---
title: "Gas Constant (R) — AP Chem Definition & Values"
description: "The gas constant R links P, V, n, and T in PV = nRT. Learn its two AP values (0.08206 and 8.314), when to use each, and where R shows up beyond Unit 3."
canonical: "https://fiveable.me/ap-chem/key-terms/gas-constant"
type: "key-term"
subject: "AP Chemistry"
unit: "Unit 3"
---

# Gas Constant (R) — AP Chem Definition & Values

## Definition

The gas constant (R) is the proportionality constant in the ideal gas law PV = nRT that ties together pressure, volume, moles, and absolute temperature; on AP Chem you use R = 0.08206 L·atm/(mol·K) for gas law math and R = 8.314 J/(mol·K) for energy-based equations.

## What It Is

The gas constant, written as R, is the number that makes the [ideal gas law](/ap-chem/unit-3/ideal-gas-law/study-guide/XINb2AUU6e3c1rGlhBXg "fv-autolink") work. [PV = nRT](/ap-chem/key-terms/pv-nrt "fv-autolink") says pressure times volume is proportional to moles times absolute temperature, and R is the conversion factor that makes both sides match. Per essential knowledge 3.4.A.1, this single equation relates all the macroscopic properties of an ideal gas, and R is the glue holding it together.

Here's the part that trips people up. R doesn't have one value; it has one value *per set of units*. If your pressure is in atmospheres and your volume is in liters, R = 0.08206 L·atm/(mol·K). If you're working in energy units (joules), R = 8.314 J/(mol·K). Both are on the AP equations sheet, so you never memorize them, but you absolutely have to pick the right one. Think of R like an exchange rate. The underlying relationship between gas properties never changes, but the number you multiply by depends on what currency (units) you're working in.

## Why It Matters

R lives in Topic 3.4 (Ideal Gas Law) inside [Unit 3](/ap-chem/unit-3 "fv-autolink"), supporting learning objective 3.4.A, which asks you to explain how the macroscopic properties of a gas sample relate to each other. Every PV = nRT calculation, every molar mass-from-gas-density problem, and every stoichiometry problem involving a gas [volume](/ap-chem/key-terms/volume "fv-autolink") runs through R. But R is sneakily one of the most reused constants in the whole course. The same 8.314 J/(mol·K) reappears in the Arrhenius equation in kinetics (Unit 5) and in ΔG° = −RT ln K in thermodynamics (Unit 9). Learning what R does in Unit 3 pays off three more times before May.

## Connections

### [PV = nRT (Unit 3)](/ap-chem/key-terms/pv-nrt)

This is R's home. The ideal gas law is the equation, and R is the constant that makes it quantitative. Without R, PV = nRT is just a proportionality statement; with R, it's a calculator-ready formula. If you only learn one thing about R, learn that it lets you solve for any one of P, V, n, or T given the other three.

### [Dalton's Law of Partial Pressure (Unit 3)](/ap-chem/key-terms/daltons-law-of-partial-pressure)

Each gas in a [mixture](/ap-chem/key-terms/mixture "fv-autolink") obeys PV = nRT independently, with the same R. That's exactly why partial pressures are proportional to mole fractions (3.4.A.2). Since R, T, and V are identical for every gas in the container, pressure depends only on moles, so P_A = P_total × X_A falls right out of the ideal gas law.

### Kinetic Energy and Temperature (Unit 3)

R also connects temperature to molecular motion. The [average kinetic energy](/ap-chem/key-terms/average-kinetic-energy "fv-autolink") of gas particles depends only on absolute temperature, and R (in J/mol·K) is the constant that converts kelvins into joules per mole. This is why R has an energy version at all.

### Arrhenius Equation and ΔG° = −RT ln K (Units 5 and 9)

The 8.314 J/(mol·K) version of R returns in k = Ae^(−Ea/RT) for [reaction rates](/ap-chem/unit-5/reaction-rates/study-guide/4V94d3BwjoPaOOyQtDKQ "fv-autolink") and in ΔG° = −RT ln K for equilibrium thermodynamics. In both, R is doing the same job, converting temperature into energy units. Spotting that pattern makes three units feel like one idea.

## On the AP Exam

R itself is given on the AP Chem equations and constants sheet, so you're never tested on memorizing it. You're tested on using it correctly. Multiple-choice questions ask things like which value of R goes with atm, L, and K (it's 0.08206), or hand you a gas scenario and expect a quick PV = nRT setup. Graphical-analysis questions are also fair game. A plot like ln(P) vs. 1/T produces a slope containing R, and you're asked to interpret what that slope tells you physically. On free-response questions, R shows up inside calculations: finding moles of a gas collected in a lab setup, computing molar mass from gas density, or later in the course plugging into ΔG° = −RT ln K. The most common point-loser is a unit mismatch, like using kPa with the 0.08206 value or forgetting to convert Celsius to kelvin. Always check that your units cancel.

## gas constant vs 0.08206 L·atm/(mol·K) vs. 8.314 J/(mol·K)

These aren't two different constants. They're the same R expressed in different units, like the same distance in miles versus kilometers. Use 0.08206 when your problem is in pressure-volume land (atm and L) doing gas law math. Use 8.314 when your problem is in energy land (joules), like the Arrhenius equation or ΔG° = −RT ln K. Quick gut check before you compute. If joules appear anywhere in the equation, you want 8.314.

## Key Takeaways

- The gas constant R is the proportionality constant in PV = nRT that relates pressure, volume, moles, and absolute temperature for an ideal gas.
- R has different numerical values depending on units, and on the AP exam you use 0.08206 L·atm/(mol·K) for gas law calculations and 8.314 J/(mol·K) for energy-based equations.
- Both values of R are printed on the AP Chem equations sheet, so the skill being tested is choosing the right one and making your units cancel.
- Temperature must always be in kelvin when you use R, because the ideal gas law only works with absolute temperature.
- The same R from Unit 3 reappears in the Arrhenius equation in Unit 5 and in ΔG° = −RT ln K in Unit 9, always converting temperature into energy per mole.
- Because every gas in a mixture shares the same R, T, and V, partial pressure depends only on moles, which is the logic behind Dalton's Law.

## FAQs

### What is the gas constant in AP Chemistry?

It's R, the proportionality constant in the ideal gas law PV = nRT (essential knowledge 3.4.A.1). It connects a gas's [pressure](/ap-chem/key-terms/pressure "fv-autolink") and volume to its moles and absolute temperature, with a value of 0.08206 L·atm/(mol·K) or 8.314 J/(mol·K) depending on units.

### Do I have to memorize the value of R for the AP Chem exam?

No. Both values of R (0.08206 L·atm/(mol·K) and 8.314 J/(mol·K)) are printed on the equations and constants sheet you get during the exam. What you do need to know is which one matches the units in your problem.

### When do I use 8.314 instead of 0.08206 for R?

Use 8.314 J/(mol·K) whenever the equation involves energy in joules, like the Arrhenius equation k = Ae^(−Ea/RT) or ΔG° = −RT ln K. Use 0.08206 L·atm/(mol·K) when you're solving PV = nRT with pressure in atm and volume in liters.

### Is the gas constant the same as the rate constant k?

No. R is a universal constant that never changes (only its units do), while the rate constant k in kinetics is specific to a particular reaction and changes with temperature. Confusingly, R appears inside the Arrhenius equation that describes how k depends on temperature, but they are completely different quantities.

### Why does R have different values?

Because R carries units, and changing the units changes the number, just like 1 mile and 1.6 kilometers describe the same distance. R = 0.08206 L·atm/(mol·K) and R = 8.314 J/(mol·K) express the exact same physical constant; a liter-atmosphere is just a different unit of energy than a joule.

## Related Study Guides

- [3.4 Ideal Gas Law](/ap-chem/unit-3/ideal-gas-law/study-guide/XINb2AUU6e3c1rGlhBXg)

## Structured Data

```json
{"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/ap-chem/key-terms/gas-constant#resource","name":"Gas Constant (R) — AP Chem Definition & Values","url":"https://fiveable.me/ap-chem/key-terms/gas-constant","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/ap-chem/key-terms/gas-constant#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-06-11T05:27:14.797Z","isPartOf":{"@type":"Collection","name":"AP Chemistry Key Terms","url":"https://fiveable.me/ap-chem/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/ap-chem/key-terms/gas-constant#term","name":"gas constant","description":"The gas constant (R) is the proportionality constant in the ideal gas law PV = nRT that ties together pressure, volume, moles, and absolute temperature; on AP Chem you use R = 0.08206 L·atm/(mol·K) for gas law math and R = 8.314 J/(mol·K) for energy-based equations.","url":"https://fiveable.me/ap-chem/key-terms/gas-constant","inDefinedTermSet":{"@type":"DefinedTermSet","name":"AP Chemistry Key Terms","url":"https://fiveable.me/ap-chem/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is the gas constant in AP Chemistry?","acceptedAnswer":{"@type":"Answer","text":"It's R, the proportionality constant in the ideal gas law PV = nRT (essential knowledge 3.4.A.1). It connects a gas's [pressure](/ap-chem/key-terms/pressure \"fv-autolink\") and volume to its moles and absolute temperature, with a value of 0.08206 L·atm/(mol·K) or 8.314 J/(mol·K) depending on units."}},{"@type":"Question","name":"Do I have to memorize the value of R for the AP Chem exam?","acceptedAnswer":{"@type":"Answer","text":"No. Both values of R (0.08206 L·atm/(mol·K) and 8.314 J/(mol·K)) are printed on the equations and constants sheet you get during the exam. What you do need to know is which one matches the units in your problem."}},{"@type":"Question","name":"When do I use 8.314 instead of 0.08206 for R?","acceptedAnswer":{"@type":"Answer","text":"Use 8.314 J/(mol·K) whenever the equation involves energy in joules, like the Arrhenius equation k = Ae^(−Ea/RT) or ΔG° = −RT ln K. Use 0.08206 L·atm/(mol·K) when you're solving PV = nRT with pressure in atm and volume in liters."}},{"@type":"Question","name":"Is the gas constant the same as the rate constant k?","acceptedAnswer":{"@type":"Answer","text":"No. R is a universal constant that never changes (only its units do), while the rate constant k in kinetics is specific to a particular reaction and changes with temperature. Confusingly, R appears inside the Arrhenius equation that describes how k depends on temperature, but they are completely different quantities."}},{"@type":"Question","name":"Why does R have different values?","acceptedAnswer":{"@type":"Answer","text":"Because R carries units, and changing the units changes the number, just like 1 mile and 1.6 kilometers describe the same distance. R = 0.08206 L·atm/(mol·K) and R = 8.314 J/(mol·K) express the exact same physical constant; a liter-atmosphere is just a different unit of energy than a joule."}}]},{"@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"AP Chemistry","item":"https://fiveable.me/ap-chem"},{"@type":"ListItem","position":2,"name":"Key Terms","item":"https://fiveable.me/ap-chem/key-terms"},{"@type":"ListItem","position":3,"name":"Unit 3","item":"https://fiveable.me/ap-chem/unit-3"},{"@type":"ListItem","position":4,"name":"gas constant"}]}]}
```