12 . 13.7
7 g of nitrogen corresponds to 0.25 mol, and hence from the balanced reaction, one would
expect that 0.50 mol of NO 2 will be produced. Therefore, the volume at STP will be (0.5 mol
NO 2 ) (22.4 L/mol at STP) = 11.2 L NO 2Now find the volume under the conditions given:13 . 728
There are three different gases in this system: hydrogen, oxygen, and water vapor. Therefore,
the pressure due to hydrogen plus oxygen will be equal to the total pressure minus the vapor
pressure of the water, which is 12 mm Hg:740 mm Hg – 12 mm Hg = 728 mm Hg14 . 0.66
From Dalton’s law of partial pressures, the sum of the partial pressures is equal to the total
pressure. In question 13 it was determined that the pressure due to H 2 and O 2 is 728 mm Hg. If
the partial pressure of oxygen is 242.7 mm Hg, then the partial pressure of hydrogen is 728 mm
Hg – 242.7 mm Hg = 485.3 mm Hg. Now, the mole fraction can be calculated using the
equation