text: The first step to solving these problems is knowing what Dalton's Law of Partial Pressures actually is.  The law states that the P(total)= P(1) + P(2) ... {with the items in parentheses in subscript}.\n\nFor #1, I know that the total pressure is 760.0 Torr.  I know that the water pressure at 25 degrees Celsius is 23.8 Torr.  So according to Dalton's Law, 760.0=23.8 + P(O2).  I know that P(O2) is the only pressure left because it is the only gas in the container.  So, with some subtraction, P(O2) = 736.  There are only 3 sig figs here, so don't add the .2.\n\nFor #2, I need to employ the theory of the mole fraction. The mole fraction is found for a particular substance in the container, and is found in the same ratio as partial pressures.  To find the mole fraction, take the number of moles of the substance and divide by the total number of moles in the container.  So, by knowing the mole fraction, I can then multiply that number by the total pressure to find the partial pressure.  I will do this for you for only H2.  The mole fraction for H2 is 2.00mol/14.0mol=.143.  .143*800= 114 Torr.\n\nFor #3, since the mole fraction is the same ration as the partial pressure fraction, take 300Torr/760Torr(1atm) = .395.  This is the mole fraction.\n\nThese problems become a lot easier when you remember Dalton's Law, and its applications through mole fractions.  Also, remember to keep the right number of sig figs.