Dalton's Law says the total pressure of a nonreacting gas mixture equals the sum of each gas's partial pressure. In Intro to Chemical Engineering, you use it to analyze vapor mixtures in distillation and other gas-phase systems.
Dalton's Law is the rule that, for a mixture of nonreacting gases, the total pressure equals the sum of the partial pressures of each gas. In Intro to Chemical Engineering, that means you can treat each gas as if it were contributing its own share of the pressure, then add those shares together to get the mixture pressure.
A partial pressure is the pressure one gas would exert if it were alone in the same volume at the same temperature. That idea works cleanly when gases behave close to ideally, which is why Dalton's Law shows up so often with ideal gas calculations. If you know the mole fraction of a gas in the mixture, you can find its partial pressure using P_i = y_i P_total.
The law is especially useful in vapor-liquid equilibrium and distillation problems. For example, when a liquid mixture is heated in a distillation column, the vapor above it is not made of one pure substance. It is a gas mixture, and Dalton's Law lets you connect the vapor composition to the total pressure in the column. That connection helps you estimate boiling behavior, vapor composition, and how easily components separate.
This is not a rule about chemically reacting gases. If gases react, the mixture composition is changing, so the simple add-the-pressures picture no longer applies cleanly. It also works best when the gas phase is close to ideal, which is why it is often paired with the ideal gas law and used as a first-pass engineering approximation.
In practice, Dalton's Law is one of the shortcuts that keeps distillation calculations manageable. Instead of tracking the whole mixture as one mysterious fluid, you break it into component gases, find each contribution, and use the total to check operating conditions in a column or separator.
Dalton's Law matters because chemical engineering is full of mixtures, and distillation depends on predicting how those mixtures behave in the vapor phase. When you design or analyze a separation, you need to know which component is contributing how much to the total pressure. That information helps you connect composition, volatility, and boiling behavior.
In a distillation problem, the vapor above a boiling liquid is usually richer in the more volatile component. Dalton's Law gives you the pressure bookkeeping that makes those calculations possible. Without it, it is much harder to move between mole fractions, partial pressures, and total column pressure.
It also shows up in safety and operating decisions. If a gas stream contains multiple components, engineers need to know whether the total pressure will stay within equipment limits and how much of each component is present in the gas phase. That affects column operation, condenser loading, and separation efficiency.
The concept is also a bridge to more advanced thermodynamics. Once you get comfortable with Dalton's Law, it becomes easier to see why ideal gas assumptions work, where they break down, and why real-mixture corrections like activity coefficients matter later in the course.
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view galleryPartial Pressure
Dalton's Law is built from partial pressures. Each gas contributes its own pressure share, and the total pressure is the sum of those parts. In distillation and vapor-phase calculations, you usually solve for a component's partial pressure first, then combine it with the others to describe the mixture.
Ideal Gas Law
Dalton's Law works cleanly when gases behave ideally, so these two ideas often show up together. The ideal gas law helps relate pressure, volume, temperature, and moles, while Dalton's Law separates a mixture into component pressures. Together they make gas-mixture calculations much easier.
Vapor-Liquid Equilibrium
Vapor-liquid equilibrium is where Dalton's Law becomes especially useful in chemical engineering. Once a liquid mixture starts boiling, the vapor phase has its own composition and pressure. Dalton's Law helps connect that vapor composition to the total pressure, which is a big part of predicting separation in distillation.
Distillation Column
Inside a distillation column, vapor and liquid move through many contacting stages, and the vapor phase is a gas mixture governed by pressure relationships. Dalton's Law helps you describe the vapor leaving each stage and analyze how the column separates components over time. It is part of the pressure and composition logic behind column design.
A quiz question or problem set usually gives you a gas mixture, a total pressure, and mole fractions or component amounts, then asks you to find one partial pressure or the full mixture pressure. You use Dalton's Law directly: add the component pressures to get the total, or multiply mole fraction by total pressure to get a component's share. In distillation questions, that step often comes right before you predict vapor composition or compare boiling behavior.
If the problem includes a vapor-liquid equilibrium diagram or a column stage, Dalton's Law helps you read the gas-phase side of the process instead of treating the vapor as a single substance. A common mistake is mixing up total pressure with partial pressure, so watch your units and check whether the gas mixture is ideal enough for the shortcut to make sense.
Dalton's Law and Raoult's Law often appear together in distillation, but they do different jobs. Dalton's Law describes the total pressure of a gas mixture as the sum of partial pressures, while Raoult's Law links a liquid component's vapor pressure to its mole fraction in the liquid. Use Dalton's Law for the vapor phase and Raoult's Law for the liquid phase.
Dalton's Law says the total pressure of a nonreacting gas mixture is the sum of the partial pressures of its components.
In Intro to Chemical Engineering, you use it most often for vapor-phase calculations in distillation and vapor-liquid equilibrium.
The shortcut works best for ideal or near-ideal gases, which is why it is often paired with the ideal gas law.
A component's partial pressure can be found with P_i = y_i P_total, where y_i is its mole fraction in the gas mixture.
If gases are reacting or strongly non-ideal, you need a more careful model than Dalton's Law alone.
Dalton's Law says that the pressure of a gas mixture is the sum of the pressures each gas would exert on its own. In chemical engineering, that lets you break a vapor stream into component pressures and handle distillation calculations more cleanly. It is a basic pressure-bookkeeping rule for gas mixtures.
You use it to connect the total pressure in the vapor phase to each component's partial pressure. That helps you predict vapor composition, estimate boiling behavior, and analyze what leaves a distillation stage. It is especially useful when the vapor contains more than one component.
No. The ideal gas law relates pressure, volume, temperature, and moles for a gas, while Dalton's Law adds up the pressures of gases in a mixture. They often work together because Dalton's Law is easiest to use when the gases behave ideally.
The total pressure is P_total = P_1 + P_2 + P_3 + ... for all nonreacting gases in the mixture. You can also write each partial pressure as P_i = y_i P_total, where y_i is the mole fraction of that gas in the vapor. That second form is the one you see a lot in distillation problems.