The vapor pressure of water at 18 degrees C is 15.5 torr. Based on Dalton's Law of Partial Pressures, this means that the fraction of water is 15.5 torr / 742 torr = 0.02089. Then the fraction of dry hydrogen is 1 - 0.02089 = 0.97911. To calculate for the volume of dry hydrogen, we multiply the total volume of 425 mL by 0.97911 to get the answer of 416.12 mL of dry H2.
