What this calculator does
The Mass Percent to Moles Calculator converts a solution's mass percent (also written % w/w) into the number of moles of dissolved solute. You give it three things — the mass percent, the total mass of the solution, and the molar mass of the solute — and it returns the mass of solute, the moles of solute, and the equivalent amount in millimoles.
This is a common step in the chemistry lab when a reagent bottle is labelled by concentration (for example "36.5% w/w hydrochloric acid") but your calculation needs an amount in moles.
How to use it
Enter the mass percent as a plain number (a 10% w/w solution is entered as 10, not 0.10). Enter the mass of the solution in grams — this is the total mass of solute plus solvent, not the solvent alone. Enter the molar mass of the solute in grams per mole, which you find by adding up the atomic masses of every atom in its chemical formula. The result updates as soon as all three boxes are filled.
The formula explained
The calculation has two steps. First, the mass of solute is the mass percent (divided by 100 to turn it into a fraction) multiplied by the mass of the solution:
$$ m_{solute} = \frac{p}{100} \times m $$Then the number of moles is that mass of solute divided by the molar mass of the solute:
$$ n = m_{solute} \div M $$Combining the two steps into a single expression gives:
$$ n = \frac{p \times m}{100 \times M} $$Here p is the mass percent, m is the mass of the solution in grams, M is the molar mass in grams per mole, and n is the amount of substance in moles.
Worked example
Suppose you have 250 g of a 10% w/w salt solution and the solute has a molar mass of 50 g/mol. The mass of solute is 0.10 × 250 = 25 g. Dividing by the molar mass gives 25 ÷ 50 = 0.5 mol, which is the same as 500 mmol. So a quarter-kilogram of this 10% solution contains half a mole of dissolved solute.
Frequently asked questions
What does "mass percent" mean? Mass percent (% w/w) is the mass of solute divided by the total mass of the solution, multiplied by 100. A 10% w/w solution contains 10 g of solute in every 100 g of solution.
Should I enter the mass of the solution or just the solvent? Enter the total mass of the solution — solute plus solvent — because mass percent is defined relative to the whole solution, not the solvent by itself.
How do I find the molar mass of the solute? Add up the atomic masses of every atom in the compound's formula. For example, sodium chloride (NaCl) is about 58.44 g/mol and glucose is about 180.16 g/mol.
