because it's about 75% ³⁵Cl and 25% ³⁷Cl in nature.
From Molecular Weight to Empirical Formula
If you know the percent composition from elemental analysis, you can work backward to find the empirical formula. Divide each element's mass percentage by its atomic weight to get mole ratios, then reduce to the smallest whole numbers. This is the reverse of the molecular weight calculation.
Applications in Mass Spectrometry
Mass spectrometry measures molecular weight with extreme precision. The molecular ion peak in a mass spectrum corresponds to the monoisotopic mass (using the most abundant isotope of each element), which differs slightly from the standard molecular weight. High-resolution mass spectrometry can determine molecular formulas by matching exact masses to within fractions of a Dalton.
Sources & Methodology
Last updated:
Frequently Asked Questions
Molecular weight is dimensionless (relative to ¹²C), while molar mass has units of g/mol. Numerically they are identical. In practice, chemists use the terms interchangeably.
The same way as for molecular compounds: sum the atomic weights of all atoms in the formula unit. NaCl = 22.99 + 35.45 = 58.44 g/mol. For ionic compounds, we more accurately call it "formula weight."
Include the water of crystallization. For CuSO₄·5H₂O: CuSO₄ = 159.61 + 5 × 18.015 = 249.69 g/mol. Always specify whether your compound is hydrated or anhydrous.
Use the standard atomic weights from IUPAC, which represent the weighted average of all naturally occurring isotopes. These are the numbers on the periodic table, like 12.011 for carbon.
For most calculations, 2-4 decimal places suffice. For high-precision work like mass spectrometry or exact mass determination, use monoisotopic masses (mass of the most abundant isotope).
Percent composition shows the mass percentage of each element, essential for determining empirical formulas from experimental data, checking compound purity, and analytical chemistry. This keeps planning practical and lowers the chance of preventable errors.
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Convert between moles, particles, atoms, molecules, and formula units using Avogadro's number (6.022 × 10²³). Essential stoichiometry tool.