gas constant A constant, equal to 8.314 joules per mole degree Kelvin or 1.985 calories per mole degree Celsius, that is the constant of proportionality (R) in the universal gas law.
(symbol R), fundamental physical constant arising in the formulation of the general gas law. For an ideal gas (approximated by most real gases that are not highly compressed or not near the point of liquefaction), the pressure p times the volume V of the gas divided by its absolute temperature T is a constant. When one of these three is altered for a given mass of gas, at least one of the other two undergoes a change so that the expression pV/T remains constant. The constant, further, is the same for all gases, provided the mass of gas being compared is one mole, or one molecular weight in grams. For one mole, therefore, pV/T = R.