A certain quantity of a gas occupies 300 mL when collected over water at 25^@C and 745 mm pressure. It occupied 182.6 mL in dry state at S.T.P. Find the vapour pressure of water at 25^@C.

A certain quantity of a gas occupies 300 mL when collected over water

1.	A certain quantity of a gas occupies 300 mL when collected over water at 25^@C and 745 mm pressure. It occupied 182.6 mL in dry state at S.T.P. Find the vapour pressure of water at 25^@C.
Answer» Solution :Let us calculate the PRESSURE EXERTED by 182.6 mL (at S.T.P.) of the DRY GAS when its volume is 300 mL and temperature is `25^@C, i.e.`, `P_1 = 760 " mm Hg. " V_1 = 182.6 mL`, `T_1 = 273 K` (at S.T.P.) `P_2 = ?, "" V_2 = 300 ml, "" T_2 = 25 + 273 = 298 K` According to the gas equation, `(P_1V_1)/T_1 = (P_2V_2)/T_2` Substituting the values, we have `(760 xx 182.6)/273 =(P_2 xx 300)/298 or P_2 = 504.9` mm Hg Under SIMILAR conditions, the wet gas exerts a pressure of 745 mm. `:." Vapour pressure of water at " 25^@C` `= 745 - 504.9 = 240.1 mm`