The pressure of the dry gas : 687.5 torr
Further explanation
Dalton's law of partial pressures states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the component gases
Can be formulated:
P tot = P1 + P2 + P3 ....
The partial pressure is the pressure of each gas in a mixture
Since the gas mixture consists of He and H₂O :
P tot=P He + P H₂O
[tex]\tt 705=P~He+17.5\\\\P~He=705-17.5=687.5~torr[/tex]
