{
  "id": "vapor-pressure",
  "version": "35023be2474e",
  "status": "published",
  "name": "Vapor Pressure Calculator",
  "question": "What is the vapor pressure?",
  "summary": "Finds a liquid’s vapor pressure at a new temperature with the Clausius–Clapeyron equation, the temperature for a vapor pressure, or the enthalpy of vaporization from two points, or the vapor pressure from Antoine constants.",
  "category": "chemistry",
  "subcategory": "gases",
  "url": "https://www.acalculator.org/chemistry/vapor-pressure-calculator",
  "markdown": "https://www.acalculator.org/chemistry/vapor-pressure-calculator.md",
  "kind": "function",
  "method": "ln(P₂ ÷ P₁) = (ΔHvap ÷ R)(1 ÷ T₁ − 1 ÷ T₂), so P₂ = P₁ × e^((ΔHvap ÷ R)(1 ÷ T₁ − 1 ÷ T₂)), 1 ÷ T₂ = 1 ÷ T₁ − (R ÷ ΔHvap) ln(P₂ ÷ P₁) and ΔHvap = R ln(P₂ ÷ P₁) ÷ (1 ÷ T₁ − 1 ÷ T₂); Antoine: log₁₀(P ÷ bar) = A − B ÷ (T + C).",
  "assumptions": [
    "ΔHvap does not change with temperature between T₁ and T₂, and the vapor behaves as an ideal gas.",
    "Antoine constants hold only in the temperature range their source gives; the page does not check it."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "find": {
        "title": "Find",
        "description": "P₂, T₂ or ΔHvap with the Clausius–Clapeyron equation, or the pressure from Antoine constants.",
        "type": "string",
        "enum": [
          "p2",
          "t2",
          "h",
          "antoine"
        ]
      },
      "dh": {
        "title": "ΔHvap (kJ/mol)",
        "description": "The enthalpy of vaporization in kilojoules per mole, such as 40.7 for water at its boiling point.",
        "type": "number",
        "minimum": 0.001,
        "maximum": 1000
      },
      "t1": {
        "title": "Temperature T₁",
        "description": "A temperature where you know the vapor pressure, such as the normal boiling point.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "temperature",
        "minimum": 1,
        "maximum": 100000
      },
      "p1": {
        "title": "Vapor pressure P₁",
        "description": "The vapor pressure at T₁: 101.325 kPa (1 atm) at the normal boiling point.",
        "type": "number",
        "minimum": 1e-12,
        "maximum": 1000000000000
      },
      "t2": {
        "title": "Temperature T₂",
        "description": "The other temperature.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "temperature",
        "minimum": 1,
        "maximum": 100000
      },
      "p2": {
        "title": "Vapor pressure P₂",
        "description": "The vapor pressure at T₂, in the same unit as P₁.",
        "type": "number",
        "minimum": 1e-12,
        "maximum": 1000000000000
      },
      "aa": {
        "title": "Antoine A",
        "description": "NIST’s A for the substance, with P in bar and T in kelvins.",
        "type": "number",
        "minimum": -100,
        "maximum": 100
      },
      "ab": {
        "title": "Antoine B",
        "description": "NIST’s B for the substance, in kelvins.",
        "type": "number",
        "minimum": -100000,
        "maximum": 100000
      },
      "ac": {
        "title": "Antoine C",
        "description": "NIST’s C for the substance, in kelvins.",
        "type": "number",
        "minimum": -1000,
        "maximum": 1000
      },
      "t": {
        "title": "Temperature T",
        "description": "The temperature, inside the range NIST gives for the constants.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "temperature",
        "minimum": 1,
        "maximum": 100000
      },
      "pu": {
        "title": "Pressure unit",
        "description": "The unit of the pressures typed and shown.",
        "type": "string",
        "enum": [
          "kPa",
          "atm",
          "mmHg",
          "torr",
          "bar",
          "Pa",
          "psi"
        ]
      }
    }
  },
  "outputs": {
    "result": {
      "label": "Answer",
      "description": "The vapor pressure, the temperature, or ΔHvap worked out.",
      "format": "text"
    },
    "pressure": {
      "label": "Vapor pressure",
      "description": "The vapor pressure in the unit chosen.",
      "format": "number"
    },
    "pBar": {
      "label": "Vapor pressure (bar)",
      "description": "The vapor pressure in bars.",
      "format": "number"
    },
    "tK": {
      "label": "Temperature T₂ (K)",
      "description": "The temperature worked out, in kelvins.",
      "format": "number"
    },
    "tC": {
      "label": "Temperature T₂ (°C)",
      "description": "The temperature worked out, in degrees Celsius.",
      "format": "number"
    },
    "dhKj": {
      "label": "ΔHvap (kJ/mol)",
      "description": "The enthalpy of vaporization in kilojoules per mole.",
      "format": "number"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "find": "p2",
      "dh": 31.3,
      "t1": "56.5 °C",
      "p1": 101.3,
      "t2": "25 °C",
      "p2": 30.1,
      "aa": 4.6543,
      "ab": 1435.264,
      "ac": -64.848,
      "t": "25 °C",
      "pu": "kPa"
    },
    "outputs": {
      "result": "P₂ = 30.3128 kPa",
      "pressure": 30.31283347675447,
      "pBar": 0.3031283347675447,
      "dhKj": 31.3
    },
    "text": "P₂ = 30.3128 kPa."
  },
  "examples": [
    {
      "given": {
        "find": "h",
        "t1": 307.15,
        "p1": 10,
        "t2": 371.95,
        "p2": 100,
        "pu": "kPa"
      },
      "expect": {
        "dhKj": 33.752811235222296,
        "result": "ΔHvap = 33.7528 kJ/mol"
      },
      "source": "OpenStax, Chemistry 2e, §10.3 Phase Transitions (ln(P₂/P₁) = (ΔHvap/R)(1/T₁ − 1/T₂)), https://openstax.org/books/chemistry-2e/pages/10-3-phase-transitions, Example 10.7 Estimating Enthalpy of Vaporization: 33.8 kJ/mol; Python 3",
      "tolerance": 1e-12
    },
    {
      "given": {
        "find": "h",
        "t1": 293.15,
        "p1": 5.95,
        "t2": 336.65,
        "p2": 53.3,
        "pu": "kPa"
      },
      "expect": {
        "dhKj": 41.35823372388284
      },
      "source": "OpenStax, Chemistry 2e, §10.3 Phase Transitions (ln(P₂/P₁) = (ΔHvap/R)(1/T₁ − 1/T₂)), https://openstax.org/books/chemistry-2e/pages/10-3-phase-transitions, Example 10.7 Check Your Learning: 41,360 J/mol; Python 3",
      "tolerance": 1e-12
    },
    {
      "given": {
        "find": "t2",
        "dh": 30.8,
        "t1": 353.25,
        "p1": 101.3,
        "p2": 83.4,
        "pu": "kPa"
      },
      "expect": {
        "tK": 346.8194160158882,
        "tC": 73.66941601588819
      },
      "source": "OpenStax, Chemistry 2e, §10.3 Phase Transitions (ln(P₂/P₁) = (ΔHvap/R)(1/T₁ − 1/T₂)), https://openstax.org/books/chemistry-2e/pages/10-3-phase-transitions, Example 10.8 Estimating Temperature (or Vapor Pressure): 346.9 K (73.8 °C) from 353.3 K; Python 3",
      "tolerance": 1e-12
    },
    {
      "given": {
        "find": "p2",
        "dh": 31.3,
        "t1": 329.65,
        "p1": 101.3,
        "t2": 298.15,
        "pu": "kPa"
      },
      "expect": {
        "pressure": 30.312833476754474,
        "result": "P₂ = 30.3128 kPa"
      },
      "source": "OpenStax, Chemistry 2e, §10.3 Phase Transitions (ln(P₂/P₁) = (ΔHvap/R)(1/T₁ − 1/T₂)), https://openstax.org/books/chemistry-2e/pages/10-3-phase-transitions, Example 10.8 Check Your Learning: 30.1 kPa (see content.mdx); Python 3",
      "tolerance": 1e-12
    },
    {
      "given": {
        "find": "antoine",
        "aa": 4.6543,
        "ab": 1435.264,
        "ac": -64.848,
        "t": 298.15,
        "pu": "kPa"
      },
      "expect": {
        "pBar": 0.03179380098485294,
        "pressure": 3.179380098485294,
        "result": "P = 3.17938 kPa (0.0317938 bar)"
      },
      "source": "NIST Chemistry WebBook, SRD 69, Water: Antoine equation parameters (log₁₀(P) = A − B/(T + C), P in bar, T in K; Stull 1947, 255.9 to 373 K: A = 4.6543, B = 1435.264, C = −64.848), https://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185&Mask=4&Type=ANTOINE&Plot=on; Python 3",
      "tolerance": 1e-12
    }
  ],
  "sources": [
    "OpenStax, Chemistry 2e, §10.3 Phase Transitions: the Clausius–Clapeyron equation (ln P = −ΔHvap/RT + ln A and ln(P₂/P₁) = (ΔHvap/R)(1/T₁ − 1/T₂), T in kelvins, any matching pressure units), Example 10.7 (isooctane: 33.8 kJ/mol) and its Check Your Learning (ethanol: 41,360 J/mol), Example 10.8 (benzene at 83.4 kPa: 346.9 K, 73.8 °C) and its Check Your Learning (acetone at 25.0 °C: 30.1 kPa). https://openstax.org/books/chemistry-2e/pages/10-3-phase-transitions (retrieved 2026-10-19)",
    "NIST Chemistry WebBook, SRD 69, Water, phase change data: Antoine equation parameters, log₁₀(P) = A − (B/(T + C)) with P in bar and T in K; 255.9 to 373 K (Stull, 1947): A = 4.6543, B = 1435.264, C = −64.848. https://webbook.nist.gov/cgi/cbook.cgi?ID=C7732185&Mask=4&Type=ANTOINE&Plot=on (retrieved 2026-10-19)",
    "NIST, CODATA value: molar gas constant R = 8.314 462 618… J/(mol·K), exact (N_A × k). https://physics.nist.gov/cgi-bin/cuu/Value?r (retrieved 2026-10-19)",
    "NIST, Special Publication 811, Appendix B.8: standard atmosphere = 101 325 Pa, torr = 101 325/760 Pa, conventional millimeter of mercury = 133.322 387 415 Pa, bar = 100 000 Pa. https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8 (retrieved 2026-10-19)"
  ],
  "related": [
    "ideal-gas-law",
    "partial-pressure",
    "combined-gas-law",
    "enthalpy"
  ],
  "changelog": []
}