ChemistryQuantitative ChemistryA-Level

Number of Moles (Mass and Molar Mass) Calculator

Calculates the number of moles (n) of a substance by dividing its mass (m) by its molar mass (M).

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Number of moles

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Overview

This fundamental relationship serves as the bridge between the macroscopic world of laboratory measurements (grams) and the microscopic world of atoms and molecules. By determining the molar mass from the periodic table, chemists can precisely quantify the amount of substance present in a sample, which is essential for stoichiometry and chemical equations.

Symbols

Variables

n = Number of moles, m = Mass (g), M = Molar mass (g/mol)

Number of moles
Variable
Mass (g)
Variable
Molar mass (g/mol)
Variable

Apply it well

When To Use

When to use: Use this equation when you are given a specific mass of a pure substance and need to determine the amount of substance in moles to perform further stoichiometric calculations.

Why it matters: It is the essential conversion factor for all chemical manufacturing, pharmacy compounding, and analytical research where reactant ratios must be controlled.

Avoid these traps

Common Mistakes

  • Forgetting to calculate the total molar mass of a molecule (e.g., using only the mass of one element in CO2).
  • Using atomic mass units (u) instead of molar mass (g/mol) in calculations.
  • Confusing molar mass (M) with the mass of a single atom.

One free problem

Practice Problem

Calculate the number of moles in 10.0 g of sodium hydroxide (NaOH), given a molar mass of 40.0 g/mol.

Mass (g)10
Molar mass (g/mol)40

Solve for:

Hint: Divide the mass by the molar mass.

The full worked solution stays in the interactive walkthrough.

References

Sources

  1. Zumdahl, S. S., & Zumdahl, S. A. (2017). Chemistry (10th ed.). Cengage Learning.
  2. Atkins, P., & de Paula, J. (2014). Elements of Physical Chemistry.
  3. A-Level Chemistry (OCR/AQA/Edexcel) Specification
  4. Fundamentals of Chemistry (General Chemistry textbooks)
  5. AQA/OCR/Edexcel A-Level Chemistry Specification - Section 1.1: Amount of substance