You Have a Flask Containing 10 Grams of Sodium Chloride and Need to Know How Many Moles You Have
Chemistry is fundamentally about counting molecules and atoms. But molecules are impossibly tiny. Instead of counting individual particles, chemists use the mole-a fixed number of particles (6.022 × 10²³, Avogadro's number). The mass of one mole of a substance is the molar mass, measured in grams per mole. To convert between grams and moles, you must know the molar mass of your compound.
What This Calculator Does
This calculator computes the molar mass (molecular weight) of any chemical compound. You type the chemical formula (like H₂O or NaCl), and the calculator instantly sums the atomic masses of all atoms in the compound. It also tells you how many moles are in any mass you provide, and converts moles to grams. Use it to prepare solutions, balance equations, or verify experimental data.
How to Use This Calculator
Enter the chemical formula: Type the formula using standard notation. Elements are capitalized (C for carbon, not c). Subscripts are written as numbers (H2O, not H₂O). Parentheses work for groups: Ca(OH)2 for calcium hydroxide. Include charges if needed, like Na+ or SO4(2-).
The calculator returns:
Examples of valid input:
The Formula Behind the Math
Molar mass is the sum of the atomic masses of all atoms in a molecule:
Molar Mass = Σ (number of atoms × atomic mass)
Atomic masses are standardized by the International Union of Pure and Applied Chemistry (IUPAC). For example:
To convert between grams and moles:
Moles = Mass (g) / Molar Mass (g/mol)
Mass (g) = Moles × Molar Mass (g/mol)
Worked Example:
Find the molar mass of glucose (C₆H₁₂O₆) and determine how many moles are in 180 grams.
Our calculator does all of this instantly, but now you understand exactly what it's computing.
Lab Preparation and Solution Making
Suppose you need to make 1 liter of a 0.1 M (0.1 molar) sodium chloride solution. Molar mass of NaCl is 58.44 g/mol. For 0.1 mol, you need 0.1 × 58.44 = 5.844 grams. Dissolve 5.844 g in water and dilute to 1 liter. This is how lab protocols specify exact concentrations.
Stoichiometry and Chemical Equations
When balancing chemical equations, molar mass helps convert between mass and moles. If a reaction produces 20 grams of CO₂ (molar mass 44.01 g/mol), that's 20 / 44.01 = 0.454 moles of CO₂. If the stoichiometry shows a 1:1 ratio with another product, you know exactly how much of that product forms.
Verification and Quality Control
In analytical chemistry, you often measure a mass and need to verify purity. If you're supposed to have 10 grams of pure CaCO₃ (molar mass 100.09 g/mol) but you measure 9.5 grams, the sample is about 95% pure. Molar mass calculations let you quantify contamination and quality.
Tips and Things to Watch Out For
Use correct element symbols. Carbon is C (uppercase), not c. Calcium is Ca, not CA. A single letter difference changes the element entirely. If the calculator doesn't recognize your formula, check capitalization first.
Parentheses multiply the subscript. Ca(OH)₂ means one calcium, two oxygen, and two hydrogen. Don't forget the atoms inside parentheses when counting. Common mistake: writing Ca(OH)2 and forgetting that the 2 multiplies the entire OH group.
Atomic masses are averages. Most elements have multiple stable isotopes. The atomic mass is a weighted average. For instance, chlorine has two main isotopes: Cl-35 and Cl-37. The listed atomic mass (35.453) reflects their natural abundance. If you're working with a specific isotope, you may need to adjust the mass slightly.
Molar mass is in grams per mole by definition. The molar mass numerically equals the atomic mass unit (amu) value. The proton and neutron each have mass close to 1 amu, so H (1 proton) ≈ 1 g/mol, C (12 nucleons) ≈ 12 g/mol. This is by design-it makes calculations simple.
Significant figures matter in lab work. Atomic masses are given to 3–5 significant figures. If you measure 1.23 grams, report moles to 3 significant figures: 1.23 g / 18.015 g/mol = 0.0682 mol (not 0.06825... with extra digits you didn't measure).
Frequently Asked Questions
What's the difference between atomic mass, molar mass, and formula weight?
Atomic mass is the mass of a single atom in amu (atomic mass units). Molar mass is the mass of one mole of atoms or molecules in grams per mole. Formula weight is essentially the same as molar mass but is sometimes used for compounds that aren't truly molecular (like ionic salts). For most practical purposes, molar mass and formula weight are used interchangeably.
Why is Avogadro's number 6.022 × 10²³?
It's defined so that the mass of one mole of carbon-12 equals exactly 12 grams. This ties the atomic mass scale to the gram. Avogadro's number was chosen historically to match this relationship, and it's exact by definition as of 2019.
How do I find molar mass if I have a chemical formula I'm not sure about?
Break it into individual elements and look up atomic masses. Add them up. Or use our calculator and check the breakdown. If the result seems wrong, recheck the formula. Common mistakes: forgetting parentheses (Ca(OH)2 vs. CaOH2) or mistyping element symbols.
What if the compound is hydrated, like CuSO₄·5H₂O?
Include the water molecules in the formula. CuSO4·5H2O has Cu (63.55) + S (32.06) + 4O (4×15.999=63.996) + 5H2O (5×18.015=90.075) = 249.68 g/mol. The dot means "associated with" (not multiplying), so all atoms are counted.
How do I convert between grams and moles?
Moles = grams / molar mass. Grams = moles × molar mass. If you have 10 grams of H₂O (molar mass 18.015 g/mol), that's 10 / 18.015 = 0.555 moles. If you have 2 moles of NaCl (58.44 g/mol), that's 2 × 58.44 = 116.88 grams.
What's a mole, anyway?
A mole is simply a number: 6.022 × 10²³ particles. Like a dozen is 12, a mole is 6.022 × 10²³. For chemistry, it's convenient because the mass of one mole in grams numerically equals the mass of a single particle in amu. One mole of H atoms is 1 gram. One mole of O atoms is 16 grams. This relationship makes stoichiometry and lab calculations much simpler.
Related Calculators
Use our Dilution Calculator to prepare solutions at exact concentrations, which relies on molar mass. The pH Calculator deals with hydrogen ion concentration, which requires molar calculations. For more chemistry and physics concepts, explore our Density and Pressure Calculators.