Partial Pressure Calculator

Related Calculators

Partial pressure: The part of the total pressure caused by one gas in a mixture.

Total pressure: The pressure of the whole gas mixture after all gases are counted together.

Gas share of the mixture: The mole fraction, which means the decimal part of the whole gas mix made by the chosen gas.

Mole fraction: A number from 0 to 1 found by dividing one gas's moles by the total moles of all gases.

Moles: A chemistry counting unit for how much gas you have.

Dry gas pressure: The gas pressure left after water vapor pressure is subtracted from the measured pressure.

Water vapor pressure: The pressure from water vapor above liquid water at a given temperature.

Ideal gas law: A relation that connects pressure, moles, temperature, and volume for a gas: P = nRT/V.

Check against total pressure: The difference between the stated or rebuilt total pressure and the sum of the partial pressures used in the calculation.

Mistake: Typing 20.9 into "Gas share of the mixture" instead of 0.209.
Fix: Enter the gas share as a decimal part of the whole mix, so 20.9% becomes 0.209.

Mistake: Mixing units, like entering "Total pressure" in kPa while "Pressure unit" is set to atm.
Fix: Make the selected "Pressure unit" match every pressure number you enter in that mode.

Mistake: Using commas incorrectly in "Moles of the other gases," such as double commas or empty entries.
Fix: Enter a clean list like 1.50, 0.20 with numbers only, separated by single commas.

Mistake: Entering 25 for "Temperature (K)" when the value is really 25 degrees C.
Fix: Convert to Kelvin first and enter the value in "Temperature (K)," such as 298.15 K for 25 degrees C.

Mistake: Putting 0 or a negative value in "Container volume (L)" or "Measured pressure."
Fix: Use values greater than 0, because pressure and volume must be positive in these formulas.

Mistake: In over-water problems, entering a "Water vapor pressure" that is greater than or equal to "Measured pressure."
Fix: Check the lab data or the selected "Pressure unit," because "Dry gas pressure" must stay above 0.

• This calculator uses standard classroom formulas for Dalton's law and the ideal gas law, so real gases that strongly deviate from ideal behavior can give different results.
• In "Gas collected over water," the built-in water table only works for listed temperatures: 0, 5, 10, 15, 20, 25, 30, 35, and 40 degrees C.
• If you choose the built-in water table, temperatures between listed values are not interpolated here and must be handled by your teacher's or lab's rule.
• All pressure entries in one calculation must use the selected pressure unit consistently, or the result will be wrong.
• The "Check against total pressure" value can be slightly different from 0 after rounding, even when the setup is correct.
• For over-water work, the setup is physically impossible when water vapor pressure is greater than or equal to measured pressure, because that would leave zero or negative dry gas pressure.

Core formulas

The calculator uses Dalton's law, which says the total pressure of a gas mixture equals the sum of the partial pressures of the individual gases.

P_total = sum(P_i)

When you know the gas share of the mixture, partial pressure is found by multiplying that decimal share by the total pressure.

P_i = x_i * P_total

When you know moles in a mixture, the calculator first finds the mole fraction, which is the chosen gas's part of all gas moles.

x_i = n_i / n_total

n_total = n_i + sum(n_other)

In ideal gas mode, the chosen gas is treated as if it alone fills the same container volume at the same temperature.

P_i = (n_i * R * T) / V

For gas collected over water, the measured pressure includes both the gas and water vapor, so the water vapor part must be subtracted.

P_gas = P_measured - P_H2O

Unit handling

The selected pressure unit controls pressure inputs and outputs in the active mode. The calculator converts internal values so the formulas stay consistent. For ideal gas mode with volume in liters and temperature in Kelvin, it uses the matching gas constant for the chosen pressure unit. Useful constants include 0.082057 L atm mol^-1 K^-1, 101.325 kPa per atm, 760 mmHg per atm, 1 torr per mmHg, and 101325 Pa per atm.

Built-in over-water table

When you use the built-in water table, the calculator matches the exact water temperature to these vapor pressures in mmHg: 0 to 4.6, 5 to 6.5, 10 to 9.2, 15 to 12.8, 20 to 17.5, 25 to 23.8, 30 to 31.8, 35 to 42.2, and 40 to 55.3.

Worked mini-examples

If total pressure is 1.00 atm and the gas share is 0.209, then the partial pressure is:

P_i = 0.209 * 1.00 = 0.209 atm

If a mixture has 0.50 mol of the chosen gas and 1.50 mol and 0.20 mol of other gases, then total moles are 2.20 mol and the mole fraction is 0.50 / 2.20 = 0.22727. At 1.00 atm total pressure, the chosen gas pressure is:

P_i = 0.22727 * 1.00 = 0.22727 atm

If gas is collected over water at 25 degrees C with measured pressure 755 mmHg, the built-in water vapor pressure is 23.8 mmHg, so the dry gas pressure is:

P_gas = 755 - 23.8 = 731.2 mmHg

Checks and output meaning

The main result is "Partial pressure of the chosen gas." In moles mode, the calculator can also rebuild the total from the chosen gas pressure plus the pressure of the other gases and show "Check against total pressure." A result near 0 means the pressures add back correctly. A positive value means the listed partial pressures add to more than the total, and a negative value means they add to less.

Assumptions used

The formulas assume a standard classroom model of non-reacting gases and ideal-gas behavior. Hidden inputs are ignored in other modes. The calculator blocks impossible entries such as mole fraction below 0 or above 1, temperature at or below 0 K, volume at or below 0, and over-water cases where water vapor pressure is greater than or equal to measured pressure.