gas constant, R

Meaning: R is a universal constant that links energy, amount of substance and temperature. It equals the Boltzmann constant times Avogadro's number. It is the same for every ideal gas. Its numerical value depends only on the units used: 8.314 J/(mol·K) or 0.0821 L·atm/(mol·K).

Equation: PV = nRT, where P is pressure (Pa), V is volume (m³), n is amount of gas (mol), R is the gas constant (J/(mol·K), which equals Pa·m³/(mol·K)), and T is absolute temperature (K).

On the MCAT: In gas problems, pick the R that matches the units given. If pressure is in atm and volume is in L, use 0.0821. R also appears outside gas problems. Examples are ΔG = ΔG° + RT ln Q, ΔG° = −RT ln Keq, and the Arrhenius equation k = Ae^(−Ea/RT). In these, R is used in joules, so a ΔG° given in kJ/mol must be converted to J/mol first. For an ideal gas, the average kinetic energy per mole is (3/2)RT, which is why temperature in kelvins sets molecular energy.

Common confusion: R is not a variable that changes with the gas or the conditions. Only its units change. The usual errors are mixing unit versions, such as 8.314 with pressure in atm, and using °C instead of K. Always match R to the units of the other quantities, and convert temperature to kelvins.