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Atomic Mass Calculator

Use our Atomic Mass Calculator to find atomic mass, molar mass, and elemental percentages from a chemical formula. Learn formulas, steps, examples, and common mistakes.
Total Molar Mass / Molecular Weight
180.156 g/mol

Hill System Formula: C6H12O6 (24 total atoms)

Elemental Mass Composition

ElementAtomsAtomic WtSubtotal MassMass %
Carbon (C)612.01172.066 g/mol40%
Hydrogen (H)121.00812.096 g/mol6.71%
Oxygen (O)615.99995.994 g/mol53.28%

C6H12O6 has a molar mass of 180.156 g/mol (24 total atoms across 3 elements).

What is an Atomic Mass?

Atomic mass is the mass of one atom, based mainly on its protons and neutrons. It uses atomic mass units (uu). Each isotope has its own atomic mass.

Atomic Mass Calculator showing carbon atom structure, isotopes C 12, C 13, C 14, and atomic mass in atomic mass units.

How can you use our Atomic Mass Calculator?

Our Atomic Mass Calculator quickly calculates the total molar mass and elemental mass percentage of a chemical formula.

Step 1

Enter Your Chemical Formula

Enter the chemical formula you want to calculate.

Step 2

View Molar Mass

Get the total molar mass or molecular weight of the compound instantly.

Step 3

Check Elemental Composition

View each element's mass contribution and percentage composition.

What Can You Calculate With an Atomic Mass Calculator?

An Atomic Mass Calculator helps you find molecular weight, molar mass, elemental mass percentage, and each element's mass contribution from a formula.

Molecular Weight of Compounds

Enter a formula to find molecular weight. For CO2CO_2, calculate 12.011+2(15.999)=44.009 g/mol12.011+2(15.999)=44.009\ g/mol. The result gives the compound's mass per mole.

Molar Mass From a Chemical Formula

The calculator adds atomic weights for each atom in a formula. For NaClNaCl, calculate 22.990+35.45=58.44 g/mol22.990+35.45=58.44\ g/mol. This gives sodium chloride's molar mass.

Elemental Mass Percentage

Find each element's share of total mass. Use  mass percentage =(element mass/molar mass)×100\text{ mass percentage }=(\text{element mass}/\text{molar mass})\times100. This gives each element's mass percentage.

Individual Element Mass Contribution

See each element's mass contribution in a compound. This shows which elements add the most to total molar mass and helps you check the formula.

Compounds With Parentheses

For formulas with parentheses, multiply every atom inside the group by the outside subscript. For Ca(OH)2Ca(OH)_2, the 22 applies to OO and HH.

Hydrated Compounds

For hydrated compounds, include the water molecules after the dot. For CuSO4⋅5H2OCuSO_4\cdot5H_2O, calculate CuSO4CuSO_4 and five water molecules.

How to calculate Atomic Mass manually

You can calculate average atomic mass by multiplying each isotope mass by its fractional abundance. Then add all isotope contributions. Use A=∑(m×f)A = \sum(m \times f), where mm is isotope mass and ff is fractional abundance.

Formula

A=(m1×f1)+(m2×f2)A = (m_1 \times f_1) + (m_2 \times f_2)

Step by step guide

  1. Write each isotope mass and percentage abundance.
  2. Divide each percentage by 100.
  3. Multiply each mass by its fractional abundance.
  4. Add all the results.
  5. Round the final answer as needed.

Practical Example

For chlorine, use 34.969 u at 75.78% and 36.966 u at 24.22%.

A=(34.969×0.7578)+(36.966×0.2422)A = (34.969 \times 0.7578) + (36.966 \times 0.2422)

A≈35.45 uA \approx 35.45\ u.

Common Chemical Formulas and Their Molar Masses: Table chart

These common chemical formulas provide quick reference values for chemistry homework, lab work, and formula checks. Values use standard atomic weights.

Chemical Formula

Compound

Molar Mass

H2OH_2O

Water

18.015 g/mol

CO2CO_2

Carbon dioxide

44.009 g/mol

NaClNaCl

Sodium chloride

58.44 g/mol

NH3NH_3

Ammonia

17.031 g/mol

CH4CH_4

Methane

16.043 g/mol

CaCO3CaCO_3

Calcium carbonate

100.086 g/mol

C6H12O6C_6H_{12}O_6

Glucose

180.156 g/mol

Note: Molar masses can vary slightly with atomic weight values and rounding. Match your course or lab reference when a specific value is required.

Common Mistakes When Calculating Molar Mass

Common errors include wrong atomic weights, missed subscripts, missing parentheses, or incorrect sums. Always check each element, atom count, formula, and unit before accepting the molar mass. These small mistakes can change the final answer.

Atomic Mass vs. Atomic Weight vs. Mass Number vs. Molar Mass

Atomic mass, atomic weight, mass number, and molar mass are related but describe different measurements. The table below highlights their definitions, units, and how they are used in chemistry.

Term

What It Means

Unit

How It Is Determined

Example

Atomic Mass

The mass of a single atom, usually expressed relative to 1/12 of the mass of a carbon 12 atom.

u (amu)

Depends on the specific isotope

Carbon 12 has an atomic mass of 12 u

Atomic Weight

The weighted average of the atomic masses of an element's naturally occurring isotopes.

u (often reported as a relative value)

Based on isotope abundances

Carbon has an atomic weight of about 12.011 u

Mass Number

The total number of protons and neutrons in the nucleus of a specific isotope.

No unit

Protons + neutrons

Carbon 12 has a mass number of 12

Molar Mass

The mass of one mole of atoms, molecules, or compounds.

g/mol

Sum of the atomic weights in the chemical formula

C₆H₁₂O₆ has a molar mass of 180.156 g/mol

Quick Difference

Atomic mass refers to the mass of one specific atom or isotope. Atomic weight is the weighted average of an element's naturally occurring isotopes. Mass number simply counts the protons and neutrons in one isotope. Molar mass tells you how many grams one mole of a substance weighs.

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Frequently Asked Questions

These FAQs answer student questions about atomic mass, neutrons, average atomic mass, mass number, carbon 12, and percentage composition.