Molar Mass Calculator
Enter any chemical formula to instantly calculate its molar mass (molecular weight) in g/mol, view an element-by-element breakdown, and see the percentage composition.
Element Breakdown
| Element | Count | Atomic Mass (u) | Subtotal (g/mol) | % Composition |
|---|
What Is Molar Mass?
Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). One mole contains exactly 6.022 140 76 × 1023 particles (Avogadro's number), whether those particles are atoms, molecules, ions, or formula units.
For an element, the molar mass numerically equals its standard atomic weight listed on the periodic table. For a compound, you add up the molar masses of every atom in its chemical formula. For instance, water (H₂O) has a molar mass of approximately 18.015 g/mol because it contains two hydrogen atoms (2 × 1.008) and one oxygen atom (15.999).
Molar Mass vs. Molecular Weight vs. Formula Weight
These three terms are closely related but differ in subtle ways:
- Molar mass is the mass of one mole of a substance in grams per mole (g/mol). It applies to any chemical species -- elements, molecules, ionic compounds, or polymers.
- Molecular weight (more precisely, relative molecular mass, Mr) is a dimensionless number equal to the sum of the relative atomic masses in a molecular formula. It applies strictly to covalent molecules.
- Formula weight is the same summation but used for ionic compounds (such as NaCl) that do not form discrete molecules. It is numerically equivalent to the molar mass.
In practice, chemists often use "molecular weight" and "molar mass" interchangeably, because numerically they are the same. The key difference is that molar mass carries the unit g/mol, while molecular weight is dimensionless (relative to 1/12 the mass of a carbon-12 atom).
How to Calculate Molar Mass Step by Step
- Write the chemical formula. Identify every element and its subscript (count). If no subscript is written, the count is 1.
- Look up atomic masses. Find each element's standard atomic weight from the periodic table.
- Multiply. For each element, multiply its atomic mass by the number of atoms of that element in the formula.
- Handle parentheses. If the formula contains parentheses, multiply the count of each element inside by the subscript outside the parentheses.
- Add. Sum all the contributions to get the total molar mass in g/mol.
Worked Examples
Example 1: Water (H₂O)
H: 2 atoms × 1.008 g/mol = 2.016 g/mol
O: 1 atom × 15.999 g/mol = 15.999 g/mol
Total = 2.016 + 15.999 = 18.015 g/mol
Example 2: Sodium Chloride (NaCl)
Na: 1 atom × 22.990 g/mol = 22.990 g/mol
Cl: 1 atom × 35.453 g/mol = 35.453 g/mol
Total = 22.990 + 35.453 = 58.443 g/mol
Example 3: Carbon Dioxide (CO₂)
C: 1 atom × 12.011 g/mol = 12.011 g/mol
O: 2 atoms × 15.999 g/mol = 31.998 g/mol
Total = 12.011 + 31.998 = 44.009 g/mol
Example 4: Glucose (C₆H₁₂O₆)
C: 6 atoms × 12.011 g/mol = 72.066 g/mol
H: 12 atoms × 1.008 g/mol = 12.096 g/mol
O: 6 atoms × 15.999 g/mol = 95.994 g/mol
Total = 72.066 + 12.096 + 95.994 = 180.156 g/mol
Example 5: Sulfuric Acid (H₂SO₄)
H: 2 atoms × 1.008 g/mol = 2.016 g/mol
S: 1 atom × 32.065 g/mol = 32.065 g/mol
O: 4 atoms × 15.999 g/mol = 63.996 g/mol
Total = 2.016 + 32.065 + 63.996 = 98.077 g/mol
Example 6: Calcium Hydroxide (Ca(OH)₂)
The subscript 2 outside the parentheses applies to both O and H inside:
Ca: 1 atom × 40.078 g/mol = 40.078 g/mol
O: 2 atoms × 15.999 g/mol = 31.998 g/mol
H: 2 atoms × 1.008 g/mol = 2.016 g/mol
Total = 40.078 + 31.998 + 2.016 = 74.092 g/mol
Using the Periodic Table for Atomic Masses
Every element on the periodic table has a listed standard atomic weight, which is the weighted average mass of all naturally occurring isotopes of that element. For example, chlorine's atomic weight is 35.453 u because it is a mixture of about 75.8% chlorine-35 and 24.2% chlorine-37.
When calculating molar mass, always use the standard atomic weight (not the mass number of a single isotope) to get the most accurate result. The values used in this calculator follow the IUPAC-recommended standard atomic weights.
Percentage Composition from Molar Mass
Percentage composition tells you what fraction of a compound's total mass is contributed by each element. The formula is:
% Element = (atoms of element × atomic mass ÷ molar mass of compound) × 100%
For example, in water (H₂O, M = 18.015 g/mol):
- % H = (2 × 1.008 / 18.015) × 100 = 11.19%
- % O = (1 × 15.999 / 18.015) × 100 = 88.81%
Percentage composition is essential for converting between empirical and molecular formulas and for verifying the purity of a synthesized compound.
Applications of Molar Mass
Molar mass is one of the most fundamental quantities in chemistry. Here are its primary applications:
- Stoichiometry: Converting between grams and moles is the first step in almost every stoichiometric calculation. For any reaction, you must convert measured masses into moles using molar mass, then use mole ratios from the balanced equation.
- Solution Preparation: To prepare a solution of a specific molarity, you calculate how many grams of solute are needed using: mass (g) = molarity (mol/L) × volume (L) × molar mass (g/mol).
- Empirical and Molecular Formulas: From experimental mass-percentage data, molar mass helps determine whether a measured empirical formula is also the molecular formula, or if it needs to be multiplied by an integer.
- Gas Law Calculations: Using the ideal gas law (PV = nRT), molar mass lets you convert between moles and grams of gas at known temperature and pressure.
- Gravimetric Analysis: In analytical chemistry, molar mass is used to calculate the expected mass of a precipitate or residue from a known quantity of analyte.
- Pharmaceutical Dosing: Drug dosages are often specified in millimoles; converting to milligrams requires the molar mass of the active ingredient.
- Material Science: Molar mass helps in calculating the density, unit cell parameters, and theoretical yield of crystalline materials.
Table of Molar Masses of Common Compounds
| Compound | Formula | Molar Mass (g/mol) |
|---|---|---|
| Water | H₂O | 18.015 |
| Sodium chloride (table salt) | NaCl | 58.443 |
| Carbon dioxide | CO₂ | 44.009 |
| Glucose | C₆H₁₂O₆ | 180.156 |
| Sulfuric acid | H₂SO₄ | 98.077 |
| Sodium hydroxide | NaOH | 39.997 |
| Calcium carbonate | CaCO₃ | 100.087 |
| Ammonia | NH₃ | 17.031 |
| Hydrochloric acid | HCl | 36.461 |
| Nitric acid | HNO₃ | 63.013 |
| Acetic acid | CH₃COOH | 60.052 |
| Ethanol | C₂H₅OH | 46.069 |
| Methane | CH₄ | 16.043 |
| Oxygen gas | O₂ | 31.998 |
| Nitrogen gas | N₂ | 28.014 |
| Calcium hydroxide | Ca(OH)₂ | 74.092 |
| Potassium permanganate | KMnO₄ | 158.034 |
| Iron(III) oxide (rust) | Fe₂O₃ | 159.688 |
| Magnesium sulfate (Epsom salt) | MgSO₄ | 120.366 |
| Sucrose (table sugar) | C₁₂H₂₂O₁₁ | 342.297 |
| Aspirin (acetylsalicylic acid) | C₉H₈O₄ | 180.159 |
| Aluminum sulfate | Al₂(SO₄)₃ | 342.151 |
| Sodium bicarbonate (baking soda) | NaHCO₃ | 84.007 |
| Phosphoric acid | H₃PO₄ | 97.994 |
Frequently Asked Questions
What is the difference between molar mass and atomic mass?
Atomic mass (or atomic weight) refers to the mass of a single atom or the weighted average mass of an element's isotopes, expressed in atomic mass units (u or Da). Molar mass is the mass of one mole (6.022 × 1023 particles) of a substance expressed in g/mol. Numerically they are equal -- for example, oxygen has an atomic mass of 15.999 u and a molar mass of 15.999 g/mol -- but the units and scale differ.
How do I handle hydrates when calculating molar mass?
Hydrated compounds (e.g., CuSO₄·5H₂O) include water molecules of crystallization. To find the total molar mass, calculate the molar mass of the anhydrous salt, then add the molar mass of water multiplied by the number of water molecules. For CuSO₄·5H₂O: 159.609 + (5 × 18.015) = 249.684 g/mol.
Why do atomic masses on the periodic table have decimal values?
Most elements exist as a mixture of stable isotopes in nature. The atomic mass listed is a weighted average that reflects the natural abundance of each isotope. For example, chlorine is approximately 75.8% Cl-35 and 24.2% Cl-37, giving an average atomic mass of 35.453 u.
Can I use this calculator for polyatomic ions?
Yes. Enter the formula of the ion just as you would a molecule. For example, enter "SO4" for the sulfate ion to get its formula weight. Note that the mass of lost or gained electrons is negligible compared to the nuclear masses, so the formula weight is essentially the same as the ionic mass.
How accurate are the molar masses calculated here?
This calculator uses IUPAC-recommended standard atomic weights rounded to three decimal places for most elements. The results are accurate to approximately ±0.001 g/mol for simple formulas. For isotopically enriched or radioactive samples, you would need to use the specific isotopic mass instead.
What is Avogadro's number and why does it matter?
Avogadro's number (NA = 6.022 140 76 × 1023 mol-1) is the number of particles in exactly one mole. It serves as the bridge between the atomic scale (atomic mass units) and the laboratory scale (grams). Because of this definition, the molar mass of a substance in g/mol is numerically equal to its formula mass in atomic mass units.
How do I convert grams to moles using molar mass?
Use the formula: moles = mass (g) ÷ molar mass (g/mol). For example, 36.03 g of water ÷ 18.015 g/mol = 2.000 moles of water. Conversely, to convert moles to grams: mass = moles × molar mass.