50cm3 of hydrogen is collected over water at 17∘C and 750 mm Hg pressure. Calculate the volume of dry gas at S.T.P. The water vapour pressure at 17∘C is 14 mm Hg.
We will use the following relation
P1V1T1=P2V2T2 ------- (1)
We are given that P1 = 750 - vapour pressure of the gas
= (750 - 14) mm of Hg
= 736mm of Hg
T1 = 170 C = (273+ 17) K = 290K
V1 = 50 cm3
We know that at STP conditions pressure = 1atm = 760 mm of Hg and temperature = 273 K
Hence P2 = 760 mm of Hg and T2 = 273 K
Substituting the above values in equation (1), we get
V2 = 736×50×273760×290
= 45.58 cm3