text: does anyone know how to work out problems dealing with dalton's law of partial pressures? please show how you do it!! i want to learn the process. there are 3 problems. showing me how to do them, i am able to do the rest of the 40 problem worksheet. thanks!!! 1. a 250. ml sample of oxygen is collected over water at 25 degrees celsius and 760.0  torr pressure. what is the pressure of the dry gas alone? (vapor pressure of water at 25 degrees celsius = 23.8 torr)  2. a mixture of 2.00 moles of h2 (2 is a superscript, meaning there is 2 hydrogen), 3.00 moles of nh3 (ammonia), 4.00 moles of co2 (carbon dioxide) and 5.00 moles of n2 (nitrogen gas) exerts a total pressure of 800 torr. what is the partial pressure of each gas?  3. the partial pressure of f2 (2 is a supersript) in a mixture of gases where the total pressure is 1.00 atm is 300. torr. what is the mole fraction of f2 2 is a superscript)?  thank you very much, and please show your work!! just kidding! just tell me how you got the answer, (the process, what formulas?) thanks! 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}.  for #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.  for #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.  for #3, since the mole fraction is the same ration as the partial pressure fraction, take 300torr/760torr(1atm) = .395.  this is the mole fraction.  these 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.