Molecular Weight Calculator
Calculate molecular weight from chemical formulas including grouped ions and hydrates. This version supports parentheses, square brackets, and hydrate dots so the tool matches real classroom and lab notation better.
Edited by Gail Joyce
Gail Joyce reviews formula-based calculators for notation accuracy, atomic-mass consistency, and cleaner routing between molecular-weight and molar-mass tools.
This page is maintained as a direct formula-to-weight tool. Formula parsing guidance and reference data are reviewed so routine chemistry calculations stay practical and easy to confirm.
Molecular Weight Calculator
Enter a chemical formula to calculate its molecular weight. Parentheses, square brackets, and hydrate notation are supported.
Table of Contents
Quickly navigate to different sections of this guide.
Understanding Molecular Weight
Molecular weight (also called molecular mass or formula weight) is the sum of the atomic weights of all atoms in a molecule. It's expressed in atomic mass units (u or Da) or grams per mole (g/mol), where it's numerically equal to molar mass. Understanding molecular weight is fundamental to chemistry—it's used in stoichiometry, solution preparation, percent composition calculations, and many other applications.
The calculation is straightforward: identify each element in the formula, count how many atoms of each element are present (using subscripts), multiply each element's atomic mass by its count, and sum all contributions. For example, water (H₂O) has molecular weight = 2×1.008 + 16.00 = 18.016 u, which equals 18.016 g/mol for molar mass.
Molecular weight is crucial because it connects the microscopic world (individual molecules) with the macroscopic world (measurable masses). When you know molecular weight, you can convert between mass and moles, prepare solutions of specific concentrations, calculate reaction yields, and determine percent composition. This makes molecular weight one of the most frequently calculated values in chemistry.
Why Molecular Weight Matters
Stoichiometry
Chemical equations show mole ratios, not mass ratios. To use balanced equations, you need molecular weights to convert between mass and moles. This is essential for determining reactant and product quantities in chemical reactions.
Solution Preparation
Preparing solutions with specific molarities requires knowing molecular weight. Molarity (M) = moles / liters, and moles = mass / molecular weight. Without molecular weight, you can't prepare accurate solutions.
Percent Composition
Calculating percent composition requires molecular weight. For each element, percent = (atomic mass × count / molecular weight) × 100%. This helps verify formulas and understand compound structure.
Empirical and Molecular Formulas
Determining molecular formulas from empirical formulas requires molecular weight. The molecular formula is a multiple of the empirical formula, and the multiplier equals molecular weight / empirical formula weight.
Common Molecular Weights
| Compound | Formula | Molecular Weight (g/mol) |
|---|---|---|
| Water | H₂O | 18.02 |
| Carbon dioxide | CO₂ | 44.01 |
| Sodium chloride | NaCl | 58.44 |
| Glucose | C₆H₁₂O₆ | 180.16 |
| Sulfuric acid | H₂SO₄ | 98.08 |
How to Use the Molecular Weight Calculator
The Molecular Weight Calculator makes molecular weight calculations instant and accurate. Whether you're solving stoichiometry problems, preparing solutions, or analyzing compounds, this calculator provides precise results with detailed explanations.
- Enter Chemical Formula: Input the chemical formula using standard notation. Use element symbols (e.g., H, C, O) and subscripts for atom counts (e.g., H2O, C6H12O6). The calculator automatically parses the formula.
- Click Calculate: The calculator identifies all elements, counts atoms, multiplies by atomic masses, and sums the contributions. Results appear with detailed step-by-step explanations showing each element's contribution.
- Review Results: Examine the calculated molecular weight and the breakdown by element. Use the step-by-step breakdown to understand how the calculation was performed.
The calculator automatically handles formula parsing, element identification, and atomic mass lookup. Results are displayed with appropriate precision, and the calculation steps show exactly how each element contributes to the total molecular weight.
Formulas and Calculations
The calculation of molecular weight uses atomic masses from the periodic table. Understanding this process is essential for chemistry calculations.
Molecular Weight Formula
MW = Σ(atomic mass × count)
Where:
- MW = molecular weight (g/mol)
- atomic mass = atomic weight from periodic table
- count = number of atoms of that element in the formula
Sum the contributions from all elements in the formula. Each element's atomic mass is multiplied by the number of atoms (subscript) in the formula.
Worked Examples
Let's work through detailed examples to understand molecular weight calculations.
Example 1: Water (H₂O)
Given: Formula H₂O
Find: Molecular weight
Solution:
H: 2 atoms × 1.008 u = 2.016 u
O: 1 atom × 16.00 u = 16.00 u
Total: 2.016 + 16.00 = 18.016 u = 18.016 g/mol
Answer: The molecular weight of water is 18.016 g/mol.
Example 2: Glucose (C₆H₁₂O₆)
Given: Formula C₆H₁₂O₆
Find: Molecular weight
Solution:
C: 6 atoms × 12.01 u = 72.06 u
H: 12 atoms × 1.008 u = 12.096 u
O: 6 atoms × 16.00 u = 96.00 u
Total: 72.06 + 12.096 + 96.00 = 180.16 u = 180.16 g/mol
Answer: The molecular weight of glucose is 180.16 g/mol.
Understanding Chemical Formulas
Chemical formulas represent the composition of compounds using element symbols and subscripts. Understanding how to read formulas is essential for accurate molecular weight calculations.
Formula Notation
Formulas use element symbols (capital letter, sometimes followed by lowercase) and subscripts (numbers indicating atom count). For example, H₂O has 2 H atoms and 1 O atom. If no subscript is shown, it means 1 atom. Parentheses indicate groups that are multiplied by an outside subscript.
Frequently Asked Questions (FAQs)
Common questions about molecular weight calculations.
What is molecular weight?
Molecular weight is the sum of atomic weights of all atoms in a molecule. It's expressed in atomic mass units (u) or grams per mole (g/mol), where it equals molar mass.
How do I calculate molecular weight?
Identify each element in the formula, count atoms (using subscripts), multiply each element's atomic mass by its count, and sum all contributions. The calculator performs this automatically.
What's the difference between molecular weight and molar mass?
Molecular weight is expressed in atomic mass units (u), while molar mass is in g/mol. However, they're numerically equal—a molecule with MW = 18.02 u has molar mass = 18.02 g/mol.
How do I enter formulas with parentheses?
Use parentheses or square brackets to group atoms that are multiplied by a subscript. Examples: Ca(NO₃)₂, K₄[Fe(CN)₆], and hydrate notation like CuSO₄·5H₂O are supported.
References and Further Reading
For more in-depth information about molecular weight, chemical formulas, and molar mass calculations, consult these authoritative sources:
| Resource | Description | Category |
|---|---|---|
| IUPAC Gold Book: Molecular Mass | Standard definition of molecular mass and accepted terminology | General Chemistry |
| PubChem | Database of chemical properties including molecular weights | Chemical Data |
| IUPAC | Official definitions of molecular weight and molar mass | Standards |
| NIST Chemistry WebBook | Standard reference data for chemical compounds | Chemical Data |