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Partial Pressure Calculator

Use Dalton’s Law to find a gas’s partial pressure: Pᵢ = xᵢ · Ptotal. Calculate from mole fraction, from moles in a mixture, or for gas collected over water (Pgas = Ptotal − PH₂O). Includes steps, quick picks, and a mini pressure bar.

Background

In an ideal gas mixture, each gas behaves like it’s alone in the container. The total pressure is the sum of partial pressures: Ptotal = ΣPᵢ. Mole fraction is xᵢ = nᵢ / ntotal, so Pᵢ = (nᵢ/ntotal)·Ptotal.

Enter values

Tip: “Over water” is the classic lab setup (eudiometer) where water vapor adds pressure.

We’ll show answers in your chosen units.

Must be between 0 and 1.

One component per line. Format: name, moles (comma or spaces).

We compute: Punknown = Ptotal − ΣPknown.

This is the total pressure inside the container (gas + water vapor).

We estimate PH₂O from temperature (Antoine equation, valid for typical lab temps).

If your lab manual gives PH₂O from a table, paste it here.

Options

Chips prefill values and calculate immediately.

Result

No results yet. Enter values and click Calculate.

How to use this calculator

  • Choose a mode (mole fraction, moles list, missing partial, or over water).
  • Enter your values and select pressure units.
  • Click Calculate to get partial pressure(s), a pressure bar, and step-by-step.

How this calculator works

  • Dalton’s Law: Ptotal = ΣPᵢ
  • Mole fraction: xᵢ = nᵢ / ntotal
  • Partial pressure: Pᵢ = xᵢ·Ptotal
  • Over water: Pgas = Pmeasured − PH₂O

Example Problem & Step-by-Step Solution

Example 1 — From mole fraction (xᵢ + Ptotal → Pᵢ)

Air at sea level has total pressure Ptotal=1.00 atm. The mole fraction of oxygen is xO₂=0.209. Find the partial pressure of oxygen PO₂.

  1. Use Dalton’s Law in mole-fraction form: Pi = xi·Ptotal.
  2. PO₂ = 0.209 × 1.00 = 0.209 atm

Example 2 — From moles in a mixture (nᵢ list + Ptotal → xᵢ + Pᵢ)

A mixture has n(O₂)=0.50 mol, n(N₂)=1.50 mol, and n(CO₂)=0.20 mol at total pressure Ptotal=1.00 atm. Find each gas’s mole fraction and partial pressure.

  1. Total moles: ntotal=0.50+1.50+0.20=2.20 mol
  2. Mole fractions:
    xO₂=0.50/2.20=0.227
    xN₂=1.50/2.20=0.682
    xCO₂=0.20/2.20=0.091
  3. Partial pressures:
    PO₂=0.227×1.00=0.227 atm
    PN₂=0.682×1.00=0.682 atm
    PCO₂=0.091×1.00=0.091 atm
  4. Quick check: 0.227+0.682+0.091=1.000 atm (matches Ptotal)

Example 3 — Gas collected over water (Pmeasured − PH₂O → Pgas)

A gas is collected over water at 25 °C. The measured pressure is Pmeasured=755 mmHg. If water vapor pressure at 25 °C is PH₂O=23.8 mmHg, find the dry gas pressure Pgas.

  1. Use: Pgas = Pmeasured − PH₂O.
  2. Pgas = 755 − 23.8 = 731.2 mmHg
  3. (Optional) Convert to atm: 731.2/760 = 0.962 atm

Formula & Equation Used

Total pressure: Ptotal = ΣPᵢ

Mole fraction: xᵢ = nᵢ / ntotal

Partial pressure: Pᵢ = xᵢ·Ptotal

Over water: Pgas = Pmeasured − PH₂O(T)

Frequently Asked Questions

Q: What is partial pressure?

It’s the pressure a gas would exert if it occupied the container by itself.

Q: When do I subtract water vapor pressure?

When the gas is collected over water (water vapor adds to the measured pressure).

Q: Does this assume ideal gases?

Yes—Dalton’s Law works best for ideal (or near-ideal) gas mixtures.