Molality (m) Calculator

Molality (m) Calculator

Range 0.001–100000 g
Range 1–1000 g/mol
Accepts kg or g

Introduction

Molality describes how many moles of a dissolved substance are present per kilogram of solvent. Because it relies on mass rather than volume, it stays steady when temperature changes or when solutions expand and contract. That makes it a reliable way to compare concentrations, plan dilutions, and reason about colligative properties such as boiling point elevation or freezing point depression. This calculator lets you work in two convenient ways: either type the mass of your solute together with its molar mass to auto-convert to moles, or enter moles directly if you already know them. A unit switch beside the solvent field accepts kilograms or grams and converts to kilograms internally. Results appear instantly with a clear badge that signals whether the solution is dilute, moderate, or concentrated, along with a timestamp you can save for notes or reports. You can adjust the precision to three or six significant figures; very small values are shown in scientific notation to keep them readable. Built-in checks catch common input mistakes—like a molar mass outside realistic bounds or an implausibly tiny solvent mass—and provide gentle warnings rather than stopping your work. Use the download buttons to save a tidy text summary, or generate a compact PDF sheet that captures your inputs and outputs in a printable format.

How to use

  1. Choose Mass + Molar Mass or Moles.
  2. Enter your numbers. If using grams for the solvent, leave the unit on g; the tool converts to kilograms.
  3. Pick the precision (3 or 6 significant figures).
  4. Press Calculate to view m (mol/kg), moles of solute, solvent mass in kilograms, and a timestamp.
  5. Use Download TXT to keep a quick record, or Download PDF for a shareable one-page summary.

Worked example

5.85 g NaCl with molar mass 58.44 g/mol in 0.100 kg water → moles = 5.85/58.44 ≈ 0.1001 mol; molality = 0.1001/0.100 ≈ 1.001 mol/kg (Moderate).

Formula and notes

m = n / msolvent(kg);   n = mass / M when starting from grams and molar mass. Molality is independent of temperature; interpretation depends on the purpose of the solution. Bands used here (dilute <1; moderate 1–<5; concentrated ≥5) are instructional and not prescriptive.

How it works

The calculator computes moles either directly or by converting mass using the molar mass. Solvent entries in grams are divided by 1000 to get kilograms. Significant-figure formatting affects display only; internal math uses double precision.

FAQ
  • What is molality? The number of moles of solute per kilogram of solvent.
  • Why use molality instead of molarity? It does not change with temperature because masses don’t expand or contract.
  • Can I type the solvent in grams? Yes; it’s converted to kilograms internally.
  • How are moles calculated from mass? Moles = mass (g) ÷ molar mass (g/mol).
  • Why is my result extremely large? Very small solvent masses inflate values—check units and realism of inputs.
  • How precise are the numbers? Display is 3 or 6 significant figures; underlying math uses double precision.

Disclaimer. Educational use only. Not a substitute for professional judgment.