Molar Mass / Molecular Weight Calculator
Parse formulas, hydrates and parentheses to find molar mass, atom counts and percent composition.
What you can enter
Use the formula box the way you would write a chemistry formula in homework. Capitalization matters for element symbols, so CO means carbon monoxide while Co means cobalt. The calculator reads subscripts, nested groups, and many hydrate formulas, then turns the atom counts into a molar mass diagram.
| Input style | Example | What the calculator counts |
|---|---|---|
| Simple molecule | H2O | 2 H atoms and 1 O atom |
| Ionic compound | NaCl | 1 Na atom and 1 Cl atom |
| Parentheses | Ca(OH)2 | 1 Ca, 2 O, and 2 H |
| Nested sulfate group | Al2(SO4)3 | 2 Al, 3 S, and 12 O |
| Hydrate dot | CuSO4.5H2O | CuSO4 plus 5 water molecules |
Formula used
Worked examples
Water, H2O: 2(1.008) + 1(15.999) = 18.015 g/mol. The diagram shows oxygen carrying most of the mass even though hydrogen has more atoms.
Calcium hydroxide, Ca(OH)2: parentheses multiply O and H by 2, so the total is about 74.09 g/mol.
Copper(II) sulfate pentahydrate, CuSO4.5H2O: add CuSO4 and five H2O units. The hydrate waters add about 90.08 g/mol to the formula mass.
How the diagram helps
The atom sketch is a quick visual check of the formula. The mass contribution bars answer a different question: which element contributes most to the grams per mole? Heavy atoms can dominate molar mass even when their atom count is small, while light atoms such as hydrogen often contribute less mass than the subscript makes you expect.
Where this calculator is useful
- Turning grams into moles for stoichiometry and limiting reagent problems.
- Checking percent composition before empirical formula work.
- Preparing molarity calculations where grams of solute must become moles.
- Comparing hydrated and anhydrous salts in lab-prep questions.
- Spotting formula-counting mistakes before they spread into later calculations.
Common mistakes
- Typing CO when you mean cobalt, Co, or typing co when you mean carbon monoxide.
- Forgetting that parentheses multiply every atom inside the group.
- Leaving hydrate water out of formulas such as CuSO4.5H2O.
- Using rounded atomic masses too early when your class expects extra precision.
- Calling the answer grams instead of grams per mole.
Rounding and result checking
For most homework, keep two to four decimal places during intermediate work and round the final value to your teacher's rule. A good reasonableness check is to compare the total with the heaviest atom in the formula. The molar mass should be larger than any single atomic mass contribution, and the percent composition values should add to about 100% after rounding.
Related Chemistry Tools
FAQs
How to find molar mass?
Add up the atomic mass of every atom in the chemical formula and the result is the molar mass in g/mol. Steps: write the correct formula, count each element's atoms (subscripts outside a bracket multiply everything inside it), look up each atomic mass on the periodic table, multiply, and sum. Worked example for H2SO4: H = 2 × 1.008 = 2.016, S = 32.06, O = 4 × 16.00 = 64.00, total = 98.08 g/mol. For Ca(OH)2 the 2 outside the bracket multiplies both O and H: Ca = 40.08, O = 2 × 16.00 = 32.00, H = 2 × 1.008 = 2.016, total = 74.10 g/mol.
What is the molar mass of CO2?
44.01 g/mol. C (12.01) + 2 × O (16.00) = 44.01 g/mol. One mole of CO2 weighs about 44 g and contains 6.022 × 10^23 molecules. CO2 is denser than air (which averages around 29 g/mol), which is why it pools at low points and works as a smothering agent in fire extinguishers.
What is the molar mass of H2O?
18.02 g/mol. 2 × H (1.008) + O (16.00) = 18.016 g/mol, usually rounded to 18 g/mol. At 25 °C and 1 atm, water has a density of 1 g/mL, so one mole occupies about 18 mL.
What is the molar mass of NaCl?
58.44 g/mol. Na (22.99) + Cl (35.45) = 58.44 g/mol, often rounded to 58.5 g/mol. NaCl is ionic and forms a crystal lattice rather than discrete molecules, so this number is more accurately a formula mass. One formula unit dissociates into one Na+ and one Cl- in water, so a 1 M NaCl solution is also 1 M in each ion.
What is the molar mass of water?
18.02 g/mol — same calculation as H2O above. Practical reference: 1 L of pure water weighs 1000 g, so it contains 1000 / 18.02 ≈ 55.5 moles of H2O. That is why pure water has a 'concentration' of about 55.5 M and why dilute aqueous solutions are mostly water by mole fraction.
What is the molar mass of NaOH?
40.00 g/mol. Na (22.99) + O (16.00) + H (1.008) = 39.998 g/mol. To prepare 1 L of 1 M NaOH, dissolve 40.0 g in water and make up to 1 L. NaOH is hygroscopic and reacts slowly with atmospheric CO2 to form Na2CO3, so solutions used as titrant must be standardized against a primary standard such as potassium hydrogen phthalate before precise work.
Is molar mass the same as molecular weight?
Numerically yes; conceptually they differ. Molecular weight (relative molecular mass, Mr) is a unitless ratio comparing a molecule's mass to 1/12 the mass of a carbon-12 atom. Molar mass is the mass of one mole of substance, in g/mol. For water, Mr = 18.02 and molar mass = 18.02 g/mol. Molar mass is the SI-preferred term and applies equally to atoms, molecules, ions, and ionic formula units; molecular weight technically applies only to discrete molecules.
What is the molar mass of NH3?
17.03 g/mol. N (14.01) + 3 × H (1.008) = 17.034 g/mol. Ammonia is produced industrially by the Haber-Bosch process: N2 + 3H2 ⇌ 2NH3, run at about 400-500 °C and 150-300 atm over an iron catalyst. By the equation, 28 g of N2 plus 6 g of H2 yield 34 g of NH3.
What is the molar mass of oxygen?
Depends which oxygen. Atomic oxygen (O) = 16.00 g/mol. Molecular oxygen (O2), the form in air, = 32.00 g/mol. Ozone (O3) = 48.00 g/mol. When a question says 'oxygen gas' or shows O2 in a reaction, use 32 g/mol. Use 16 g/mol only when the formula explicitly shows a single O atom or refers to atomic oxygen.
What is the molar mass of Al2(SO4)3?
342.15 g/mol. The 3 outside the bracket multiplies every atom in (SO4), so the formula contains 2 Al, 3 S, and 12 O. Calculation: (2 × 26.98) + (3 × 32.06) + (12 × 16.00) = 53.96 + 96.18 + 192.00 = 342.14 g/mol. Forgetting to apply the 3 to the oxygens is the most common mistake — count atoms before you start multiplying masses.
What is the molar mass of C6H12O6?
180.16 g/mol. (6 × 12.01) + (12 × 1.008) + (6 × 16.00) = 72.06 + 12.10 + 96.00 = 180.16 g/mol. C6H12O6 is the molecular formula for several hexose sugars — glucose, fructose, and galactose are all isomers with this formula and so all share this molar mass even though their structures differ.
What is the molar mass of glucose?
180.16 g/mol. Glucose is C6H12O6, so (6 × 12.01) + (12 × 1.008) + (6 × 16.00) = 180.16 g/mol. Reference points: a 5% w/v glucose IV drip is 50 g/L ≈ 0.278 mol/L; normal fasting blood glucose of 70-100 mg/dL works out to roughly 3.9-5.6 mmol/L.
What is the molar mass of hydrogen?
1.008 g/mol for atomic hydrogen (H); 2.016 g/mol for hydrogen gas (H2). When a problem says 'hydrogen gas' or shows H2 in a reaction, use 2.016 g/mol (often rounded to 2 g/mol). At STP, one mole of H2 occupies 22.4 L. The standard atomic mass 1.008 is a weighted average of natural isotopic abundance — almost entirely protium (1H), with trace deuterium.
What is the molar mass of nitrogen?
14.01 g/mol for atomic nitrogen (N); 28.02 g/mol for nitrogen gas (N2). Atmospheric nitrogen is N2, so use 28 g/mol whenever the question refers to the gas. The N≡N triple bond has a bond energy of about 945 kJ/mol, which is why N2 is fairly inert at room temperature and industrial fixation needs high pressure plus a catalyst.
What is the molar mass of water (H2O)?
18.02 g/mol. The formula H2O has 2 H atoms (2 × 1.008 = 2.016 g/mol) and 1 O atom (16.00 g/mol), summing to 18.016 g/mol. The molecule is bent with an H-O-H angle of about 104.5°. Hydrogen bonding is what gives liquid water its high boiling point (100 °C at 1 atm) and high specific heat capacity (4.184 J/g·°C).
Is the atomic mass the same as the molar mass?
Numerically yes, but the units differ. Atomic mass is per atom, in atomic mass units (amu or u), where 1 amu equals exactly 1/12 the mass of a carbon-12 atom. Molar mass is per mole, in g/mol. The mole is defined so that 1 amu/atom equals 1 g/mol exactly, which is why carbon's atomic mass of 12.01 amu and molar mass of 12.01 g/mol share the same number.
What is the molar mass of CaCl2?
110.98 g/mol. Ca (40.08) + 2 × Cl (35.45) = 110.98 g/mol. CaCl2 is hygroscopic and dissolves exothermically. Each formula unit gives three ions in solution — one Ca2+ and two Cl- — so the van't Hoff factor approaches 3 in dilute solutions, which is why CaCl2 is more effective per gram than NaCl at depressing freezing points.
What is the molar mass of CH4?
16.04 g/mol. C (12.01) + 4 × H (1.008) = 16.042 g/mol. Methane is tetrahedral (H-C-H angle ≈ 109.5°). Combustion: CH4 + 2O2 → CO2 + 2H2O, releasing about 890 kJ per mole of CH4 burned. At STP, one mole of CH4 (or any ideal gas) occupies 22.4 L.
What is the molar mass of H2SO4?
98.08 g/mol. (2 × 1.008) + 32.06 + (4 × 16.00) = 2.016 + 32.06 + 64.00 = 98.08 g/mol. Concentrated sulfuric acid is roughly 98% by mass with density 1.84 g/mL, which works out to about 18.4 mol/L. Always add the acid to water when diluting, never the other way — the dissolution is strongly exothermic and pouring water onto concentrated acid can cause violent splashing.
What is the molar mass of HCl?
36.46 g/mol. H (1.008) + Cl (35.45) = 36.458 g/mol. Concentrated hydrochloric acid is about 37% by mass with a density near 1.18 g/mL, giving roughly 12 mol/L. HCl is a strong acid — it ionizes essentially completely in water — so a 0.1 M solution has a pH of about 1.0.