Calculate the molecular weight and molar mass of any chemical compound from its molecular formula, quickly and accurately.
| Element | Atomic Mass | Atoms | Total | % |
|---|
Click any compound to calculate its molecular weight instantly.
A molecular weight calculator sums the standard atomic weights of every atom in a chemical formula and returns the result in grams per mole (g/mol) or atomic mass units (amu), also called Daltons (Da). This molecular weight calculation tool uses abridged standard atomic weights from the International Union of Pure and Applied Chemistry (IUPAC).
To calculate molecular weight, the calculator follows these steps:
To find molecular weight from a chemical formula, enter the formula using standard chemical notation. The molecular weight calculator reads element symbols (case sensitive), subscript numbers, parentheses, and hydrate dots. Each symbol and number has a specific role in defining the compound's composition.
Chemical formulas follow 4 core rules that the molecular weight finder processes:
Molecular weight and molar mass are two closely related measurements that chemists often use interchangeably, but they have distinct formal definitions. Understanding the difference between molecular weight and molar mass matters for scientific papers, lab reports, and academic exams, even though their numerical values are identical.
Every chemical compound consists of specific elements, and each element contributes a measurable portion of the total molecular weight. A molecular weight calculation breaks down a compound into its atomic contributions, representing the mass each element adds to the whole molecule. This breakdown is called percent composition.
To calculate the atomic contribution of each element, multiply the element's standard atomic weight by the number of atoms of that element in the formula, then divide by the total molecular weight and multiply by 100. The result is the mass percentage of that element in the compound.
Molecular weight is a core measurement in laboratory work. Chemists, pharmacists, and researchers use molecular weight calculations in 5 practical areas: stoichiometry, solution preparation, reaction yield analysis, mass spectrometry, and pharmaceutical dosing.
Stoichiometry uses molecular weight to convert grams of a reactant into moles. A balanced equation shows the mole ratio between reactants and products. For example, 36.03 grams of water equals 2 moles of water (36.03 ÷ 18.015 = 2.00 mol), which allows chemists to calculate how much of another substance is needed or produced.
Preparing a 1 M (one molar) solution requires dissolving one mole of solute per liter of solution. For sodium chloride (NaCl, molecular weight 58.44 g/mol), a 1 M solution needs 58.44 grams of NaCl dissolved in enough water to make 1 liter. The molecular weight directly determines the mass of solute required.
Reaction yield compares the actual mass of product to the theoretical mass. The theoretical mass comes from stoichiometric calculations using molecular weights. A reaction that produces 85 grams of product when the theoretical yield is 100 grams has an 85% yield. Molecular weight is needed to convert between moles of reactant and grams of product.
Mass spectrometry measures the mass to charge ratio (m/z) of ionized molecules. The molecular ion peak on a mass spectrum corresponds to the molecular weight of the compound. Chemists compare the measured m/z value against calculated molecular weights to identify unknown compounds in a sample.
Pharmaceutical scientists use molecular weight to convert between milligrams and micromoles when dosing active ingredients. A drug with a molecular weight of 500 g/mol at a 10 mg dose equals 20 μmol (10 ÷ 500 × 1000 = 20 μmol). This conversion is standard practice for pharmacokinetic studies and drug formulation.
Every atomic weight used in molecular weight calculation comes from the periodic table. Each element has a standard atomic weight, a weighted average based on the natural abundance of its stable isotopes, published and updated by IUPAC.
Atomic weight is not the mass of a single atom, but the weighted average across all naturally occurring isotopes. For example, hydrogen has 3 isotopes:
The weighted average gives hydrogen a standard atomic weight of 1.008.
Molecular weight calculation is straightforward, but 6 common errors cause incorrect results. Each error involves misreading the chemical formula or using wrong data. Recognizing these mistakes prevents calculation errors in academic assignments, lab work, and research.
Reference table of frequently searched chemical compounds.
| Chemical Formula | Compound Name | Molecular Weight |
|---|---|---|
| H₂O | Water | 18.015 g/mol |
| CO₂ | Carbon Dioxide | 44.009 g/mol |
| NaCl | Sodium Chloride | 58.443 g/mol |
| NH₃ | Ammonia | 17.031 g/mol |
| CH₄ | Methane | 16.043 g/mol |
| H₂SO₄ | Sulfuric Acid | 98.077 g/mol |
| CaCO₃ | Calcium Carbonate | 100.086 g/mol |
| C₆H₁₂O₆ | Glucose | 180.156 g/mol |
| C₂H₅OH | Ethanol | 46.069 g/mol |
| HCl | Hydrochloric Acid | 36.461 g/mol |
| NaOH | Sodium Hydroxide | 39.997 g/mol |
| Ca(OH)₂ | Calcium Hydroxide | 74.092 g/mol |
| HNO₃ | Nitric Acid | 63.012 g/mol |
| KMnO₄ | Potassium Permanganate | 158.032 g/mol |
| CuSO₄ | Copper(II) Sulfate | 159.607 g/mol |
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