PPM to Molality Calculator
Molarity changes with temperature; molality does not — which is why freezing-point and boiling-point work uses mol/kg. Converting PPM there takes the molecular weight: PPM ÷ (MW × 1,000) for dilute aqueous solutions.
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PPM to Molality Calculator
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Molarity changes with temperature; molality does not — which is why freezing-point and boiling-point work uses mol/kg. Converting PPM there takes the molecular weight: PPM ÷ (MW × 1,000) for dilute aqueous solutions.
How to use this calculator
- Enter PPM.
- Enter molecular weight (g/mol).
- Get molality (mol/kg).
The formula
PPM (mg/L) → g/L (÷1,000) → mol/L (÷MW) ≈ mol/kg in dilute water.
What the units mean
PPM (parts per million) — one part of solute per million parts of the total mixture. The workhorse unit for trace concentrations in water, air, and food.
Molality (mol/kg) — moles of solute per kilogram of solvent. Unlike molarity it does not change with temperature, which is why freezing-point and boiling-point calculations use it.
Assumption: This conversion assumes a dilute water-like solution (density ≈ 1 kg/L). For concentrated solutions or other solvents, measure density and convert properly.
Worked example
1,000 PPM NaCl (MW 58.44): 1,000 ÷ (58.44 × 1,000) ≈ 0.0171 mol/kg.
Quick reference
| PPM NaCl | Molality |
|---|---|
| 585 | 0.010 |
| 5,844 | 0.100 |
Dilute aqueous approximation.
Where you will actually use this
Colligative property calculations demand molality, not molarity.
- Freezing-point depression
- Boiling-point elevation
- Osmotic pressure estimates
Mistakes that give wrong answers
- Using it for concentrated solutions (density shifts)
- Confusing molality with molarity
Key takeaways
- Formula: Molality ≈ PPM ÷ (MW × 1,000) [dilute aqueous]
- Remember: Dilute aqueous approximation.
- Watch out for: Using it for concentrated solutions (density shifts)
- Next step: run your own numbers in the calculator above, then verify with a measurement — calculated values are only as good as the inputs.
Frequently asked questions
What is the difference between molality and molarity?
Molality is per kg of solvent (temperature-independent); molarity is per liter of solution.
When does the approximation break?
Above ~10,000 PPM or in non-aqueous solvents.