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Chemistry tool

Mole calculator

Type what you have — a mass and a formula, or just a formula. You get the answer, and underneath it the working, written the way it has to be handed in.

Grams, moles, or a bare formula. Lower case is fine.

Answer

1.39 mol

H2O has a molar mass of 18.015 g/mol, and the answer keeps 3 significant figures because that is what you typed.

Mass
25.0 g
what you can weigh
Amount of substance
1.39 mol
n, the quantity
Particles
8.36 × 10²³
molecules

What the numbers are describing

Four identical watch glasses in a row, each holding one mole: a small heap of copper turnings, a shallow pool of water, a low heap of salt crystals, and a tall mound of powdered sugar.
One mole of copper, of water, of table salt, of sugar. Every dish holds the same number of particles — and the piles are nothing like each other. That is the whole reason molar mass exists: the count is fixed, so the mass is what has to move.
A single water droplet at left, magnification lines opening to the right, and a dense field of individual water molecules running off the right edge of the paper.
A drop of water, magnified until the molecules are separate objects you could in principle count. The field runs off the edge of the plate because the count does too — a mole is 602,214,076,000,000,000,000,000 of them, and no drawing gets to the end of it.
A single bent water molecule alone at left; at right a cut-away cube of salt lattice, larger chloride and smaller sodium spheres alternating in a regular array.
Water comes in molecules: one oxygen, two hydrogens, a finished object. Table salt does not. It is an endless alternating lattice of sodium and chloride ions with no NaCl unit anywhere inside it — which is why the answer above counts formula units for salt and molecules for water.

How the conversion works

  1. What you have, and what you want

    Write down the formula of the substance, the quantity you were given, and the unit the question is asking for. Going from grams to moles and going from moles to grams use the same molar mass — which direction you are heading decides whether you divide by it or multiply.

  2. Find the molar mass

    Add up the atomic weight of every atom the formula contains, multiplying any bracketed group by the subscript outside it. Calcium nitrate, Ca(NO3)2, is one calcium at 40.078 plus two nitrate groups at 62.004 each, giving 164.086 g/mol.

  3. Get to moles

    Multiply the mass by a conversion factor written so the grams cancel: 25.0 g H2O × (1 mol H2O / 18.015 g H2O) = 1.39 mol. Whichever unit is left standing after the cancelling is the unit of the answer, which is how you check you picked the factor the right way up.

Questions

How do you calculate moles from grams?
Divide the mass in grams by the molar mass in grams per mole. For example, 25.0 g of water divided by 18.015 g/mol gives 1.39 mol. Written as a conversion factor it is 25.0 g H2O × (1 mol H2O / 18.015 g H2O), where the grams cancel and moles are left.
How do you find the molar mass of a compound?
Add the atomic weight of every atom in the formula. Calcium nitrate, Ca(NO3)2, is one calcium at 40.078 plus two nitrate groups at 62.004 each, giving 164.086 g/mol. The subscript outside the parentheses multiplies everything inside it.
Is the molar mass of oxygen 16 or 32?
Both, depending on what you mean. A single oxygen ATOM has a molar mass of 15.999 g/mol. Oxygen GAS is diatomic, written O2, so it is 31.998 g/mol. If a question says "oxygen gas" or writes O2, use 32. If it asks about the element or an atom of oxygen, use 16.
How many significant figures should the answer have?
As many as the least precise measurement you started with. If you weighed 25.0 g, that is three significant figures, so the answer is 1.39 mol rather than 1.38774. Molar masses taken from the periodic table are treated as exact enough not to limit the answer.
What is the difference between molar mass and molecular weight?
They land on the same number. Molecular MASS is the mass of one molecule, measured in unified atomic mass units; molar mass is the mass of a mole of them, in grams per mole. Water is 18.015 either way, which is exactly what makes the mole useful. "Molecular weight" is the older name and is still used for both — strictly it means relative molecular mass, which is a ratio and carries no units at all.

Getting the answer is not the same as being able to get it next time. Bond Forge makes you answer a chemistry question before you can take your turn, and deals the next one at your level.

Play Bond Forge freeWhat is a mole?

Atomic weights are the standard values from the CRC Handbook of Chemistry and Physics, 104th edition (2023), covering all 118 elements. Molar masses are rounded to three decimals, which is the precision printed on a periodic table, and every conversion on this page is computed with the same rounded value it shows you.

This reference is free for teachers and students to use and share under the CC BY-NC-ND 4.0 license. Licensing details