Molar mass is the bridge between the formula you can write on paper and the amount of substance you can weigh out in the laboratory. This molar mass calculator breaks a chemical formula down into its elements, adds up the atomic masses and gives you the molar mass along with the mass of a single molecule and the mole ratio of each element.
What Is the Molar Mass Calculator?
The molar mass of a compound is the mass of one mole of it, in grams per mole, and it is found by adding up the atomic masses of the elements in the formula multiplied by how many of each there are.
This calculator parses the formula for you, including parentheses and multipliers, and returns the molar mass, the mass of a single molecule and the element breakdown so you can check the working.
How to Work It Out
Molar mass = sum of (atomic mass × number of atoms of that element)
For glucose, C6H12O6, that is six carbons at 12.011, twelve hydrogens at 1.008 and six oxygens at 15.999, giving 180.16 grams per mole. The arithmetic is easy; the part that catches people out is remembering to use the atomic mass of the hydrate if that is what is on the shelf.
Parentheses change the count. Calcium hydroxide, Ca(OH)2, contains one calcium, two oxygens and two hydrogens, not one of each, because the subscript applies to the whole group.
How Molar Mass Is Used
The single most common use is converting between mass and moles, which is what turns a weight on the balance into an amount of substance for a reaction. The relationship is mass equals moles multiplied by molar mass.
It also underpins solution preparation. To make a 0.1 molar solution of sodium chloride you need 5.844 grams of it in a litre of solution, which you get by multiplying 0.1 by 58.44. Stoichiometry relies on the same conversion in the other direction, turning a reactant mass into moles and then into a coefficient ratio.
Practical Points Worth Knowing
Round your final answer to match the precision of your data rather than to the full number of digits. A molar mass quoted to nine decimal places suggests a precision that no balance can deliver.
Atomic masses are averages over natural isotopes, so a molar mass is a weighted mean rather than a fixed value. For most purposes this does not matter, but for work on isotopes, or on elements with widely varying natural abundance, it can.
Frequently Asked Questions
What is the molar mass of water?
18.015 grams per mole, calculated as two hydrogens at 1.008 plus one oxygen at 15.999. A mole of water is therefore 18.015 grams.
How do I calculate molar mass from a formula?
Multiply each element’s atomic mass by its subscript, then add the results. For Ca(OH)2 that is 40.078 plus twice 15.999 plus twice 1.008, giving 74.092 grams per mole.
What is the difference between molar mass and molecular mass?
They are the same number expressed in different units. Molecular mass is the mass of one molecule in atomic mass units, and molar mass is the mass of one mole in grams, so a molar mass of 18.015 corresponds to a molecular mass of 18.015.
How do I convert grams to moles?
Divide the mass by the molar mass. 36 grams of water is 36 divided by 18.015, which is very close to 2 moles.
What if I am using a hydrated salt?
Use the molar mass of the hydrate including the water of crystallisation, because that is the material on the balance. Copper sulfate pentahydrate is 249.68 grams per mole, not 159.61 for the anhydrous salt.