Updated Chemistry

Molecular Weight Calculator

Enter a chemical formula to compute molar mass, convert between mass and moles, find the number of particles using Avogadro’s number, and calculate percent composition by element.

Formula → Molar Mass Mass ⇄ Moles Moles → Particles Percent Composition

All-in-One Molecular Weight And Composition Suite

Switch between formula-based molar mass, mass–mole conversions, particles from moles and element-by-element percent composition.

Enter a chemical formula such as H2O, C6H12O6, Mg(OH)2 or CuSO4·5H2O. Parentheses and hydrate dots are supported.

Use standard element symbols with optional counts (defaults to 1).

Convert between mass and moles when you know molar mass. You can copy the molar mass from the first tab.

For Mass → Moles use g or mg. For Moles → Mass use mol.

Use Avogadro’s number NA ≈ 6.022 × 10²³ particles per mole to convert moles to particles.

Percent composition shows how much of the molar mass comes from each element.

Molecular Weight Calculator – Molar Mass, Mass, Moles And Composition

This Molecular Weight Calculator helps you move between chemical formulas, molar mass, mass, moles and percent composition. It is designed for lab work, assignments and quick checks when working with chemical substances. You can compute molar mass from a formula, convert between mass and moles, use Avogadro’s number to find the number of particles and break down a compound into element-by-element mass percentages.

Molar Mass From Chemical Formula

Molar mass (sometimes called molecular weight) is the mass of one mole of a substance, expressed in grams per mole (g/mol). The calculator uses a built-in periodic table of atomic weights and applies your chemical formula. It understands:

  • Simple formulas such as H2O, CO2 and NaCl
  • Organic examples such as C6H12O6 or C3H8
  • Parentheses such as Mg(OH)2 or Al2(SO4)3
  • Hydrated compounds such as CuSO4·5H2O

The total molar mass is the sum of each element’s atomic weight multiplied by its count in the formula. The breakdown table shows how much each element contributes to the final value, which is helpful for cross-checking hand calculations.

Mass And Moles – Using Molar Mass

Once you know molar mass, you can change between mass and moles. The key relationships are:

n = m ÷ M
m = n × M

Here, n is the amount in moles, m is the mass in grams and M is the molar mass in g/mol. The mass–mole tab lets you choose which direction you want, accepts grams or milligrams for mass and returns the converted value with a clear summary. This is useful when preparing solutions, weighing samples or interpreting balanced equations.

Moles To Particles – Avogadro’s Number

A mole is a counting unit. One mole of any substance contains the same number of particles, given by Avogadro’s number:

NA ≈ 6.022 × 10²³ particles per mole

To find how many molecules, atoms or ions correspond to a given amount in moles, you multiply by NA. The calculator performs this step for you and shows both a full decimal value and a compact scientific notation format, which is usually how very large counts are presented in chemistry and physics.

Percent Composition By Mass

Percent composition shows how the molar mass of a compound is shared among its elements. For each element in the formula, the calculator works out:

Element mass = (count × atomic weight)
Mass % = (element mass ÷ total molar mass) × 100%

For example, in water (H2O) the oxygen contribution to molar mass is much larger than hydrogen, so oxygen has a higher mass percentage even though there are two hydrogen atoms for every oxygen atom. This kind of analysis is frequently used when checking empirical formulas or working with gravimetric data.

Working With Formulas, Molar Mass And Moles Together

The four calculator modes can be used together as a small concentration and composition toolkit. You might start by entering a formula to obtain molar mass, then use that value to turn a mass into moles, and finally move from moles to a particle count. For composition questions, the dedicated tab shows how each element contributes to the total, making it easier to confirm results from textbook style exercises.

Formula And Constant Summary

  • Molar mass: sum of (atomic weight × count) for all elements
  • Moles from mass: n = m ÷ M
  • Mass from moles: m = n × M
  • Particles from moles: N = n × NA
  • Percent composition: (element mass ÷ total molar mass) × 100%

Molecular Weight Calculator – Frequently Asked Questions

What units does the molar mass use?

Molar mass is reported in grams per mole (g/mol). Atomic weights for the elements are also in g/mol, so multiplying by the number of atoms and adding them together keeps everything in the same unit.

Can I use parentheses and hydrates in the formula?

Yes. Parentheses are interpreted as grouped units with a trailing count, and a dot in a hydrate such as CuSO4·5H2O is treated as an additional group with its own multiplier. As long as the symbols and counts are clear, the calculator will resolve the nested structure.

Why is my formula not accepted?

Most errors come from misspelled element symbols or incomplete groups. Element symbols always start with an uppercase letter and may be followed by one or more lowercase letters. If an element is not present in the built-in periodic table, the input will be treated as invalid.

How accurate are the atomic weights?

The tool uses commonly accepted standard atomic weights rounded to a reasonable number of decimal places. For teaching, assignments and general lab calculations this level of precision is usually more than enough.

Can I use this with polyatomic ions and large organic molecules?

Yes, as long as the formula uses standard symbols, counts and parentheses. For very large structures it may be easier to work from the empirical formula or a simplified representation, but the same approach to molar mass still applies.