Molecular Weight Calculator
Calculate Molar Mass from Chemical Formula - IUPAC Standard Atomic Weights
⚛️ Pro Tips for Molecular Weight Calculations:
Chemical formula is case-sensitive (H2O not h2o). Include all atoms including hydrogen. Use subscripts correctly: CO2 has 3 atoms. Use IUPAC standard atomic weights for accuracy. Round final answer appropriately. Verify by checking known compounds. Account for parentheses: Ca(OH)2 is Ca + 2O + 2H.
Calculate Molecular Weight from Formula
✅ Molecular Weight Results:
Common Chemicals & Molecular Weights
Periodic Table & Atomic Weights
IUPAC Standard Atomic Weights (2023):
These are internationally recognized standard atomic weights with uncertainty values in parentheses.
| Element | Symbol | Atomic # | Atomic Weight (g/mol) |
|---|
Molecular Weight Reference & Examples
Calculation Examples:
| Compound | Formula | MW (g/mol) | Calculation |
|---|---|---|---|
| Water | H₂O | 18.02 | (1.008×2) + 15.999 |
| Sodium Chloride | NaCl | 58.44 | 22.990 + 35.45 |
| Glucose | C₆H₁₂O₆ | 180.16 | (12.01×6) + (1.008×12) + (16.00×6) |
| Sulfuric Acid | H₂SO₄ | 98.08 | (1.008×2) + 32.065 + (16.00×4) |
| Methane | CH₄ | 16.04 | 12.01 + (1.008×4) |
| Calcium Hydroxide | Ca(OH)₂ | 74.09 | 40.08 + (16.00×2) + (1.008×2) |
Key Concepts:
Relative Atomic Mass: Weighted average of all isotopes. Accounts for natural abundance. Used in standard atomic weights. Not to be confused with mass of single atom.
Molar Mass vs Molecular Mass: Molar mass = g/mol. Molecular mass = daltons. Numerically equal but different units. One mole contains Avogadro's number of particles.
Isotope Consideration: Most calculations use standard (average) atomic weights. Purified isotopes give different values. Specify isotope if using non-standard material.
Significant Figures: IUPAC weights typically 4-5 significant figures. Final answer: 3-4 decimal places. Match precision to input data.
Molar Mass vs Molecular Mass: Molar mass = g/mol. Molecular mass = daltons. Numerically equal but different units. One mole contains Avogadro's number of particles.
Isotope Consideration: Most calculations use standard (average) atomic weights. Purified isotopes give different values. Specify isotope if using non-standard material.
Significant Figures: IUPAC weights typically 4-5 significant figures. Final answer: 3-4 decimal places. Match precision to input data.
🧪 Understanding Molecular Weight & Molar Mass
What is Molecular Weight?:
Molecular weight (MW), also called molar mass, is the sum of atomic weights of all atoms in molecule. Expressed in g/mol (grams per mole). Calculated using IUPAC standard atomic weights including isotope distributions. Fundamental concept for chemical calculations, stoichiometry, and laboratory preparations.
Key Relationships:- Atomic Weight: Weighted average of naturally occurring isotopes
- Molecular Weight: Sum of atomic weights in molecule
- Molar Mass: Mass of one mole (numerically equals MW)
- Avogadro's Number: 6.022 × 10²³ particles per mole
- Identify correct chemical formula
- Count atoms of each element
- Look up atomic weight for each element
- Multiply atomic weight by atom count
- Sum all contributions
- Express result in g/mol with appropriate precision
- Chemical Reactions: Calculate reagent quantities from stoichiometry
- Solution Preparation: Determine mass needed for specific molarity
- Laboratory Analysis: Convert between moles and grams
- Pharmaceutical: Calculate drug dosages and concentrations
- Industrial: Scale reactions for production
❓ Frequently Asked Questions
How do you calculate molecular weight of H2O?
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H: 1.008 g/mol × 2 = 2.016 g/mol. O: 15.999 g/mol × 1 = 15.999 g/mol. Total: 2.016 + 15.999 = 18.015 g/mol ≈ 18.02 g/mol. This is water's molar mass.
What does subscript 2 in H2O mean?
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Subscript indicates number of atoms of preceding element. H2 means 2 hydrogen atoms. O (no subscript) means 1 oxygen atom. H2O has total 3 atoms. Different subscripts = different compounds: H2O (water) vs H2O2 (hydrogen peroxide).
How do you handle parentheses in Ca(OH)2?
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Multiply everything inside parentheses by subscript outside. Ca(OH)2 means 1 Ca + 2 O + 2 H. Ca: 40.08. O: 16.00 × 2 = 32.00. H: 1.008 × 2 = 2.016. Total: 40.08 + 32.00 + 2.016 = 74.10 g/mol.
Why is formula case-sensitive?
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Capital letter is element symbol: H (hydrogen), Co (cobalt). Lowercase is part of previous element or invalid. CO (carbon monoxide) vs Co (cobalt) - different compounds! This is strict chemical naming convention.
Where do you find IUPAC atomic weights?
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IUPAC Periodic Table (www.iupac.org) is official source. Updated periodically. Found in chemistry textbooks and periodic tables. Calculator databases usually include standard values. Always check date - values update as measurement precision improves.
What decimal places should molecular weight have?
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Use 2-4 decimal places for most purposes. IUPAC atomic weights: 4-5 significant figures. Final MW: 3-4 decimal places appropriate. Match precision to application: 18.02 g/mol typical for water, not 18.015348.
How does isotope composition affect molecular weight?
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Standard atomic weights already include natural isotope distributions. Hydrogen: 99.9855% ¹H (1.008 Da), 0.0145% ²H (deuterium), trace ³H (tritium). Average = 1.008 g/mol. Purified isotopes give different MW.