Yes, R is constant in the ideal gas law for a given set of units, but its numerical value changes depending on the units used for pressure, volume, and temperature.
Does R change in ideal gas law?
Yes, R changes in the ideal gas law because its numerical value depends entirely on the units chosen for pressure, volume, and temperature.
Think of R as a currency converter between measurement systems. Switch from liters and atmospheres to cubic meters and pascals, and suddenly R’s number changes—but the actual gas behavior stays identical. It’s the same physical constant, just expressed in different "currencies." ChemGuide has a great breakdown of these unit conversions.
Why is the ideal gas constant R?
The ideal gas constant R is named after the French chemist Henri Victor Regnault, who conducted precise experiments on gas behavior in the 19th century.
Regnault’s meticulous work helped standardize gas measurements, and scientists honored him by slapping his name on this constant. Fun fact: R also equals the Boltzmann constant (kB) multiplied by Avogadro’s number (NA), which ties together tiny particle behavior with big-picture gas properties. It’s basically the bridge between "energy per molecule" and "energy per mole." Wikipedia has the full historical timeline.
What is the constant R equal to in our ideal gas law equation?
In the ideal gas law PV = nRT, R equals 0.082057 L·atm·mol–1·K–1 when using liters for volume, atmospheres for pressure, moles for amount, and Kelvin for temperature.
This specific value is like a perfectly calibrated measuring cup—it only works if you use the right units. Plug in P in atm, V in L, n in mol, and T in K, and the units cancel out neatly. Mess up the units, though, and your calculation will be as wrong as using teaspoons when the recipe calls for milliliters. Always triple-check your units before hitting "calculate." LibreTexts Chemistry has a handy unit conversion guide.
What is the constant R in PV nRT?
In the equation PV = nRT, R is the universal gas constant, typically valued at 8.314 J·mol–1·K–1 when using SI units.
This version works when pressure is in pascals (Pa), volume in cubic meters (m³), temperature in kelvin (K), and amount in moles (mol). For air, engineers often use R = 287 J·kg–1·K–1 because it accounts for air’s molar mass. The golden rule? Always match R to your units—mixing them up is the fastest way to ruin your day. Omni Calculator has an interactive unit converter to save you the headache.
What is the constant R equal to?
| Unit System | Value of R | Units |
|---|---|---|
| SI | 8.3144598 | m³·Pa·K⁻¹·mol⁻¹ |
| Pressure in Torr | 62.363577 | L·Torr·K⁻¹·mol⁻¹ |
| Thermal Energy | 1.9872036 × 10⁻³ | kcal·K⁻¹·mol⁻¹ |
| Pressure in atm | 8.2057338 × 10⁻⁵ | m³·atm·K⁻¹·mol⁻¹ |
Each row gives you an R value tailored for a specific unit system—use the one that matches your measurements. For example, if your pressure is in torr, just grab the 62.36 value and plug it straight in. No conversions needed. Engineering Toolbox lists R values for tons of unit combinations, so you’re never stuck guessing.
What is PV is equal to nRT?
PV = nRT is the ideal gas law, stating that the product of pressure and volume equals the number of moles times the gas constant times the temperature.
This equation is the Swiss Army knife of gas behavior. Need to know what happens when you heat a gas in a sealed container? Pressure goes up. Compress a gas into a smaller space? Pressure spikes. It’s a compact way to predict how gases react to changes in conditions. University of California, Los Angeles walks through the math and real-world applications.
What does N mean in the ideal gas law?
In the form PV = NkBT, N represents the actual number of atoms or molecules in the gas sample.
This version swaps out R for the Boltzmann constant (kB), making it perfect for microscopic calculations. Instead of dealing with moles, you’re counting individual molecules—like switching from bags of marbles to counting each marble one by one. To flip between moles and molecules, just multiply or divide by Avogadro’s number. Physics of the Universe explains how particle count ties into temperature.
What is r in PV nRT for Torr?
When pressure is measured in Torr (mmHg), R equals 62.363 L·Torr·K⁻¹·mol⁻¹ in the ideal gas law PV = nRT.
This pre-scaled R value saves you the hassle of converting Torr to other units. Just plug in P in Torr, V in liters, n in moles, and T in kelvin, and the math works like a charm. It’s the gas constant equivalent of using a measuring cup that’s already marked for your recipe. Need to double-check conversions? ConvertUnits has you covered.
How do you find R in PV nRT?
To find R in PV = nRT, rearrange the equation to R = PV/(nT) and plug in your measured values with consistent units.
Say you measure pressure in atmospheres, volume in liters, moles by mass divided by molar mass, and temperature in kelvin. Plug those numbers in, and R should pop out as 0.0821 L·atm·mol⁻¹·K⁻¹. The catch? Units must match perfectly—mix liters with cubic meters, and your answer will be garbage. Think of R as a recipe: the ingredients have to be in the right proportions, or the whole thing falls apart. Khan Academy has step-by-step examples to walk you through it.
How do you find N in PV nRT?
To find the number of moles n in PV = nRT, rearrange the equation to n = PV/(RT).
Start by measuring pressure, volume, and temperature, then divide their product by R and T. For example, if P = 2 atm, V = 10 L, T = 300 K, and R = 0.0821 L·atm·mol⁻¹·K⁻¹, n = (2 × 10)/(0.0821 × 300) ≈ 0.81 mol. Always double-check units—mixing them up is the fastest way to get nonsense results. This formula is gold for gas stoichiometry problems. LibreTexts Gas Laws has worked examples to study.
What is the value of R in kJ?
| Context | R in joules | R in kilojoules |
|---|---|---|
| per gram-mole per kelvin | 8.31447 J·mol⁻¹·K⁻¹ | 0.00831447 kJ·mol⁻¹·K⁻¹ |
| per pound-mole per kelvin | 3771.38 J·lb-mol⁻¹·K⁻¹ | 3.77138 kJ·lb-mol⁻¹·K⁻¹ |
Use these values when your calculations demand energy in kilojoules instead of joules. For example, in thermodynamics problems involving heat transfer, converting R to kJ keeps the units consistent with other energy terms. Just be sure your software or textbook expects kJ—getting this wrong can lead to answers that are off by a factor of 1000. Engineering Toolbox has even more variations if you need them.
What is a in real gas equation?
In the van der Waals real gas equation (P + an²/V²)(V − nb) = nRT, the constant a accounts for intermolecular attractive forces between gas molecules.
The a term bumps up the effective pressure because attraction softens the collisions with container walls. Bigger, stickier molecules (think CO₂) have higher a values than tiny, slippery ones (like He). Meanwhile, the b term corrects for molecular volume, subtracting the space molecules physically occupy. Honestly, this is the most intuitive way to model real gases when ideal gas assumptions start to crack. LibreTexts Real Gases breaks down the physics beautifully.
What is R in pV nRT for kPa?
When pressure is in kilopascals (kPa), volume in liters (L), temperature in kelvin (K), and amount in moles (mol), R equals 8.314 kPa·L·K⁻¹·mol⁻¹ in the ideal gas law pV = nRT.
This version is basically the SI gas constant tweaked for common lab units. Many pressure sensors spit out kPa, and lab glassware often uses liters, so this R value is super convenient. Just remember: if your volume is in cubic meters, divide by 1000 first. Mixing volume units is a classic way to mess up calculations. Omni Calculator has a kPa-specific solver to save you the trouble.
