Grams to Moles Calculator

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Understanding Moles

What is a mole?

A mole is 6.022 × 1023 particles of a substance. It is the bridge between a mass you can weigh and the number of molecules that actually react. To find how many moles you have, divide the mass in grams by the molar mass in grams per mole.

n = m / M

The formula as a triangle

Cover the quantity you want and what is left is the formula. Cover n and you get m / M.

6.022 × 1023

particles in one mole. One mole of water is about 18 mL, a little more than a tablespoon (a tablespoon is 15 mL), and that spoonful holds 6.022 × 1023 molecules.

A worked example, live

This uses the values in the calculator above. Change the mass or the molar mass and it updates.

n = m / M

Enter a mass and a molar mass above to see this worked out.

Common mistakes

The formula n = m / M is simple. Three things still trip people up.

Atomic mass is one atom from the periodic table. Molar mass is the whole formula added up, so for a compound you cannot just read one element’s value.
Subscripts multiply through the whole group: H₂O has 2 hydrogens, so it’s 2×1.008 + 15.999, not 1.008 + 15.999.
Units have to match: type 500 mg into a grams field and the answer is off by 1000× with no error shown.

FAQs

How do I convert grams to moles?

  1. Measure the mass in grams.
  2. Find the molar mass.
  3. Divide the mass by the molar mass.

For example, 36 grams of water divided by its molar mass, 18.015 g/mol, gives about 2 moles (1.998, rounded):

36 g ÷ 18.015 g/mol = 2 mol

How do I convert moles to grams?

  1. Find the number of moles.
  2. Find the molar mass.
  3. Multiply the moles by the molar mass.

For example, 3 moles of sodium chloride multiplied by its molar mass, 58.44 g/mol, gives 175.32 grams:

3 mol × 58.44 g/mol = 175.32 g

What is molar mass?

Molar mass is the mass of one mole of a substance, measured in grams per mole. It tells you how much 6.022 × 1023 particles of that substance weigh together. It varies a lot across the periodic table — a mole of hydrogen weighs about 1.008 grams, while a mole of uranium weighs about 238.03 grams.

How do I find the molar mass of a compound?

Add up the atomic mass of every atom in the compound. For carbon dioxide (CO2): one carbon atom (12.011 g/mol) plus two oxygen atoms (15.999 g/mol each) equals 44.01 g/mol total.

How is molar mass determined in a lab?

  • For an unknown solid, chemists use the colligative properties of a solution.
  • For a gas, equal volumes of any gas contain the same number of moles at the same temperature and pressure.
  • For a known compound, calculate it directly by summing atomic masses.