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In a certain constant-volume ideal-gas thermometer, the measured pressure at the ice point (see Section 1.11) of water, 0°C, 1s 110.9 kPa and at the steam point, 100°C, it1s 151.5 kPa. Extrapolating, at what Celsius temperature does the pressure go to zero (i.e., zero absolute temperature)? Analysis From the ideal-gas equation of state PV = m RT at constant mass and volume, pressure is directly proportional to temperature, as shown in Fig. 5.22. P = CT, where T is the absolute ideal-gas temperature J 1515 = = = = - ———— | 1109 = = = = = — — — /—4/: T I i l | Pl l | - I | oL” I I ? 0°C 100°C T FIGURE 5.22 Plot for Example 5.3. Solution AP 151.5-110.9 Slope — = = 0.406 kPa/°C N 100 —0 Extrapolating from the 0°C point to P = 0, 110.9 kPa T=0- =125 |H (@ 0.406 kPa/°C establishing the relation between absolute ideal-gas Kelvin and Celsius temperature scales. (Note: Compatible with the subsequent present-day definitions of the Kelvin scale and the Celsius scale in Section 1.11.) T T . — HL1E N =~ e