Find a gas's partial pressure with Dalton's law from its moles, the total moles and the total pressure, plus mole fraction and percent in atm, kPa, mmHg or bar.
mol
mol
Results
Calculated
Mole fraction χ
—
Moles of gas / total moles
Partial pressure
—
In the unit you chose
Share of the mixture
—
Percent by moles (= volume % for ideal gases)
Pressure of the other gases
—
Total minus this gas
Ready
Enter moles of the gas, total moles and total pressure, then press Calculate.
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What this calculator finds
In a mixture of gases, each gas contributes its own share of the total pressure, called its partial pressure. Dalton's law says the total pressure is the sum of these partial pressures. Enter the moles of one gas, the total moles in the mixture and the total pressure, and this calculator returns the mole fraction, the partial pressure, its percentage share and the pressure left over for the other gases.
It suits general chemistry problems on gas mixtures, breathing gases, and reaction vessels.
The equations
χi = ni / ntotal, the mole fraction of the gas.
Pi = χi × Ptotal, Dalton's law of partial pressures.
ΣPi = Ptotal, so the pressure of all other gases is Ptotal − Pi.
Worked example
A vessel holds 0.40 mol of oxygen in a 2.00 mol gas mixture at 1.00 atm total pressure, the default inputs.
The mole fraction is 0.40 / 2.00 = 0.2000. The partial pressure of oxygen is 0.2000 × 1.00 atm = 0.2000 atm, which is 20.00% of the mixture. The other gases account for 1.00 − 0.20 = 0.8000 atm. Switching the unit to kPa with a total of 101.325 kPa would give 20.27 kPa for oxygen.
Common mistakes and how to read the result
Using moles of the wrong gas as the total. The total must include every gas, so it can never be smaller than the gas's own moles.
Mixing pressure units. The answer is in whatever unit you pick for the total pressure; converting afterwards is safer than mixing inputs.
Ignoring water vapor. For gases collected over water, subtract the vapor pressure first.
Applying it to reactive mixtures after reaction. Recount moles after any reaction changes the composition.
Frequently Asked Questions
Does this only work for ideal gases?
Dalton's law is exact for ideal gases and a very good approximation for real gases at ordinary pressures. At high pressure the mole fraction and partial pressure drift apart from measurements.
Is mole fraction the same as volume fraction?
For ideal gases, yes, because equal moles occupy equal volumes at the same temperature and pressure. That is why gas percentages such as 21% oxygen in air can be used directly as mole fractions.
What if I know volume and temperature instead of total pressure?
Find the total pressure first with the ideal gas law, P = nRT/V, using the total moles, then enter it here. Each gas's partial pressure can also be found directly from P = nRT/V using its own moles.
Can I use it for humid gas collected over water?
Yes, in two steps. Subtract the water vapor pressure from the total pressure to get the pressure of the dry gas, then enter that as the total pressure with the gas's own mole fraction.
Partial Pressure Calculator is most useful when the inputs reflect the situation you are actually planning around, not a best-case estimate. Treat the result as a decision aid: it gives you a structured way to compare assumptions, spot outliers, and decide what to verify next. For Chemistry work, the most important review lens is units, concentration, limiting assumptions, temperature, precision, and significant figures.
Start with a baseline run using values you can defend. Then change one assumption at a time and watch which output moves the most. If one input dominates the result, spend your verification time there first. If several inputs have similar influence, use a conservative scenario and an optimistic scenario to create a practical range instead of relying on a single exact number.
Before acting on the result, verify inputs against lab notes, reagent labels, and the expected reaction or solution model. This is especially important when the calculator supports a purchase, project plan, performance target, or operational decision. The calculator can make the math consistent, but the quality of the conclusion still depends on current data, clear units, and assumptions that match your real constraints.
When the output looks surprising, slow down and inspect each input in order. A small change in one high-leverage field can move the final number more than several low-leverage fields combined. For Partial Pressure Calculator, that means you should first confirm the value with the greatest scale, then confirm the value with the greatest uncertainty, then rerun the calculator with conservative and optimistic assumptions. This sequence turns the calculator from a single answer into a practical decision range.
Review Checklist
Confirm every input uses the unit and time period requested by the calculator.
Run a low, expected, and high scenario so the answer has a useful range.
Check whether rounding or a missing decimal place changes the decision.
Update the calculation for every new mixture, batch, reaction, or homework data set.
How to Validate the Result
Use Partial Pressure Calculator as a repeatable checkpoint rather than a one-time answer. The safest workflow is to record the original inputs, save the output, and write down which assumption you are testing. Then rerun the calculator with one changed value. If the result changes sharply, that input deserves more attention before you act on the number.
For this topic, the main validation lens is units, concentration, limiting assumptions, temperature, precision, and significant figures. That means a result can be mathematically correct and still be misleading if the inputs come from the wrong time period, use inconsistent units, or mix expected values with best-case values. Keep baseline, conservative, and optimistic runs separate so the final decision is easier to explain later.
When you share the result with someone else, include the assumptions and the date of the calculation. Many calculator outputs become stale after prices, schedules, measurements, or constraints change. A short note about the source of each input makes the calculation auditable and prevents later confusion about why the answer moved.
Label the source for each input before comparing scenarios.
Use the same rounding method across every run.
Flag any input that is estimated rather than measured.
Recalculate for every new mixture, batch, reaction, or homework data set.