Molecular Weight and Molar Mass Calculator
Enter a chemical formula such as H2O, C6H12O6, Ca(OH)2 or CuSO4·5H2O to calculate molar mass in g/mol and each element’s mass percentage. Add a mass in grams to calculate moles and particles. Parentheses and hydrate dots are supported; charges, isotope notation and decimal atom counts are not.
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Molecular weight calculator
| Element | Atoms | Atomic weight | Mass | Mass % |
|---|
Use the middle dot (·) for hydrates, for example CuSO4·5H2O. Decimal atom counts and leading isotope numbers are not supported.
Common compounds: formula input examples
These values use the calculator’s built-in weights, checked for the elements below against CIAAW Abridged Standard Atomic Weights 2024. Sum all atom contributions first, then round to three decimal places in g/mol. The displayed decimals are a calculation convention, not a measurement uncertainty.
| Substance | Formula to enter | Molar mass (g/mol) |
|---|---|---|
| Sodium chloride | NaCl | 58.440 |
| Water | H2O | 18.015 |
| Carbon dioxide | CO2 | 44.009 |
| Sodium hydroxide | NaOH | 39.997 |
| Hydrogen chloride | HCl | 36.458 |
| Sulfuric acid | H2SO4 | 98.072 |
| Calcium carbonate | CaCO3 | 100.086 |
| Ammonia | NH3 | 17.031 |
| Ethanol | C2H5OH | 46.069 |
| Glucose | C6H12O6 | 180.156 |
| Ammonium sulfate | (NH4)2SO4 | 132.134 |
| Copper(II) sulfate pentahydrate | CuSO4·5H2O | 249.677 |
Atomic weights used here: C 12.011; Ca 40.078; Cl 35.45; Cu 63.546; H 1.008; N 14.007; Na 22.99; O 15.999; S 32.06.
Chemistry Molar Mass Practice Pack
Common molar mass references, 8 molar-mass questions and 6 conversion questions with answer keys, plus a solution-preparation sheet. Four PDFs are free individually; the $5 ZIP adds 2 editable Word files and 2 Excel files.
- Common molar masses (PDF/XLSX, static reference)
- Molar mass practice: 8 questions and answers (PDF/DOCX)
- Grams and moles: 6 questions and answers (PDF/DOCX)
- Solution preparation (static PDF and formula-based XLSX)
Formats: PDF, XLSX, DOCX. Instant download after payment (link valid 72 hours, up to 5 downloads). AI-assisted: the templates were drafted with AI help and reviewed and laid out before publishing.
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Printable Solution Calculator SheetWhat molecular weight means
For water, H₂O: 2 × 1.008 + 15.999 = 18.015. Its relative molecular mass is unitless; its molar mass is 18.015 g/mol. Molar mass is the mass of one mole — 6.022 × 10²³ molecules or formula units. Add the atomic weights of all atoms in the formula to find the relative molecular mass, then use the same numerical value in g/mol for molar mass. Ionic compounds such as sodium chloride are described using relative formula mass because they do not form discrete molecules.
How to calculate molecular weight by hand
- Write the formula and count the atoms of each element, multiplying through any brackets.
- Look up each element’s standard atomic weight on the periodic table.
- Multiply each atomic weight by its number of atoms.
- Add the results.
Example — calcium hydroxide, Ca(OH)₂: Ca 40.078 + O 2 × 15.999 + H 2 × 1.008 = 74.092 g/mol.
Formula syntax the calculator understands
| Feature | Example | Meaning |
|---|---|---|
| Element symbols | NaCl | Capital first letter, lowercase second: Na, Cl |
| Subscripts | C6H12O6 | Numbers after an element are its count |
| Parentheses | Ca(OH)2, (NH4)2SO4 | A group multiplied by the number after the bracket |
| Square brackets | K4[Fe(CN)6] | Nested groups |
| Hydrates | CuSO4·5H2O | Water of crystallisation added to the formula |
| Unicode subscripts | H₂O | Converted automatically |
Capitalisation matters: “Co” is cobalt, but “CO” is carbon monoxide.
Mass percent composition
The table shows how much each element contributes to the total mass. In copper(II) sulfate pentahydrate, CuSO₄·5H₂O (249.68 g/mol), copper makes up about 25.45% of the mass, and the five water molecules together account for about 36% — which is why the blue crystals lose so much mass when heated to drive off the water. Percent composition is used to check purity, work out empirical formulas from lab data and calculate how much of an element a fertiliser or supplement supplies.
Grams, moles and molecules
| Convert | Formula |
|---|---|
| Grams → moles | moles = grams ÷ molar mass |
| Moles → grams | grams = moles × molar mass |
| Moles → molecules | molecules = moles × 6.022 × 10²³ |
| Molecules → moles | moles = molecules ÷ 6.022 × 10²³ |
Enter a mass to see the moles and number of formula units. For example, 10 g of CuSO₄·5H₂O is 0.04005 mol, about 2.41 × 10²² formula units.
Atomic weights and precision
The element breakdown shows the exact weights stored in this calculator. The common-compound table above identifies the weights it uses and their reference. The wider built-in table is a fixed set of rounded values and is not a promise of the latest value for every element; for example, its zirconium value is 91.224 while CIAAW’s 2024 abridged table lists 91.222. Isotope-specific masses are not supported.
Uses
- Preparing solutions of a given molarity in the lab.
- Stoichiometry: converting between masses of reactants and products.
- Checking homework and worked examples.
- Calculating the percentage of an active ingredient or nutrient.
- Finding empirical formulas from percent composition.
Empirical and molecular formulas
The molecular formula gives the actual number of atoms in a molecule (glucose, C₆H₁₂O₆), while the empirical formula gives the simplest whole-number ratio (CH₂O). Molecular weight is calculated from the molecular formula. To find a molecular formula from an empirical one, divide the measured molar mass by the empirical formula mass and multiply the subscripts by the result: glucose’s molar mass 180.16 ÷ 30.03 (CH₂O) = 6, so C₆H₁₂O₆.
Molecular weight in the lab
Laboratory recipes usually specify concentrations in molarity (moles per litre). To make 250 mL of 0.5 M sodium chloride solution, you need 0.5 × 0.250 = 0.125 mol, which is 0.125 × 58.44 = 7.31 g of NaCl. For hydrated salts, use the molar mass of the hydrate you are weighing: 0.1 mol of copper sulfate pentahydrate is 24.97 g, whereas 0.1 mol of anhydrous copper sulfate is 15.96 g. Mixing these up is one of the most common errors in preparing solutions.
Common mistakes
- Writing element symbols in the wrong case (CO for cobalt).
- Forgetting to multiply everything inside brackets by the subscript outside.
- Leaving out water of crystallisation for hydrates.
- Using mass numbers (whole numbers) instead of standard atomic weights.
- Rounding too early in multi-step calculations.
Privacy
All calculations run in your browser; formulas you type are not stored or sent anywhere.
Frequently asked questions
What is the difference between molecular weight and molar mass?
Numerically the same; molar mass is expressed in g/mol and molecular weight is often stated as a unitless relative value.
What is the molecular weight of water?
Its relative molecular mass is about 18.015 (unitless); its molar mass is about 18.015 g/mol.
How do I enter a hydrate?
Use a middle dot, for example CuSO4·5H2O. A period is rejected because decimal atom counts are not supported. Parentheses and matching square brackets can be nested.
Why does NACL give an error?
Element symbols are case-sensitive: write NaCl.
How do I convert grams to moles?
Divide the mass by the molar mass; enter the mass in the calculator to do it automatically.
Which atomic weights are used?
A fixed table of rounded atomic weights is built into the calculator. Each value is visible in the element breakdown. The common-compound examples cite CIAAW 2024 for their listed elements; the full built-in table is not updated automatically.
Can I enter isotopes or charges?
No. Enter the neutral formula with element symbols; ionic charges do not change the molar mass meaningfully.
Is molar mass the same as density?
No. Molar mass is grams per mole; density is mass per volume.
May I print and share these worksheets with my class?
License: personal and classroom use. Teachers may print and photocopy these files for the students in their own classes, and share them in a password-protected class space (e.g. Google Classroom). Please do not resell, redistribute, or post the files on public websites. One purchase covers one teacher.
Is Cl the same formula as Cl2?
No. Cl contains one chlorine atom, while Cl2 contains two. With the stored chlorine weight of 35.45, they give 35.450 and 70.900 g/mol respectively. Enter the formula required by your problem.
Can molar mass alone tell me grams or moles?
No. To find moles you also need a mass in grams; enter it in the optional Mass box. To find grams from moles, use grams = moles × molar mass separately: this form has no moles-input box.
Which formula inputs are rejected?
Unknown elements, mismatched or empty brackets, ionic charge symbols, leading isotope or coefficient numbers, decimal atom counts and zero atom counts are rejected. Use ordinary case-sensitive chemical formulas.