# 18.3: The Universal Gas Constant

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If you had an ideal gas, all you would have to do is to measure its pressure, its temperature, and the volume occupied by a mole, for then *PV* = *RT*. (Measuring *P* and *T* is relatively easy. Measuring the volume occupied by a mole is less so.) In real life, however, we have to make measurements on real gases. What has to be done is to measure the product *PV* (at a given temperature) at progressively lower and lower pressures, and extrapolate the value of *PV/T* to the limit of zero pressure. (See notes in Chapter 6 on the compression factor.)