How do you calculate the number of molecules?

Calculate the number of molecules of water (H2O) in 1120 L at STP and.....?

• Calculate the number of molecules of water (H2O) in 1120 L at STP. Calculate the volume of 143.03 g of carbon dioxide gas (CO2) at STP. These are for an online practice test ...show more

• Answer:

  I've answered this question before..... For Melody. Do you happen to know Melody? "Calculate the number of molecules of water (H2O) in 1120 L at STP." If you have H2O at STP, then it must be water vapor. This is a nonsense question because at standard temperature (273K or 0C) water freezes, and the vapor pressure of water vapor is quite low. It certainly won't be at 1.00 atm, which is standard pressure. Whoever wrote the question may be as confused as you are. The question should be about a substance which is a gas at STP, like oxygen or nitrogen or some other gas. Nonetheless, the process is the same for any ideal gas at STP (which wouldn't include water). More on the mole concept here: http://c7chemistry.wikispaces.com/file/v... and here" http://c7chemistry.wikispaces.com/file/v... Use the unit-factor method for solving the problem. 1120L x (1 mol gas / 22.4L) x (6.022x10^23 molecules gas / 1 mol gas) = 3.01x10^25 molecules gas ......... to three significant digits "Calculate the volume of 143.03 g of carbon dioxide gas (CO2) at STP." 143.03g CO2 x (1 mol CO2 / 44.009g CO2) x (22.414L CO2 / 1 mol CO2) = 72.846L CO2 ....... to five significant digits