Calculadora de Peso Molecular
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
Formula
PM = m / n
About this calculator
The Molecular Weight Calculator is a tool that determines the molecular mass of a substance from the total sample mass and the amount of substance in moles. Molecular weight, also called molecular mass, is a fundamental property that relates the mass of a substance to the number of particles, expressed in grams per mole (g/mol). This calculator uses the formula MW = m / n, where m is the mass in grams and n is the number of moles, providing the result quickly and accurately.
To use the calculator, enter the values of mass and amount in moles in the appropriate fields. The tool is useful for chemistry students checking stoichiometry exercises, for laboratory professionals preparing solutions with specific concentrations, or for anyone needing to convert between mass and moles of a pure substance. Ensure units are correct: mass in grams and amount in moles, as the result will automatically be in g/mol.
Common cautions include confusion between molecular weight and atomic mass. Molecular weight refers to molecules, while atomic mass is for individual atoms. Also, ensure the sample is homogeneous and pure, as impurities can distort the calculation. For ionic compounds like NaCl, the correct term is formula mass, but the principle is the same. This calculator simplifies the process, but the user must have accurate mass and mole values.
Frequently asked questions
What is molecular weight?
Molecular weight is the mass of a molecule, expressed in atomic mass units (u) or grams per mole (g/mol). It is calculated by summing the atomic masses of all atoms in the molecule.
Can I use this calculator for ionic compounds?
Yes, but for ionic compounds the correct term is formula mass. The calculator works the same way, using total mass and amount in moles.
How do I convert mass to moles without the calculator?
Use the formula n = m / MW, where n is the number of moles, m is mass, and MW is molecular weight. The calculator does the reverse: given m and n, it finds MW.
Does the calculator work for gases?
Yes, as long as you have the gas mass and amount in moles. The molecular weight of gases is calculated the same way.
What is the difference between molecular weight and molar mass?
In practice, they are numerically the same. Molecular weight is the mass of one molecule in u, while molar mass is the mass of one mole in g/mol. The calculator provides molar mass.