My Easy Science
Molecular Weight and Molarity Calculator
Type a formula, get the molecular weight and the element breakdown, then work out how much to weigh for any molarity.
Inputs
Worked example
Loads a hand-worked test case from the spec, then shows the expected answer.
The formula
The molar mass is worked out from the formula above, with a per-element breakdown in the table below the chart.
Grams ↔ moles
Make a solution
Dilution helper (C1V1 = C2V2)
Change a mass and the result updates. These are the same values the Periodic Table uses.
Mass share by element
| Element | Atoms | Mass (g/mol) | Share (%) |
|---|---|---|---|
| Tap Show weight and molarity to see the numbers. | |||
Molecular weight
— g/mol
Molecular weight of the formula.
Checked against eight hand-worked examples (see the page source comments).
Results are shown to 2 decimal places.
Type a formula, then tap Show weight and molarity.
| Element | Atoms | Atomic weight | Mass (g/mol) | Share (%) |
|---|---|---|---|---|
| Tap Show weight and molarity to see the breakdown. | ||||
| Value | Result | Unit |
|---|---|---|
| Mass to weigh (target solution) | — | g |
| Mass to weigh (target solution) | — | mg |
| Moles in the weighed mass | — | mol |
| Molarity of the weighed mass | — | M |
| Grams from the moles entered | — | g |
| Dilution: final volume (V2) | — | mL |
For learning only. Not for patient care decisions. Always check with a qualified professional and the product label.
How it works
The molecular weight is the sum of every atom in the formula, each counted at its atomic weight. Water is 2 hydrogens at 1.008 plus 1 oxygen at 15.999, which is 18.015 g/mol. Glucose, C6H12O6, is 6 carbons, 12 hydrogens and 6 oxygens: 72.066 + 12.096 + 95.994, which is 180.156 g/mol. Brackets multiply a whole group, so Ca(OH)2 is calcium at 40.078 plus two OH groups at 17.007 each, which is 74.092 g/mol.
Once you have the molecular weight, moles and molarity follow. Moles equal mass divided by molecular weight. Molarity is moles per litre. And the mass to weigh for a solution is molarity times litres times molecular weight. So 1 litre of 1 M salt water needs 1 × 1 × 58.44, which is 58.44 g of NaCl. And 250 mL of 0.5 M glucose needs 0.5 × 0.25 × 180.156, which is 22.52 g. Work the other way: 5 g of NaCl in 250 mL is 5 / 58.44 = 0.0856 mol in 0.25 L, which is 0.342 M.
Hydrates carry water in the crystal: CuSO4·5H2O is the anhydrous salt at 159.602 plus five waters at 18.015 each, which is 249.68 g/mol. A number at the start multiplies the whole formula, so 2H2O is 36.03 g/mol. Brackets multiply a group, so K4[Fe(CN)6] is 368.35 g/mol. Charges do not change the mass, so Ca2+ and Fe3+ are read as Ca and Fe.