Moles Calculator

About the Moles Calculator

The mole is the SI unit for amount of substance. Given any two of mass (g), molar mass (g/mol), or moles, this calculator finds the third using n = m / M.

How to use this calculator

1. Enter the mass, molar mass, or moles, depending on which value you know.
2. Use the calculator to apply the relationship between mass, molar mass, and moles.
3. Check that your units match, for example grams for mass and grams per mole for molar mass.
4. Read the result and, if needed, use it to plan a reaction or convert to another unit.

The formula explained

The formula \( n = \frac{m}{M} \) computes the number of moles \( n \) from the mass \( m \) and the molar mass \( M \). If you know two of the values, you can rearrange the relationship to find the third.

• n = number of moles
• m = mass of the substance
• M = molar mass of the substance

Step by step method

1. Write down the mass and the molar mass with the correct units.
2. Substitute the values into \( n = \frac{m}{M} \).
3. Divide the mass by the molar mass to get the number of moles.

Worked example

Problem. Find the number of moles in \( 18.0 \) grams of water, where the molar mass of water is \( 18.0 \) grams per mole.

1. Use \( n = \frac{m}{M} \).
2. Substitute the values, \( n = \frac{18.0}{18.0} \).
3. Calculate \( n = 1.0 \) mole.

Tips and common mistakes

• Always check that mass and molar mass use matching units, so the units cancel correctly.
• If your answer seems too large or too small, recheck the molar mass, because a small unit error can change the result a lot.

Frequently asked questions