Note 24 — Chapter VII · MOLECULES AND ATOMS. THE LAWS OF GAY-LUSSAC AND AVOGADRO-GERHARDT

The above formula may be directly applied in order to ascertain the molecular weight from the data; weight of vapour m grms., its volume v c.c., pressure p cm., and temperature t°; for s = the weight of vapour m, divided by the volume v, and consequently M = 6,200m(273 + t)/pv. Therefore, instead of the formula (see Chapter II., Note 34), pv = R(273 + t), where R varies with the mass and nature of a gas, we may apply the formula pv = 6,200(m/M)(273 + t). These formulæ simplify the calculations in many cases. For example, required the volume v occupied by 5 grms. of aqueous vapour at a temperature t = 127° and under a pressure p = 76 cm. According to the formula M = 6,200m(273 + t)/pv, we find that v = 9,064 c.c., as in the case of water M = 18, m in this instance = 5 grms. (These formulæ, however, like the laws of gases, are only approximate.)