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Molecular Weight Calculator

· Last updated：2026-06-13

Calculate molar mass, elemental composition, and mass percentages from a chemical formula

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Written by the Elysia Tools team
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Weight50
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Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category
Math & Numbers
Input types
text, number
Output type
json
Sample coverage
4
API ready
Yes

Overview

The Molecular Weight Calculator computes the molar mass, elemental composition, and mass percentages of any chemical formula. Simply input a formula, including hydrates or complex compounds, to instantly retrieve precise molecular weights and stoichiometric details.

When to use

  • When preparing chemical solutions in a laboratory and needing to calculate the exact mass of a solute required for a specific molarity.
  • When analyzing stoichiometry problems to determine theoretical yields or mass percentages of individual elements in a compound.
  • When verifying the molecular weight of complex chemical formulas, including hydrates with dot notation.

How it works

  1. 1Enter the chemical formula (e.g., CuSO4·5H2O) into the input field.
  2. 2Specify the desired number of decimal places for the calculation precision.
  3. 3The tool parses the formula, identifies the constituent elements, multiplies their atomic weights by their quantities, and outputs the total molar mass.

Use cases

  • Calculating the molar mass of complex hydrates for laboratory reagent preparation.
  • Determining the mass percentage of specific elements within a compound for stoichiometric analysis.
  • Verifying chemical formulas and molecular weights during academic study or research.

Examples

1. Calculating Molar Mass of a Hydrate

Chemistry Lab Technician

Background

A technician needs to prepare a standard solution of copper(II) sulfate pentahydrate and needs its exact molar mass to calculate the required mass.

Problem

Manually calculating the molecular weight of CuSO4·5H2O is time-consuming and prone to rounding errors.

How to use

Enter 'CuSO4·5H2O' in the Chemical Formula field and set the decimal places to 4.

Formula: CuSO4·5H2O, Decimal Places: 4

Outcome

The tool outputs a molar mass of 249.6774 g/mol along with the elemental composition.

2. Determining Mass Percentage of Ethanol

High School Chemistry Teacher

Background

A teacher is preparing a lesson on mass percentage composition and needs quick, accurate values for ethanol.

Problem

Needs to show students the exact mass percentage of Carbon, Hydrogen, and Oxygen in C2H5OH.

How to use

Enter 'C2H5OH' in the Chemical Formula field and set the decimal places to 2.

FAQ

Does the calculator support hydrates?

Yes, you can input hydrates using dot notation, such as CuSO4·5H2O.

How does the tool handle parentheses in formulas?

It parses nested parentheses, multiplying the subscripts outside the parentheses by the elements inside.

Can I adjust the precision of the output?

Yes, you can set the decimal places option to any value between 0 and 8.

What atomic weights does the calculator use?

It uses standard IUPAC atomic weights for all elements on the periodic table.

Is case sensitivity important when entering formulas?

Yes, element symbols must use correct capitalization, such as Co for Cobalt versus CO for Carbon Monoxide.

Hub

Chemistry Lab Calculations and Reference

Prepare a traceable stoichiometry or solution plan by checking references, balancing equations, calculating molar quantities, normalizing units, and controlling significant figures.

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Formula: C2H5OH, Decimal Places: 2

Outcome

The tool calculates the molar mass of 46.07 g/mol and provides the mass percentage breakdown for C, H, and O.