{
  "id": "molality",
  "version": "6f354d6c03ed",
  "status": "published",
  "name": "Molality Calculator",
  "question": "What is the molality of my solution?",
  "summary": "Computes molality, moles of solute per kilogram of solvent, from the solute mass and molar mass or its moles, or works out the solute mass needed for a target molality.",
  "category": "chemistry",
  "subcategory": "solutions",
  "url": "https://www.acalculator.org/chemistry/molality-calculator",
  "markdown": "https://www.acalculator.org/chemistry/molality-calculator.md",
  "kind": "function",
  "method": "molality = moles of solute ÷ kilograms of solvent; moles = grams ÷ molar mass. For a target molality: moles = molality × kg of solvent; grams = moles × molar mass.",
  "assumptions": [
    "The solvent mass is the solvent alone, not the solution. Molality does not change with temperature, because it uses masses, not volumes."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "find": {
        "title": "Find",
        "description": "The molality from the solute mass or its moles, or the solute mass needed for a molality.",
        "type": "string",
        "enum": [
          "mass",
          "mol",
          "solute"
        ]
      },
      "ms": {
        "title": "Solute mass",
        "description": "The mass of the dissolved substance.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "mass",
        "minimum": 1e-12,
        "maximum": 1000000000
      },
      "n": {
        "title": "Solute, mol",
        "description": "The amount of the dissolved substance in moles.",
        "type": "number",
        "minimum": 1e-15,
        "maximum": 1000000000000000
      },
      "b": {
        "title": "Molality, mol/kg",
        "description": "The molality you want, in moles of solute per kilogram of solvent.",
        "type": "number",
        "minimum": 1e-12,
        "maximum": 1000000
      },
      "M": {
        "title": "Molar mass, g/mol",
        "description": "The molar mass of the solute in grams per mole (58.44 for sodium chloride).",
        "type": "number",
        "minimum": 0.001,
        "maximum": 10000000
      },
      "mw": {
        "title": "Solvent mass",
        "description": "The mass of the solvent alone (for example the water), not of the whole solution.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "mass",
        "minimum": 1e-12,
        "maximum": 1000000000
      }
    }
  },
  "outputs": {
    "result": {
      "label": "Answer",
      "description": "The molality or the solute mass, to 6 significant figures.",
      "format": "text"
    },
    "molality": {
      "label": "Molality (mol/kg)",
      "description": "Moles of solute per kilogram of solvent.",
      "format": "number"
    },
    "moles": {
      "label": "Solute (mol)",
      "description": "The amount of solute in moles.",
      "format": "number"
    },
    "grams": {
      "label": "Solute (g)",
      "description": "The solute mass in grams (when the molar mass is known).",
      "format": "number"
    },
    "mmolal": {
      "label": "Molality (mmol/kg)",
      "description": "The molality in millimoles per kilogram.",
      "format": "number"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "find": "mass",
      "ms": "2220 g",
      "n": 1,
      "b": 0.5,
      "M": 62.07,
      "mw": "2000 g"
    },
    "outputs": {
      "result": "17.883 mol/kg",
      "molality": 17.883035282745286,
      "moles": 35.76607056549057,
      "grams": 2220,
      "mmolal": 17883.035282745288
    },
    "text": "The answer is 17.883 mol/kg."
  },
  "examples": [
    {
      "given": {
        "find": "mass",
        "ms": 2.22,
        "M": 62.07,
        "mw": 2
      },
      "expect": {
        "b": 17.883035282745286,
        "result": "17.883 mol/kg"
      },
      "source": "OpenStax, Chemistry 2e, §11.4 Colligative Properties (molality m = mol solute ÷ kg solvent), https://openstax.org/books/chemistry-2e/pages/11-4-colligative-properties, Example 11.3 Calculating Mole Fraction and Molality: 17.9 m; Python 3"
    },
    {
      "given": {
        "find": "solute",
        "b": 0.5,
        "M": 58.44,
        "mw": 0.25
      },
      "expect": {
        "grams": 7.305,
        "n": 0.125,
        "result": "7.305 g of solute (0.125 mol)"
      },
      "source": "OpenStax, Chemistry 2e, §11.4 Colligative Properties (molality m = mol solute ÷ kg solvent), https://openstax.org/books/chemistry-2e/pages/11-4-colligative-properties; hand calculation in content.mdx: n = 0.500 × 0.250 = 0.125 mol; 0.125 × 58.44 = 7.305 g"
    },
    {
      "given": {
        "find": "mol",
        "n": 0.3,
        "mw": 0.6
      },
      "expect": {
        "b": 0.5,
        "mmolal": 500
      },
      "source": "OpenStax, Chemistry 2e, §11.4 Colligative Properties (molality m = mol solute ÷ kg solvent), https://openstax.org/books/chemistry-2e/pages/11-4-colligative-properties; hand calculation in content.mdx: 0.3 mol ÷ 0.6 kg = 0.5 mol/kg"
    }
  ],
  "sources": [
    "OpenStax, Chemistry 2e, section 11.4 Colligative Properties: molality m = mol solute ÷ kg solvent; Example 11.3 (ethylene glycol, 17.9 m). CC BY 4.0, retrieved 2026-10-03. https://openstax.org/books/chemistry-2e/pages/11-4-colligative-properties",
    "NIST Special Publication 811, Appendix B.8: 1 lb = 0.45359237 kg; 1 oz = 0.028349523125 kg. Retrieved 2026-10-03. https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8"
  ],
  "related": [
    "molarity",
    "concentration",
    "molar-mass"
  ],
  "changelog": []
}