The pressure of a moist gas at 27 ° C is 4atm . The volume of the...

Dalton's Law states that the total pressure $P$ of moist air is the sum of partial pressures of dry air ${P}_{a}$ and water vapor ${P}_{v}$: $P = {P}_{a} + {P}_{v}$. Only the absolute pressure is the state parameter of moist air.

${P}_{gas} + {P}_{{H}_{2}O} = 4$

${P}_{gas} = 4 - 0 . 4 = 3 . 6atm$

As volume is doubled, ${\left[{P}_{gas}\right]}_{new} = 1 . 8atm$

New Total Pressure $= 1 . 8 + 0 . 4 = 2 . 2atm$