Helium gas is collected over water at 25◦c. what is the partial pressure of the helium, given a barometric pressure of 720 mm hg? answer in units of mm hg.

The total pressure in the container is thus 720 mmHg which is given by the sum of the partial pressure of helium plus pressure of water;
That is; P (helium) + P (water) = P (total)
Therefore; P (helium) = P (total) - P (water) 
Assuming the pressure of water at 25 °C is 23.76 mm Hg
Hence; P (helium) = 720 - 23.76 
  = 696.24 mm Hg    

Rn in the right answer