Answer
$0.36334\text{ psia}$
Work Step by Step
The saturation temperature of water at $70^{\circ} \mathrm{F}$ is
$$
P_{\text {sat }@ 70^\circ \text{F}}=0.36334\ \mathrm{psia}
$$ Since the mole fraction of the air in the liquid water is essentially zero,
$$ P_{v, \max }=1 \times P_{\text {sat }@ 70^\circ \text{F}}=0.36334 \text { psia }
$$
